A COMPOSITE MATERIAL is a macroscopic combination of two or more distinct materials, having a
recognizable interface between them. Composites are used not only for their structural properties, but
also for electrical, thermal, tribological, and environmental applications. Modern composite materials
are usually optimized to achieve a particular balance of properties for a given range of applications.
Given the vast range of materials that may be considered as composites and the broad range of uses for
which composite materials may be designed, it is difficult to agree upon a single, simple, and useful
definition. However, as a common practical definition, composite materials may be restricted to
emphasize those materials that contain a continuous matrix constituent that binds together and
provides form to an array of a stronger, stiffer reinforcement constituent.
The resulting composite material has a balance of structural properties that is superior to either
constituent material alone. The improved structural properties generally result from a load-sharing
mechanism. Although composites optimized for other functional properties (besides high structural
efficiency) could be produced from completely different constituent combinations than fit this structural
definition, it has been found that composites developed for structural applications also provide
attractive performance in these other functional areas as well. As a result, this simple definition for
structural composites provides a useful definition for most current functional composites.
in general there are two phases in a composite material the outer material which keeps the other
material stiff and un damaged form the external force is called matrix. the main function of matrix is to
provide a best cover for the secondary phase particles.based on the type of secondary phase the
composite materials are classified in to
1) Fiber reinforced composite material
2) Flake rein forced composite materials
3) particle reinforced composite materials
some of the desirable properties of the matrix material are ductile and soft where as the properties of
secondary phase are quite opposite. they should posses high hardness and also highly brittle in nature
due to high hardness.
why are composite materials needed when there are more than 100 variety of materials available and
numerous number of alloys and ceramics available? this may be the question which had raised in many
minds? here is the solution for it. when you take the life of a air crafts whose body is developed by
duralumin, its life time is a maximum of 10 years after then the air crafts becomes useless. but due to
introduction of sintered aluminum the life time of air crafts hiked by 80 years. this development is
observed in latest air crafts. this indicates the importance of the composite materials.
Fiber reinforced composites:- in these composite materials the secondary phase is made up of fibers.
these composite materials are again sub classified in to sub categories based on the length of it they are
namely
1) continuous fiber reinforced composite materials
2) discontinuous fiber reinforced composite materials
continuous fiber reinforced composite materials have a continuous fibers and do not have any breaks
through out the material. where as it is not the same in case of dis continuous materials. they are very
discontinuous and again classified based on the orientation of the fibers. aligned and random are the
two types.
flake reinforced composites contain secondary particles in the form of flakes. a flake is a piece of
material having no particular shape or orientation.
last but not least of this classification is particle reinforced composite materials. these have particles
sprinkled in the primary phase. the classifications in this are large particle reinforced composite
materials and dispersion strengthened materials.
finally i would like to thank in advance for all the readers of my artilce and ISC for giving me a great
opportunity for expressing my learnt knowledge in a great platform like this
Introduction to Materials and Processes
Introduction
Introduction
General Material Classifications
Metals
Ceramics
Polymers
Composites
Structure of Materials
Atomic Bonds
Solid State Structure
Primary Metallic Crystalline Structures
Solidification
Anisotropy and Isotropy
Crystal Defects
Elastic/Plastic Deformation
Fatigue Crack Initiation
Diffusion
Property Modification
Ceramic Structures
Polymer Structure
Composite Structures
Physical and Chemical
Properties
Section Introduction
Phase Transformation Temperature
Density
Specific Gravity
Thermal Conductivity
Thermal Expansion
Electrical Conductivity
Magnetic Properties
Oxidation and Corrosion
Mechanical Properties
Section Introduction
- Loading
- Stress & Strain
Tensile
Compression, Bearing, & Shear
Hardness
Creep & Stress Rupture
Toughness
- Impact Toughness
- Notch Toughness
- Fracture Toughness
Fatigue
- S-N Fatigue
- Fatigue Crack Growth Rate
Selection of Materials
Specific Metals
Metal Ores
Iron and Steel
Decarburization
Aluminum and Aluminum Alloys
Nickel and Nickel Alloys
Titanium and Titanium Alloys
Polymers
Composites
General Manufacturing
Processes
Metallic
Wrought
Castings
Welding
Brazing and Soldering
Forming
Machining
Powdered Metal Processes
Heat Treatment
Surface Treatment
Ceramic and Glass
Basic Materials Processing
Polymers/Plastic
Basic Materials Processing
Adhesive Joining
Composites
Basic Materials Processing
Adhesive Joining
Origins of Discontinuities
Inherent Discontinuities
Process-Induced Discontinuities
Service-Induced Discontinuities
Material Specifications
Component Design,
Performance and NDE
Strength
Durability
Fracture Mechanics
Nondestructive Evaluation
Introduction to Materials
This section will provide a basic introduction to materials and material fabrication processing. It
is important that NDT personnel have some background in material science for a couple of
reasons. First, nondestructive testing almost always involves the interaction of energy of some
type (mechanics, sound, electricity, magnetism or radiation) with a material. To understand how
energy interacts with a material, it is necessary to know a little about the material. Secondly,
NDT often involves detecting manufacturing defects and service induced damage and, therefore,
it is necessary to understand how defects and damage occur.
This section will begin with an introduction to the four common types of engineering materials.
The structure of materials at the atomic level will then be considered, along with some atomic
level features that give materials their characteristic properties. Some of the properties that are
important for the structural performance of a material and methods for modifying these
properties will also be covered.
In the second half of this text, methods used to shape and form materials into useful shapes will
be discussed. Some of the defects that can occur during the manufacturing process, as well as
service induced damage will be highlighted. This section will conclude with a summary of the
role that NDT plays in ensuring the structural integrity of a component.
General Material Classifications
There are thousands of materials available for use in
engineering applications. Most materials fall into one of three
classes that are based on the atomic bonding forces of a
particular material. These three classifications are metallic,
ceramic and polymeric. Additionally, different materials can
be combined to create a composite material. Within each of
these classifications, materials are often further organized
into groups based on their chemical composition or certain
physical or mechanical properties. Composite materials are
often grouped by the types of materials combined or the way
the materials are arranged together. Below is a list of some of the commonly classification of
materials within these four general groups of materials.
Metals
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Ferrous metals and alloys (irons,
carbon steels, alloy steels,
stainless steels, tool and die
steels)
Nonferrous metals and alloys
(aluminum, copper, magnesium,
nickel, titanium, precious metals,
refractory metals, superalloys)
Polymeric
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Ceramics
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Glasses
Thermoplastics plastics
Thermoset plastics
Elastomers
Composites
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Reinforced plastics
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Glass ceramics
Graphite
Diamond
Metal-matrix composites
Ceramic-matrix composites
Sandwich structures
Concrete
Each of these general groups will be discussed in more detail in the following pages.
Metals
Metals account for about two thirds of all the elements and about 24% of the mass of the planet.
Metals have useful properties including strength, ductility, high melting points, thermal and
electrical conductivity, and toughness. From the periodic table, it can be seen that a large number
of the elements are classified as being a metal. A few of the common metals and their typical
uses are presented below.
Common Metallic Materials
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Iron/Steel - Steel alloys are used for strength critical applications
Aluminum - Aluminum and its alloys are used because they are easy to form, readily
available, inexpensive, and recyclable.
Copper - Copper and copper alloys have a number of properties that make them useful,
including high electrical and thermal conductivity, high ductility, and good corrosion
resistance.
Titanium - Titanium alloys are used for strength in higher temperature (~1000° F)
application, when component weight is a concern, or when good corrosion resistance is
required
Nickel - Nickel alloys are used for still higher temperatures (~1500-2000° F) applications
or when good corrosion resistance is required.
Refractory materials are used for the highest temperature (> 2000° F) applications.
The key feature that distinguishes metals from non-metals is their bonding. Metallic materials
have free electrons that are free to move easily from one atom to the next. The existence of these
free electrons has a number of profound consequences for the properties of metallic materials.
For example, metallic materials tend to be good electrical conductors because the free electrons
can move around within the metal so freely. More on the structure of metals will be discussed
later.
Ceramics
A ceramic has traditionally been defined as “an inorganic, nonmetallic solid that is prepared
from powdered materials, is fabricated into products through the application of heat, and displays
such characteristic properties as hardness, strength, low electrical conductivity, and brittleness."
The word ceramic comes the from Greek word "keramikos", which means "pottery." They are
typically crystalline in nature and are compounds formed between metallic and nonmetallic
elements such as aluminum and oxygen (alumina-Al2O3), calcium and oxygen (calcia - CaO),
and silicon and nitrogen (silicon nitride-Si3N4).
Depending on their method of formation, ceramics can be
dense or lightweight. Typically, they will demonstrate
excellent strength and hardness properties; however, they
are often brittle in nature. Ceramics can also be formed to
serve as electrically conductive materials or insulators.
Some ceramics, like superconductors, also display
magnetic properties. They are also more resistant to high
temperatures and harsh environments than metals and
polymers. Due to ceramic materials wide range of
properties, they are used for a multitude of applications.
The broad categories or segments that make up the ceramic industry can be classified as:
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Structural clay products (brick, sewer pipe, roofing and wall tile, flue linings, etc.)
Whitewares (dinnerware, floor and wall tile, electrical porcelain, etc.)
Refractories (brick and monolithic products used in metal, glass, cements, ceramics,
energy conversion, petroleum, and chemicals industries)
Glasses (flat glass (windows), container glass (bottles), pressed and blown glass
(dinnerware), glass fibers (home insulation), and advanced/specialty glass (optical
fibers))
Abrasives (natural (garnet, diamond, etc.) and synthetic (silicon carbide, diamond, fused
alumina, etc.) abrasives are used for grinding, cutting, polishing, lapping, or pressure
blasting of materials)
Cements (for roads, bridges, buildings, dams, and etc.)
Advanced ceramics
o Structural (wear parts, bioceramics, cutting tools, and engine components)
o Electrical (capacitors, insulators, substrates, integrated circuit packages,
piezoelectrics, magnets and superconductors)
o Coatings (engine components, cutting tools, and industrial wear parts)
o Chemical and environmental (filters, membranes, catalysts, and catalyst supports)
The atoms in ceramic materials are held together by a chemical bond which will be discussed a
bit later. Briefly though, the two most common chemical bonds for ceramic materials are
covalent and ionic. Covalent and ionic bonds are much stronger than in metallic bonds and,
generally speaking, this is why ceramics are brittle and metals are ductile.
Polymers
A polymeric solid can be thought of as a material that contains many chemically bonded parts or
units which themselves are bonded together to form a solid. The word polymer literally means
"many parts." Two industrially important polymeric materials are plastics and elastomers.
Plastics are a large and varied group of synthetic materials which are processed by forming or
molding into shape. Just as there are many types of metals such as aluminum and copper, there
are many types of plastics, such as polyethylene and nylon. Elastomers or rubbers can be
elastically deformed a large amount when a force is applied to them and can return to their
original shape (or almost) when the force is released.
Polymers have many properties that make them attractive to use in certain conditions. Many
polymers:
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are less dense than metals or ceramics,
resist atmospheric and other forms of corrosion,
offer good compatibility with human tissue, or
exhibit excellent resistance to the conduction of electrical current.
The polymer plastics can be divided into two classes,
thermoplastics and thermosetting plastics, depending on
how they are structurally and chemically bonded.
Thermoplastic polymers comprise the four most
important commodity materials – polyethylene,
polypropylene, polystyrene and polyvinyl chloride. There
are also a number of specialized engineering polymers.
The term ‘thermoplastic’ indicates that these materials
melt on heating and may be processed by a variety of
molding and extrusion techniques. Alternately,
‘thermosetting’ polymers can not be melted or remelted.
Thermosetting polymers include alkyds, amino and
phenolic resins, epoxies, polyurethanes, and unsaturated
polyesters.
Rubber is a natural occurring polymer. However, most
polymers are created by engineering the combination of hydrogen and carbon atoms and the
arrangement of the chains they form. The polymer molecule is a long chain of covalent-bonded
atoms and secondary bonds then hold groups of polymer chains together to form the polymeric
material. Polymers are primarily produced from petroleum or natural gas raw products but the
use of organic substances is growing. The super-material known as Kevlar is a man-made
polymer. Kevlar is used in bullet-proof vests, strong/lightweight frames, and underwater cables
that are 20 times stronger than steel.
Composites
A composite is commonly defined as a combination of two or more distinct materials, each of
which retains its own distinctive properties, to create a new material with properties that cannot
be achieved by any of the components acting alone. Using this definition, it can be determined
that a wide range of engineering materials fall into this category. For example, concrete is a
composite because it is a mixture of Portland cement and aggregate. Fiberglass sheet is a
composite since it is made of glass fibers imbedded in a polymer.
Composite materials are said to have two phases. The reinforcing phase is the fibers, sheets, or
particles that are embedded in the matrix phase. The reinforcing material and the matrix material
can be metal, ceramic, or polymer. Typically,
reinforcing materials are strong with low densities
while the matrix is usually a ductile, or tough,
material.
Some of the common classifications of
composites are:
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Reinforced plastics
Metal-matrix composites
Ceramic-matrix composites
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Sandwich structures
Concrete
Composite materials can take many forms but they can be separated into three categories based
on the strengthening mechanism. These categories are dispersion strengthened, particle
reinforced and fiber reinforced. Dispersion strengthened composites have a fine distribution of
secondary particles in the matrix of the material. These particles impede the mechanisms that
allow a material to deform. (These mechanisms include dislocation movement and slip, which
will be discussed later). Many metal-matrix composites would fall into the dispersion
strengthened composite category. Particle reinforced composites have a large volume fraction of
particle dispersed in the matrix and the load is shared by the particles and the matrix. Most
commercial ceramics and many filled polymers are particle-reinforced composites. In fiberreinforced composites, the fiber is the primary load-bearing component. Fiberglass and carbon
fiber composites are examples of fiber-reinforced composites.
If the composite is designed and fabricated correctly, it combines the strength of the
reinforcement with the toughness of the matrix to achieve a combination of desirable properties
not available in any single conventional material. Some composites also offer the advantage of
being tailorable so that properties, such as strength and stiffness, can easily be changed by
changing amount or orientation of the reinforcement material. The downside is that such
composites are often more expensive than conventional materials.
Structure of Materials
It should be clear that all matter is made of atoms. From the periodic table, it can be seen that
there are only about 100 different kinds of atoms in the entire Universe. These same 100 atoms
form thousands of different substances ranging from the air we breathe to the metal used to
support tall buildings. Metals behave differently than ceramics, and ceramics behave differently
than polymers. The properties of matter depend on which atoms are used and how they are
bonded together.
The structure of materials can be classified by the general magnitude of various features being
considered. The three most common major classification of structural, listed generally in
increasing size, are:
Atomic structure, which includes features that cannot be seen, such as the types of bonding
between the atoms, and the way the atoms are arranged.
Microstructure, which includes features that can be seen using a microscope, but seldom with the
naked eye.
Macrostructure, which includes features that can be seen with the naked eye)
The atomic structure primarily affects the chemical, physical, thermal, electrical, magnetic, and
optical properties. The microstructure and macrostructure can also affect these properties but
they generally have a larger effect on mechanical properties and on the rate of chemical reaction.
The properties of a material offer clues as to the structure of the material. The strength of metals
suggests that these atoms are held together by strong bonds. However, these bonds must also
allow atoms to move since metals are also usually formable. To understand the structure of a
material, the type of atoms present, and how the atoms are arranged and bonded must be known.
Let’s first look at atomic bonding.
Atomic Bonding
(Metallic, Ionic, Covalent, and van der Waals Bonds)
From elementary chemistry it is known that the atomic
structure of any element is made up of a positively charged
nucleus surrounded by electrons revolving around it. An
element’s atomic number indicates the number of
positively charged protons in the nucleus. The atomic
weight of an atom indicates how many protons and
neutrons in the nucleus. To determine the number of
neutrons in an atom, the atomic number is simply
subtracted from the atomic weight.
Atoms like to have a balanced electrical charge. Therefore,
they usually have negatively charged electrons surrounding
the nucleus in numbers equal to the number of protons. It is also known that electrons are present
with different energies and it is convenient to consider these electrons surrounding the nucleus in
energy “shells.” For example, magnesium, with an atomic number of 12, has two electrons in the
inner shell, eight in the second shell and two in the outer shell.
All chemical bonds involve electrons. Atoms will stay close together if they have a shared
interest in one or more electrons. Atoms are at their most stable when they have no partiallyfilled electron shells. If an atom has only a few electrons in a shell, it will tend to lose them to
empty the shell. These elements are metals. When metal atoms bond, a metallic bond occurs.
When an atom has a nearly full electron shell, it will try to find electrons from another atom so
that it can fill its outer shell. These elements are usually described as nonmetals. The bond
between two nonmetal atoms is usually a covalent bond. Where metal and nonmetal atom come
together an ionic bond occurs. There are also other, less common, types of bond but the details
are beyond the scope of this material. On the next few pages, the Metallic, Covalent and Ionic
bonds will be covered in more detail.
Ionic Bonds
Ionic bonding occurs between charged particles. These may be atoms or groups of atoms, but
this discuss will be conducted in terms of single atoms. Ionic bonding occurs between metal
atoms and nonmetal atoms. Metals usually have 1, 2, or 3 electrons in their outermost shell.
Nonmetals have 5, 6, or 7 electrons in their outer shell. Atoms with outer shells that are only
partially filled are unstable. To become stable, the metal atom wants to get rid of one or more
electrons in its outer shell. Losing electrons will either result in an empty outer shell or get it
closer to having an empty outer shell. It would like to have an empty outer shell because the next
lower energy shell is a stable shell with eight electrons.
Since electrons have a negative charge, the atom that gains electrons becomes a negatively
charged ions (aka anion) because it now has more electrons than protons. Alternately, an atom
that loses electrons becomes a positively charged ion (aka cations). The particles in an ionic
compound are held together because there are oppositely charged particles that are attracted to
one another.
The images above schematically show the process that takes place during the formation of an
ionic bond between sodium and chlorine atoms. Note that sodium has one valence electron that it
would like to give up so that it would become stable with a full outer shell of eight. Also note
that chlorine has seven valence electrons and it would like to gain an electron in order to have a
full shell of eight. The transfer of the electron causes the previously neutral sodium atom to
become a positively charged ion (cation), and the previously neutral chlorine atom to become a
negatively charged ion (anion). The attraction for the cation and the anion is called the ionic
bond.
Generally, solid materials with ionic bonds:
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are hard because particles cannot easily slide past one another.
are good insulators because there are no free electrons or ions (unless dissolved or
melted).
are transparent because their electrons are not moving from atom to atom and less likely
to interact with light photons.
are brittle and tend to cleave rather than deform because bonds are strong.
have high melting point because ionic bonds are relatively strong.
Covalent Bonding
Where a compound only contains nonmetal atoms, a covalent bond is formed by atoms sharing
two or more electrons. Nonmetals have 4 or more electrons in their outer shells (except boron).
With this many electrons in the outer shell, it would require more energy to remove the electrons
than would be gained by making new bonds. Therefore, both the atoms involved share a pair of
electrons. Each atom gives one of its outer electrons to the electron pair, which then spends some
time with each atom. Consequently, both atoms are held near each other since both atoms have a
share in the electrons.
More than one electron pair can be formed with half of the electrons coming from one atom and
the rest from the other atom. An important feature of this bond is that the electrons are tightly
held and equally shared by the participating atoms. The atoms can be of the same element or
different elements. In each molecule, the bonds between the atoms are strong but the bonds
between molecules are usually weak. This makes many solid materials with covalent bonds
brittle. Many ceramic materials have covalent bonds.
Compounds with covalent bonds may be solid, liquid or gas at room temperature depending on
the number of atoms in the compound. The more atoms in each molecule, the higher a
compound’s melting and boiling temperature will be. Since most covalent compounds contain
only a few atoms and the forces between molecules are weak, most covalent compounds have
low melting and boiling points. However, some, like carbon compounds, can be very large. An
example is the diamond in which carbon atoms each share four electrons to form giant lattices.
Some Common Features of Materials with Covalent Bonds:
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Hard
Good insulators
Transparent
Brittle or cleave rather than deform
Metallic Bonding
A common characteristic of metallic elements is they contain only one to
three electrons in the outer shell. When an element has only one, two or
three valence electrons (i.e. electrons in the outer shell), the bond
between these electrons and the nucleus is relatively weak. So, for
example, when aluminum atoms are grouped together in a block of
metal, the outer electrons leave individual atoms to become part of
common “electron cloud.” In this arrangement, the valence electrons
have considerable mobility and are able to conduct heat and electricity
easily. Also, the delocalized nature of the bonds, make it possible for the
atoms to slide past each other when the metal is deformed instead of fracturing like glass or other
brittle material.
Since the aluminum atoms lose two electrons, they end up having a positive charge and are
designated Al3+ ions (cations). These ions repel each other but are held together in the block
because the negative electrons are attracted to the positively charged ions. A result of the sharing
of electrons is the cations arrange themselves in a regular pattern. This regular pattern of atoms is
the crystalline structure of metals. In the crystal lattice, atoms are packed closely together to
maximize the strength of the bonds. An actual piece of metal consists of many tiny crystals
called grains that touch at grain boundaries.
Some Common Features of Materials with Metallic Bonds:
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Good electrical and thermal conductors due to their free valence electrons
Opaque
Relatively ductile
Solid State Structure
In the previous pages, some of the mechanisms that bond together the multitude of individual
atoms or molecules of a solid material were discussed. These forces may be primary chemical
bonds, as in metals and ionic solids, or they may be secondary van der Waals’ forces of solids,
such as in ice, paraffin wax and most polymers. In solids, the way the atoms or molecules
arrange themselves contributes to the appearance and the properties of the materials.
Atoms can be gathered together as an aggregate through a number of different processes,
including condensation, pressurization, chemical reaction, electrodeposition, and melting. The
process usually determines, at least initially, whether the collection of atoms will take to form of
a gas, liquid or solid. The state usually changes as its temperature or pressure is changed.
Melting is the process most often used to form an aggregate of atoms. When the temperature of a
melt is lowered to a certain point, the liquid will form either a crystalline solid or and amorphous
solid.
Amorphous Solids
A solid substance with its atoms held apart at equilibrium spacing, but with no long-range
periodicity in atom location in its structure is an amorphous solid. Examples of amorphous solids
are glass and some types of plastic. They are sometimes described as supercooled liquids
because their molecules are arranged in a random manner some what as in the liquid state. For
example, glass is commonly made from silicon dioxide or quartz sand, which has a crystalline
structure. When the sand is melted and the liquid is cooled rapidly enough to avoid
crystallization, an amorphous solid called a glass is formed. Amorphous solids do not show a
sharp phase change from solid to liquid at a definite melting point, but rather soften gradually
when they are heated. The physical properties of amorphous solids are identical in all directions
along any axis so they are said to have isotropic properties, which will be discussed in more
detail later
.
Crystalline Solids
More than 90% of naturally occurring and artificially prepared solids are crystalline. Minerals,
sand, clay, limestone, metals, carbon (diamond and graphite), salts ( NaCl, KCl etc.), all have
crystalline structures. A crystal is a regular, repeating arrangement of atoms or molecules. The
majority of solids, including all metals, adopt a crystalline arrangement because the amount of
stabilization achieved by anchoring interactions between neighboring particles is at its greatest
when the particles adopt regular (rather than random) arrangements. In the crystalline
arrangement, the particles pack efficiently together to minimize the total intermolecular energy.
The regular repeating pattern that the atoms arrange in is called the crystalline lattice. The
scanning tunneling microscope (STM) makes it possible to image the electron cloud associated
individual atoms at the surface of a material. Below is an STM image of a platinum surface
showing the regular alignment of atoms.
Courtesy: IBM Research, Almaden Research Center.
Crystal Structure
Crystal structures may be conveniently specified by describing the arrangement within the solid
of a small representative group of atoms or molecules, called the ‘unit cell.’ By multiplying
identical unit cells in three directions, the location of all the particles in the crystal is determined.
In nature, 14 different types of crystal structures or lattices are found. The simplest crystalline
unit cell to picture is the cubic, where the atoms are lined up in a square, 3D grid. The unit cell is
simply a box with an atom at each corner. Simple cubic crystals are relatively rare, mostly
because they tend to easily distort. However, many crystals form body-centered-cubic (bcc) or
face-centered-cubic (fcc) structures, which are cubic with either an extra atom centered in the
cube or centered in each face of the cube. Most metals form bcc, fcc or Hexagonal Close Packed
(hpc) structures; however, the structure can change depending on temperature. These three
structures will be discussed in more detail on the following page.
Crystalline structure is important because it contributes to the properties of a material. For
example, it is easier for planes of atoms to slide by each other if those planes are closely packed.
Therefore, lattice structures with closely packed planes allow more plastic deformation than
those that are not closely packed. Additionally, cubic lattice structures allow slippage to occur
more easily than non-cubic lattices. This is because their symmetry provides closely packed
planes in several directions. A face-centered cubic crystal structure will exhibit more ductility
(deform more readily under load before breaking) than a body-centered cubic structure. The bcc
lattice, although cubic, is not closely packed and forms strong metals. Alpha-iron and tungsten
have the bcc form. The fcc lattice is both cubic and closely packed and forms more ductile
materials. Gamma-iron, silver, gold, and lead have fcc structures. Finally, HCP lattices are
closely packed, but not cubic. HCP metals like cobalt and zinc are not as ductile as the fcc
metals.
Primary Metallic Crystalline Structures
(BCC, FCC, HCP)
As pointed out on the previous page, there are 14 different types of crystal
unit cell structures or lattices are found in nature. However most metals
and many other solids have unit cell structures described as body center
cubic (bcc), face centered cubic (fcc) or Hexagonal Close Packed (hcp).
Since these structures are most common, they will be discussed in more
detail.
Body-Centered Cubic (BCC) Structure
The body-centered cubic unit cell has atoms at each of the eight corners of a cube (like the cubic
unit cell) plus one atom in the center of the cube (left image below). Each of the corner atoms is
the corner of another cube so the corner atoms are shared among eight unit cells. It is said to
have a coordination number of 8. The bcc unit cell consists of a net total of two atoms; one in the
center and eight eighths from corners atoms as shown in the middle image below (middle image
below). The image below highlights a unit cell in a larger section of the lattice.
The bcc arrangement does not allow the atoms to pack together as closely as the fcc or hcp
arrangements. The bcc structure is often the high temperature form of metals that are closepacked at lower temperatures. The volume of atoms in a cell per the total volume of a cell is
called the packing factor. The bcc unit cell has a packing factor of 0.68.
Some of the materials that have a bcc structure include lithium, sodium, potassium, chromium,
barium, vanadium, alpha-iron and tungsten. Metals which have a bcc structure are usually harder
and less malleable than close-packed metals such as gold. When the metal is deformed, the
planes of atoms must slip over each other, and this is more difficult in the bcc structure. It should
be noted that there are other important mechanisms for hardening materials, such as introducing
impurities or defects which make slipping more difficult. These hardening mechanisms will be
discussed latter.
Face Centered Cubic (FCC) Structure
The face centered cubic structure has atoms located at each of the corners and the centers of all
the cubic faces (left image below). Each of the corner atoms is the corner of another cube so the
corner atoms are shared among eight unit cells. Additionally, each of its six face centered atoms
is shared with an adjacent atom. Since 12 of its atoms are shared, it is said to have a coordination
number of 12. The fcc unit cell consists of a net total of four atoms; eight eighths from corners
atoms and six halves of the face atoms as shown in the middle image above. The image below
highlights a unit cell in a larger section of the lattice.
In the fcc structure (and the hcp structure) the atoms can pack closer together than they can in the
bcc structure. The atoms from one layer nest themselves in the empty space between the atoms
of the adjacent layer. To picture packing arrangement, imagine a box filled with a layer of balls
that are aligned in columns and rows. When a few additional balls are tossed in the box, they will
not balance directly on top of the balls in the first layer but instead will come to rest in the pocket
created between four balls of the bottom layer. As more balls are added they will pack together
to fill up all the pockets. The packing factor (the volume of atoms in a cell per the total volume
of a cell) is 0.74 for fcc crystals. Some of the metals that have the fcc structure include
aluminum, copper, gold, iridium, lead, nickel, platinum and silver.
Hexagonal Close Packed (HPC) Structure
Another common close packed structure is the hexagonal close pack. The hexagonal structure of
alternating layers is shifted so its atoms are aligned to the gaps of the preceding layer. The atoms
from one layer nest themselves in the empty space between the atoms of the adjacent layer just
like in the fcc structure. However, instead of being a cubic structure, the pattern is hexagonal.
(See image below.) The difference between the HPC and FCC structure is discussed later in this
section.
The hcp structure has three layers of atoms. In each the top and bottom layer, there are six atoms
that arrange themselves in the shape of a hexagon and a seventh atom that sits in the middle of
the hexagon. The middle layer has three atoms nestle in the triangular "grooves" of the top and
bottom plane. Note that there are six of these "grooves" surrounding each atom in the hexagonal
plane, but only three of them can be filled by atoms.
As shown in the middle image above, there are six atoms in the hcp unit cell. Each of the 12
atoms in the corners of the top and bottom layers contribute 1/6 atom to the unit cell, the two
atoms in the center of the hexagon of both the top and bottom layers each contribute ½ atom and
each of the three atom in the middle layer contribute 1 atom. The image on the right above
attempts to show several hcp unit cells in a larger lattice.
The coordination number of the atoms in this structure is 12. There are six nearest neighbors in
the same close packed layer, three in the layer above and three in the layer below. The packing
factor is 0.74, which is the same as the fcc unit cell. The hcp structure is very common for
elemental metals and some examples include beryllium, cadmium, magnesium, titanium, zinc
and zirconium.
Similarities and Difference Between the
FCC and HCP Structure
The face centered cubic and hexagonal close packed structures both have a packing factor of
0.74, consist of closely packed planes of atoms, and have a coordination number of 12. The
difference between the fcc and hcp is the stacking sequence. The hcp layers cycle among the two
equivalent shifted positions whereas the fcc layers cycle between three positions. As can be seen
in the image, the hcp structure contains only two types of planes with an alternating ABAB
arrangement. Notice how the atoms of the third plane are in exactly the same position as the
atoms in the first plane. However, the fcc structure contains three types of planes with a
ABCABC arrangement. Notice how the atoms in rows A and C are no longer aligned.
Remember that cubic lattice structures allow slippage to occur more easily than non-cubic
lattices, so hcp metals are not as ductile as the fcc metals.
The table below shows the stable room temperature crystal structures for several elemental
metals.
Metal
Aluminum
Cadmium
Chromium
Cobalt
Copper
Gold
Iron (Alpha)
Lead
Magnesium
Molybdenum
Nickel
Platinum
Silver
Tantalum
Titanium (Alpha)
Tungsten
Zinc
Crystal Structure
FCC
HCP
BCC
HCP
FCC
FCC
BCC
FCC
HCP
BCC
FCC
FCC
FCC
BCC
HCP
BCC
HCP
Atomic Radius (nm)
0.1431
0.1490
0.1249
0.1253
0.1278
0.1442
0.1241
0.1750
0.1599
0.1363
0.1246
0.1387
0.1445
0.1430
0.1445
0.1371
0.1332
A nanometer (nm) equals 10-9 meter or 10 Angstrom units.
Solidification
The crystallization of a large amount of material from a single point of nucleation results in a
single crystal. In engineering materials, single crystals are produced only under carefully
controlled conditions. The expense of producing single crystal materials is only justified for
special applications, such as turbine engine blades, solar cells, and piezoelectric materials.
Normally when a material begins to solidify, multiple crystals begin to grow in the liquid and a
polycrystalline (more than one crystal) solid forms.
The moment a crystal begins to grow is know as nucleation and the point where it occurs is the
nucleation point. At the solidification temperature, atoms of a liquid, such as melted metal, begin
to bond together at the nucleation points and start to form crystals. The final sizes of the
individual crystals depend on the number of nucleation points. The crystals increase in size by
the progressive addition of atoms and grow until they impinge upon adjacent growing crystal.
a) Nucleation of crystals, b) crystal growth, c) irregular grains form as crystals grow together,
d) grain boundaries as seen in a microscope.
In engineering materials, a crystal is usually referred to as a grain. A grain is merely a crystal
without smooth faces because its growth was impeded by contact with another grain or a
boundary surface. The interface formed between grains is called a grain boundary. The atoms
between the grains (at the grain boundaries) have no crystalline structure and are said to be
disordered.
Grains are sometimes large enough to be visible under an ordinary light microscope or even to
the unaided eye. The spangles that are seen on newly galvanized metals are grains. Rapid cooling
generally results in more nucleation points and smaller grains (a fine grain structure). Slow
cooling generally results in larger grains which will have lower strength, hardness and ductility.
Dendrites
In metals, the crystals that form in the liquid during
freezing generally follow a pattern consisting of a main
branch with many appendages. A crystal with this
morphology slightly resembles a pine tree and is called a
dendrite, which means branching. The formation of
dendrites occurs because crystals grow in defined planes
due to the crystal lattice they create. The figure to the right
shows how a cubic crystal can grow in a melt in three
dimensions, which correspond to the six faces of the cube.
For clarity of illustration, the adding of unit cells with
continued solidification from the six faces is shown simply
as lines. Secondary dendrite arms branch off the primary
arm, and tertiary arms off the secondary arms and etcetera.
During freezing of a polycrystalline material, many
dendritic crystals form and grow until they eventually
become large enough to impinge upon each other.
Eventually, the interdendriticspaces between the dendrite
arms crystallize to yield a more regular crystal. The
original dendritic pattern may not be apparent when
examining the microstructure of a material. However,
dendrites can often be seen in solidification voids that
sometimes occur in castings or welds, as shown to the
right..
Shrinkage
Most materials contract or shrink during solidification and
cooling. Shrinkage is the result of:



Contraction of the liquid as it cools prior to its
solidification
Contraction during phase change from a liquid to solid
Contraction of the solid as it continues to cool to ambient temperature.
Shrinkage can sometimes cause cracking to occur in component as it solidifies. Since the coolest
area of a volume of liquid is where it contacts a mold or die, solidification usually begins first at
this surface. As the crystals grow inward, the material continues to shrink. If the solid surface is
too rigid and will not deform to accommodate the internal shrinkage, the stresses can become
high enough to exceed the tensile strength of the material and cause a crack to form. Shrinkage
cavitation sometimes occurs because as a material solidifies inward, shrinkage occurred to such
an extent that there is not enough atoms present to fill the available space and a void is left.
Anisotropy and Isotropy
In a single crystal, the physical and mechanical properties often differ with orientation. It can be
seen from looking at our models of crystalline structure that atoms should be able to slip over
one another or distort in relation to one another easier in some directions than others. When the
properties of a material vary with different crystallographic orientations, the material is said to be
anisotropic.
Alternately, when the properties of a material are the same in all directions, the material is said to
be isotropic. For many polycrystalline materials the grain orientations are random before any
working (deformation) of the material is done. Therefore, even if the individual grains are
anisotropic, the property differences tend to average out and, overall, the material is isotropic.
When a material is formed, the grains are usually distorted and elongated in one or more
directions which makes the material anisotropic. Material forming will be discussed later but
let’s continue discussing crystalline structure at the atomic level.
Crystal Defects
A perfect crystal, with every atom of the same type in the correct position, does not exist. All
crystals have some defects. Defects contribute to the mechanical properties of metals. In fact,
using the term “defect” is sort of a misnomer since these features are commonly intentionally
used to manipulate the mechanical properties of a material. Adding alloying elements to a metal
is one way of introducing a crystal defect. Nevertheless, the term “defect” will be used, just keep
in mind that crystalline defects are not always bad. There are basic classes of crystal defects:



point defects, which are places where an atom is missing or irregularly placed in the
lattice structure. Point defects include lattice vacancies, self-interstitial atoms,
substitution impurity atoms, and interstitial impurity atoms
linear defects, which are groups of atoms in irregular positions. Linear defects are
commonly called dislocations.
planar defects, which are interfaces between homogeneous regions of the material. Planar
defects include grain boundaries, stacking faults and external surfaces.
It is important to note at this point that plastic deformation in a material occurs due to the
movement of dislocations (linear defects). Millions of dislocations result for plastic forming
operations such as rolling and extruding. It is also important to note that any defect in the regular
lattice structure disrupts the motion of dislocation, which makes slip or plastic deformation more
difficult. These defects not only include the point and planer defects mentioned above, and also
other dislocations. Dislocation movement produces additional dislocations, and when
dislocations run into each other it often impedes movement of the dislocations. This drives up the
force needed to move the dislocation or, in other words, strengthens the material. Each of the
crystal defects will be discussed in more detail in the following pages.
Point Defects
Point defects are where an atom is missing or is in an
irregular place in the lattice structure. Point defects
include self interstitial atoms, interstitial impurity
atoms, substitutional atoms and vacancies. A self
interstitial atom is an extra atom that has crowded its
way into an interstitial void in the crystal structure.
Self interstitial atoms occur only in low
concentrations in metals because they distort and
highly stress the tightly packed lattice structure.
A substitutional impurity atom is an atom of a
different type than the bulk atoms, which has
replaced one of the bulk atoms in the lattice.
Substitutional impurity atoms are usually close in
size (within approximately 15%) to the bulk atom.
An example of substitutional impurity atoms is the
zinc atoms in brass. In brass, zinc atoms with a
radius of 0.133 nm have replaced some of the copper
atoms, which have a radius of 0.128 nm.
Interstitial impurity atoms are much smaller than the
atoms in the bulk matrix. Interstitial impurity atoms fit into the open space between the bulk
atoms of the lattice structure. An example of interstitial impurity atoms is the carbon atoms that
are added to iron to make steel. Carbon atoms, with a radius of 0.071 nm, fit nicely in the open
spaces between the larger (0.124 nm) iron atoms.
Vacancies are empty spaces where an atom should be, but is missing. They are common,
especially at high temperatures when atoms are frequently and randomly change their positions
leaving behind empty lattice sites. In most cases diffusion (mass transport by atomic motion) can
only occur because of vacancies.
Linear Defects - Dislocations
Dislocations are another type of defect in crystals. Dislocations are areas were the atoms are out
of position in the crystal structure. Dislocations are generated and move when a stress is applied.
The motion of dislocations allows slip – plastic deformation to occur.
Before the discovery of the dislocation by Taylor, Orowan and Polyani in 1934, no one could
figure out how the plastic deformation properties of a metal could be greatly changed by solely
by forming (without changing the chemical composition). This became even bigger mystery
when in the early 1900’s scientists estimated that metals undergo plastic deformation at forces
much smaller than the theoretical strength of the forces that are holding the metal atoms together.
Many metallurgists remained skeptical of the dislocation theory until the development of the
transmission electron microscope in the late 1950’s. The TEM allowed experimental evidence to
be collected that showed that the strength and ductility of metals are controlled by dislocations.
There are two basic types of dislocations, the edge dislocation and the screw dislocation.
Actually, edge and screw dislocations are just extreme forms of the possible dislocation
structures that can occur. Most dislocations are probably a hybrid of the edge and screw forms
but this discussion will be limited to these two types.
Edge Dislocations
The edge defect can be easily visualized as an extra half-plane of atoms in a lattice. The
dislocation is called a line defect because the locus of defective points produced in the lattice by
the dislocation lie along a line. This line runs along the top of the extra half-plane. The interatomic bonds are significantly distorted only in the immediate vicinity of the dislocation line.
Understanding the movement of a dislocation is key to understanding why dislocations allow
deformation to occur at much lower stress than in a perfect crystal. Dislocation motion is
analogous to movement of a caterpillar. The caterpillar would have to exert a large force to move
its entire body at once. Instead it moves the rear portion of its body forward a small amount and
creates a hump. The hump then moves forward and eventual moves all of the body forward by a
small amount.
As shown in the set of images above, the dislocation moves similarly moves a small amount at a
time. The dislocation in the top half of the crystal is slipping one plane at a time as it moves to
the right from its position in image (a) to its position in image (b) and finally image (c). In the
process of slipping one plane at a time the dislocation propagates across the crystal. The
movement of the dislocation across the plane eventually causes the top half of the crystal to
move with respect to the bottom half. However, only a small fraction of the bonds are broken at
any given time. Movement in this manner requires a much smaller force than breaking all the
bonds across the middle plane simultaneously.
Screw Dislocations
There is a second basic type of dislocation,
called screw dislocation. The screw dislocation
is slightly more difficult to visualize. The motion
of a screw dislocation is also a result of shear
stress, but the defect line movement is
perpendicular to direction of the stress and the
atom displacement, rather than parallel. To
visualize a screw dislocation, imagine a block of
metal with a shear stress applied across one end
so that the metal begins to rip. This is shown in
the upper right image. The lower right image
shows the plane of atoms just above the rip. The
atoms represented by the blue circles have not
yet moved from their original position. The
atoms represented by the red circles have moved
to their new position in the lattice and have
reestablished metallic bonds. The atoms
represented by the green circles are in the
process of moving. It can be seen that only a
portion of the bonds are broke at any given time.
As was the case with the edge dislocation,
movement in this manner requires a much
smaller force than breaking all the bonds across the middle plane simultaneously.
If the shear force is increased, the atoms will continue to slip to the right. A row of the green
atoms will find there way back into a proper spot in the lattice (and become red) and a row of the
blue atoms will slip out of position (and become green). In this way, the screw dislocation will
move upward in the image, which is perpendicular to direction of the stress. Recall that the edge
dislocation moves parallel to the direction of stress. As shown in the image below, the net plastic
deformation of both edge and screw dislocations is the same, however.
The dislocations move along the densest planes of atoms in a material, because the stress needed
to move the dislocation increases with the spacing between the planes. FCC and BCC metals
have many dense planes, so dislocations move relatively easy and these materials have high
ductility. Metals are strengthened by making it more difficult for dislocations to move. This may
involve the introduction of obstacles, such as interstitial atoms or grain boundaries, to “pin” the
dislocations. Also, as a material plastically deforms, more dislocations are produced and they
will get into each others way and impede movement. This is why strain or work hardening
occurs.
In ionically bonded materials, the ion must move past an area with a repulsive charge in order to
get to the next location of the same charge. Therefore, slip is difficult and the materials are
brittle. Likewise, the low density packing of covalent materials makes them generally more
brittle than metals.
Planar Defects
Stacking Faults and Twin Boundaries
A disruption of the long-range stacking sequence can produce two other common types of crystal
defects: 1) a stacking fault and 2) a twin region. A change in the stacking sequence over a few
atomic spacings produces a stacking fault whereas a change over many atomic spacings produces
a twin region.
A stacking fault is a one or two layer interruption in the stacking sequence of atom planes.
Stacking faults occur in a number of crystal structures, but it is easiest to see how they occur in
close packed structures. For example, it is know from a previous discussion that face centered
cubic (fcc) structures differ from hexagonal close packed (hcp) structures only in their stacking
order. For hcp and fcc structures, the first two layers arrange themselves identically, and are said
to have an AB arrangement. If the third layer is placed so that its atoms are directly above those
of the first (A) layer, the stacking will be ABA. This is the hcp structure, and it continues
ABABABAB. However it is possible for the third layer atoms to arrange themselves so that they
are in line with the first layer to produce an ABC arrangement which is that of the fcc structure.
So, if the hcp structure is going along as ABABAB and suddenly switches to ABABABCABAB,
there is a stacking fault present.
Alternately, in the fcc arrangement the pattern is ABCABCABC. A stacking fault in an fcc
structure would appear as one of the C planes missing. In other words the pattern would become
ABCABCAB_ABCABC.
If a stacking fault does not corrects itself immediately but continues over some number of atomic
spacings, it will produce a second stacking fault that is the twin of the first one. For example if
the stacking pattern is ABABABAB but switches to ABCABCABC for a period of time before
switching back to ABABABAB, a pair of twin stacking faults is produced. The red region in the
stacking sequence that goes ABCABCACBACBABCABC is the twin plane and the twin
boundaries are the A planes on each end of the highlighted region.
Grain Boundaries in Polycrystals
Another type of planer defect is the grain boundary. Up to this point, the discussion has focused
on defects of single crystals. However, solids generally consist of a number of crystallites or
grains. Grains can range in size from nanometers to millimeters across and their orientations are
usually rotated with respect to neighboring grains. Where one grain stops and another begins is
know as a grain boundary. Grain boundaries limit the lengths and motions of dislocations.
Therefore, having smaller grains (more grain boundary surface area) strengthens a material. The
size of the grains can be controlled by the cooling rate when the material cast or heat treated.
Generally, rapid cooling produces smaller grains whereas slow cooling result in larger grains.
For more information, refer to the discussion on solidification.
Elastic/Plastic Deformation
When a sufficient load is applied to a metal or other structural material, it will cause the material
to change shape. This change in shape is called deformation. A temporary shape change that is
self-reversing after the force is removed, so that the object returns to its original shape, is called
elastic deformation. In other words, elastic deformation is a change in shape of a material at low
stress that is recoverable after the stress is removed. This type of deformation involves stretching
of the bonds, but the atoms do not slip past each other.
When the stress is sufficient to
permanently deform the metal, it is called
plastic deformation. As discussed in the
section on crystal defects, plastic
deformation involves the breaking of a
limited number of atomic bonds by the
movement of dislocations. Recall that the
force needed to break the bonds of all the
atoms in a crystal plane all at once is very
great. However, the movement of
dislocations allows atoms in crystal planes
to slip past one another at a much lower
stress levels. Since the energy required to
move is lowest along the densest planes of
atoms, dislocations have a preferred
direction of travel within a grain of the
material. This results in slip that occurs
along parallel planes within the grain.
These parallel slip planes group together to
form slip bands, which can be seen with an
optical microscope. A slip band appears as
a single line under the microscope, but it is
in fact made up of closely spaced parallel
slip planes as shown in the image.
Fatigue Crack Initiation
While on the subject of dislocations, it is appropriate to briefly
discuss fatigue. Fatigue is one of the primary reasons for the
failure of structural components. The life of a fatigue crack has
two parts, initiation and propagation. Dislocations play a major
role in the fatigue crack initiation phase. It has been observed
in laboratory testing that after a large number of loading cycles
dislocations pile up and form structures called persistent slip
bands (PSB). An example
of a PSB is shown in the
micrograph image to the
right.
PSBs are areas that rise
(intrusion) the surface of the
material along slip planes.
surface that serve as stress
above (extrusion) or fall below
component due to movement of
This leaves tiny steps in the
risers where fatigue cracks can
initiate. A crack at the edge of a PSB is shown in the image below taken with a scanning electron
microscope (SEM).
Diffusion
Diffusion is the migration of atoms from a region of high concentration to
a region of low concentration. In a homogeneous material, atoms are
routinely moving around but the movement is random (i.e. there is
always an equal number of atoms moving in all directions). In an
inhomogeneous material, all the atoms are moving near randomly, but
there is a migration of atoms to areas where their concentrations are
lower. In other words, there is a net diffusion.
Atom diffusion can occur by the motion of host or substitutional atoms to
vacancies (vacancy diffusion), or interstitial impurities atoms to different
interstitial positions (interstitial diffusion). In order to move, an atom
must overcome the bond energy due to nearby atoms. This is more easily
achieved at high temperatures when the atoms are vibrating strongly.
Carburizing, which will be discussed later, is an example of diffusion is
used.
Strengthening/Hardening Mechanisms
As discussed in the previous section, the ability of a crystalline material to plastically deform
largely depends on the ability for dislocation to move within a material. Therefore, impeding the
movement of dislocations will result in the strengthening of the material. There are a number of
ways to impede dislocation movement, which include:

controlling the grain size (reducing continuity of atomic planes)


strain hardening (creating and tangling dislocations)
alloying (introducing point defects and more grains to pin dislocation)
Control of Grain Size
The size of the grains within a material also has an effect
on the strength of the material. The boundary between
grains acts as a barrier to dislocation movement and the
resulting slip because adjacent grains have different
orientations. Since the atom alignment is different and slip
planes are discontinuous between grains. The smaller the
grains, the shorter the distance atoms can move along a
particular slip plane. Therefore, smaller grains improve the
strength of a material. The size and number of grains within a material is controlled by the rate of
solidification from the liquid phase.
Strain Hardening
Strain hardening (also called work-hardening or cold-working) is the process of making a metal
harder and stronger through plastic deformation. When a metal is plastically deformed,
dislocations move and additional dislocations are generated. The more dislocations within a
material, the more they will interact and become pinned or tangled. This will result in a decrease
in the mobility of the dislocations and a strengthening of the material. This type of strengthening
is commonly called cold-working. It is called cold-working because the plastic deformation must
occurs at a temperature low enough that atoms cannot rearrange themselves. When a metal is
worked at higher temperatures (hot-working) the dislocations can rearrange and little
strengthening is achieved.
Strain hardening can be easily demonstrated with piece of wire or a paper clip. Bend a straight
section back and forth several times. Notice that it is more difficult to bend the metal at the same
place. In the strain hardened area dislocations have formed and become tangled, increasing the
strength of the material. Continued bending will eventually cause the wire to break at the bend
due to fatigue cracking. (After a large number of bending cycles, dislocations form structures
called Persistent Slip Bands (PSB). PSBs are basically tiny areas where the dislocations have
piled up and moved the material surface out leave steps in the surface that act as stress risers or
crack initiation points.)
It should be understood, however, that increasing the
strength by cold-working will also result in a reduction in
ductility. The graph to the right shows the yield strength
and the percent elongation as a function of percent coldwork for a few example materials. Notice that for each
material, a small amount of cold-working results in a
significant reduction in ductility.
Effects of Elevated Temperature on Strain Hardened Materials
When strain hardened materials are exposed to elevated temperatures, the strengthening that
resulted from the plastic deformation can be lost. This can be a bad thing if the strengthening is
needed to support a load. However, strengthening due to strain hardening is not always desirable,
especially if the material is being heavily formed since ductility will be lowered.
Heat treatment can be used to remove the effects of strain hardening. Three things can occur
during heat treatment:
1. Recovery
2. Recrystallization
3. Grain growth
Recovery
When a stain hardened material is held at
an elevated temperature an increase in
atomic diffusion occurs that relieves
some of the internal strain energy.
Remember that atoms are not fixed in
position but can move around when they
have enough energy to break their bonds.
Diffusion increases rapidly with rising
temperature and this allows atoms in
severely strained regions to move to
unstrained positions. In other words,
atoms are freer to move around and
recover a normal position in the lattice
structure. This is known as the recovery
phase and it results in an adjustment of
strain on a microscopic scale. Internal residual stresses are lowered due to a reduction in the
dislocation density and a movement of dislocation to lower-energy positions. The tangles of
dislocations condense into sharp two-dimensional boundaries and the dislocation density within
these areas decrease. These areas are called subgrains. There is no appreciable reduction in the
strength and hardness of the material but corrosion resistance often improves.
Recrystallization
At a higher temperature, new, strain-free grains nucleate and grow inside the old distorted grains
and at the grain boundaries. These new grains grow to replace the deformed grains produced by
the strain hardening. With recrystallization, the mechanical properties return to their original
weaker and more ductile states. Recrystallization depends on the temperature, the amount of time
at this temperature and also the amount of strain hardening that the material experienced. The
more strain hardening, the lower the temperature will be at which recrystallization occurs. Also,
a minimum amount (typically 2-20%) of cold work is necessary for any amount of
recrystallization to occur. The size the new grains is also partially dependant on the amount of
strain hardening. The greater the stain hardening, the more nuclei for the new grains, and the
resulting grain size will be smaller (at least initially).
Grain Growth
If a specimen is left at the high temperature beyond the time needed for complete
recrystallization, the grains begin to grow in size. This occurs because diffusion occurs across the
grain boundaries and larger grains have less grain boundary surface area per unit of volume.
Therefore, the larger grains lose fewer atoms and grow at the expense of the smaller grains.
Larger grains will reduce the strength and toughness of the material.
Alloying
Only a few elements are widely used commercially in their pure form. Generally, other elements
are present to produce greater strength, to improve corrosion resistance, or simply as impurities
left over from the refining process. The addition of other elements into a metal is called alloying
and the resulting metal is called an alloy. Even if the added elements are nonmetals, alloys may
still have metallic properties.
Copper alloys were produced very early in our history. Bronze, an alloy of copper and tin, was
the first alloy known. It was easy to produce by simply adding tin to molten copper. Tools and
weapons made of this alloy were stronger than pure copper ones. The typical alloying elements
in some common metals are presented in the table below.
Alloy
Brass
Bronze
Pewter
Cast Iron
Steel
Stainless Steel
Composition
Copper, Zinc
Copper, Zinc, Tin
Tin, Copper, Bismuth, Antimony
Iron, Carbon, Manganese, Silicon
Iron, Carbon (plus small amounts of other elements)
Iron, Chromium, Nickel
The properties of alloys can be manipulated by varying composition. For example steel formed
from iron and carbon can vary substantially in hardness depending on the amount of carbon
added and the way in which it was processed.
When a second element is added, two basically different structural changes are possible:
1. Solid solution strengthening occurs when the atoms of the new element form a solid
solution with the original element, but there is still only one phase. Recall that the term
‘phase’ refers to that region of space occupied by a physically homogeneous material.
2. The atoms of the new elements form a new second phase. The entire microstructure may
change to this new phase or two phases may be present.
Solid Solution Strengthening
Solid solution strengthening involves the addition of other metallic elements that will dissolve in
the parent lattice and cause distortions because of the difference in atom size between the parent
metal and the solute metal. Recall from the section on crystal point defects that it is possible to
have substitutional impurity atoms, and interstitial impurity atoms. A substitutional impurity
atom is an atom of a different type than the bulk atoms, which has replaced one of the bulk atoms
in the lattice. Substitutional impurity atoms are usually close in size (within approximately 15%)
to the bulk atom. Interstitial impurity atoms are much smaller than the atoms in the bulk matrix.
Interstitial impurity atoms fit into the open space between the bulk atoms of the lattice structure.
Since the impurity atoms are smaller or larger than the surrounding atoms they introduce tensile
or compressive lattice strains. They disrupt the regular arrangement of ions and make it more
difficult for the layers to slide over each other. This makes the alloy stronger and less ductile
than the pure metal. For example, an alloy of 30% nickel raises the cast tensile strength of copper
from 25,000 PSI to 55,000 PSI.
Multiphase Metals
Still another method of strengthening the metal is adding elements that have no or partial
solubility in the parent metal. This will result in the appearance of a second phase distributed
throughout the crystal or between crystals. These secondary phases can raise or reduce the
strength of an alloy. For example, the addition of tin, zinc, or aluminum to copper will result in
an alloy with increased strength, but alloying with lead or bismuth with result in a lower strength
alloy. The properties of a polyphase (two of more phase) material depend on the nature, amount,
size, shape, distribution, and orientation of the phases. Greek letters are commonly used to
distinguish the different solid phases in a given alloy.
Phases can be seen on a microscopic scale with an optical microscope after the surface has been
properly polished and etched. Below is a micrograph take at 125x of lead-tin alloy composed of
two phases. The light colored regions are a tin-rich phase and the dark colored regions are a leadrich phase.
Alloying (continued)
Phase Diagrams
As previously stated, the phase diagram is simply a map showing the structure of phases present
as the temperature and overall composition of the alloy are varied. It is a very useful tool for
understanding and controlling the structures of polyphase materials. A binary phase diagram
shows the phases formed in differing mixtures of two elements over a range of temperatures.
When an alloy exhibits more than two phases, a different type of phase diagram must be used,
such as a ternary diagram for three phase alloys. This discussion will focus on the binary phase
diagram.
On the binary phase diagram, compositions run
from 100% Element A on the left, through all
possible mixtures, to 100% Element B on the
right. The composition of an alloy is given in the
form A - x%B. For example, Cu - 20%Al is 80%
copper and 20% aluminum. Weight percentages
are often used to specify the proportions of the
alloying elements, but atomic percent are
sometimes used. Weight percentages will be used
throughout this text.
Alloys generally do not have a single melting
point, but instead melt (or alternately solidify)
over a range of temperatures. At each end of the
phase diagram only one of the elements is present
(100% A or 100% B) so a specific melting point
does exists. Additionally, there is sometimes a
mixture of the constituent elements which
produces melting at a single temperature like a
pure element. This is called the eutectic point.
At compositions other than at the pure A, pure
B and the eutectic points, when the alloy is
cooled from a high temperature it will begin to
solidify at a certain temperature but will
remain in a mushy (liquid plus solid) condition
over a range of temperatures. If experiments
are conducted over a range of compositions to
determine the temperature at which the alloys
start to solidify, this data can be potted on the phase diagram to produce a curve. This “start of
solidification curve” will join the three single solidification points and is called the liquidus line.
Up to a few percent of composition, it is possible
for one element to remain dissolve in another
while both are in the solid state. This is called
solid solubility and the solubility limit normally
changes with temperature. The extent of the solid
solubility region can be plotted onto the phase
diagram. In this example, the alpha phase is the
region of solid solution where some of B atoms
have dissolved in a matrix of A atoms. The beta
phase is the region where a small percentage of A
atoms have dissolved in a matrix of B atoms. It is
important to note that some elements have zero solid solubility in other elements. An example is
aluminum/silicon alloys, where aluminum has zero solid solubility in silicon.
If an alloy's composition does not place it within
the alpha or beta solid solution regions, the alloy
will become fully solid at the eutectic
temperature. The eutectic line on the phase
diagram indicates where this transformation will
occur over the range of compositions. At alloy
compositions and temperatures between the
liquidus temperature and the eutectic temperature,
a mushy mix of either alpha or beta phase will
exist as solid masses within a liquid mixture of A
and B. These are the alpha plus liquid and the
beta plus liquid areas on the phase diagram. The region below the eutectic line, and outside the
solid solution region, will be a solid mixture of alpha and beta.
Alloying (continued)
Tie and Lever Rules
Simply by looking at a phase diagram it is possible to tell what phase or phases an alloy will
have at a given temperature. But, it is also possible to get quantitative information from the
diagram. Consider the alloy at the temperature
shown on the phase diagram. It is easy to see that
at this temperature, it is a mixture of alpha and
liquid phases. Using a tie line it is also possible to
determine the composition of the phases at this
temperature. A tie line is an isothermal (constant
temperature) line drawn through the alloy's
position on the phase diagram when it is in a two
phase field. The points where the ends of the tie
line intersect the two adjacent solubility curves
indicate the compositions of the two phases that
exist in equilibrium at this temperature. In this
example, the tie line shows that the alpha phase is 5.2%B and the liquid phase is 34.5%B at this
temperature. It is important to keep in mind that the tie rule addresses the determination of the
compositions of the constituent phases within the sample and it does not address the overall
chemical composition of the sample, which remains unchanged.
It is also possible to determine how much of each phase exists at the given temperature using the
lever rule. It is important to know the amounts of each phase present because the properties of
the alloy depend on the amount of each phase present. The lever rule uses the tie line and the
basic scientific principle of the conservation of mass to determine the ratio of the two phases
present. The tie-line gives the chemical compositions of each of the two phases, and the
combined amounts of these two compositions must add up to the alloy's overall composition
(Co), which is known. In other words, Co must be composed of the appropriate amount of α at
composition Cα and of liquid at Cliq. So basically, the proportions of the phases present are given
by the relative lengths of the two sections of the tie line.
The fraction of alpha phase present is the given by the ratio of the Co to Cliq portion of the tie line
and the total length of the tie line (Cliq to Cα). Mathematically the relationships can be written as
fα << (Cliq – Co)/(Cliq - Cα). The fraction of liquid phase present is given by the ratio of the Co to
Cα portion of the tie line and the total length of the tie line (Cliq to Cα). Mathematically this
relationships can be written as fliq << (Co - Cα)/(Cliq - Cα). Of course, the two values must total to
equal one.
Note that the right side of the tie line gives the proportion of the phase on the left (α phase in this
example) and left side of the tie line gives the proportion of the phase to the right (liquid phase in
this example). It is easy to keep this relationship straight by simply considering what the ratio
would be near one of the tie line intersect points. For example, if Co were near the liquidus line
the ratio of the liquid section of the line to the total length of the line will be nearly one.
Alloying (continued)
Composition, Microstructure, and the Phase Diagram
Let’s finish this discussion on phase diagrams by briefly looking at three different compositions
of elements A and B, and how their microstructures will differ because of their positions on the
phase diagram. First a eutectic alloy, which is an alloy with composition right at the eutectic
point, will be considered. Then compositions on both sides of the eutectic point will be
discussed. An alloy with a composition that lies to the left of the eutectic point on the phase
diagram is called a hypoeutectic alloy, and an alloy with a composition that lies to the right of
the eutectic point is called hypereutectic alloy. At this point, only the condition of slow cooling,
which will allow the alloy to solidify into it equilibrium condition, will be considered. The
microstructure can be controlled by manipulating the speed of cooling the alloy, but this will be
covered in the section on heat treatments.
Eutectic Alloys
First, consider the eutectic alloy of elements A
and B as it is cooled from a temperature at
location 1 to location 4 on the phase diagram. At
location 1, the alloy is at a high enough
temperature to make the mixture fully liquid. The circles below show a representation of the
alloy's microstructure at each of the locations numbered on the phase diagram.
At location 1, there is nothing of interest as the alloy is completely liquid. As the alloy is slow
cooled, it remains liquid until it reaches the eutectic temperature (location 2) where it starts to
solidify at any favorable nucleation sites. From the microstructure image 2, it can be see that as
the alloy solidifies it forms into alternate layers of alpha and beta phase. This layered
microstructure is known as lamellar microstructure and the layers are often only of the order of 1
micron across. The reason that a eutectic alloy forms in this way has to do with the diffusion
times required to form the solid.
The grains grow by adding alpha to alpha and beta to beta until they encounter another grain
(location 3). Further nucleation sites will also continue to form within the liquid parts of the
mixture. This solidification happens very rapidly as any given volume of liquid in the melt
reaches the eutectic temperature. Remember that a eutectic composition solidifies at a single
temperature like a pure element and not over a temperature range.
As the now sold alloy cools to location 4, the composition of the layers of alpha and beta
continue to change as it cools. Atoms of A and B will diffuse between the two phases to produce
the equilibrium compositions of alpha and beta phase at a given temperature. By drawing tie
lines at various temperatures the eutectic point on the phase diagram, it can be seen that the
solubility of A in the beta phase and B in the alpha phase decreases as the temperature decreases.
Since this phase composition change is due to diffusion, which is a relatively a slow process), it
is important that eutectic alloys be allowed to cool slowly to produce the correct microstructure.
Hypoeutectic Alloys
Next, consider an alloy of A and B that has an
overall composition that places it to the left of the
eutectic point. When an alloy falls to the left of
the eutectic point it is called a hypoeutectic alloy.
At location 1, the alloy is at a temperature that is
high enough to put it in a fully liquid phase.
When the alloy is cooled, it remains in the liquid
state until it reaches the temperature where it
crosses the liquidus line (location 2). At this
temperature, the alpha phase starts to solidify at
any favorable nucleation sites. The alpha solidifies as dendrites which grow to become grains of
alpha. The first solid phase to form is called the primary phase so, in this case, primary alpha is
formed.
As the alloy continues to cool (location 3) the existing nucleation sites will grow as dendrites and
further nucleation sites will form within the liquid part of the mixture. The melt will have that
mushy consistency of chunks in liquid while it is in the “alpha + liquid” region of the phase
diagram. Since the alpha phase is mostly element A (with a small amount of B atoms in solid
solution), the remaining liquid becomes slightly richer in B as the liquid cools, which is indicated
by the liquidus line. The composition of the solid alpha phase also becomes slightly richer in B
atoms as the solid solution line shows.
This primary alpha phase growth and the accompanying phase composition shifts continue until
enough A atoms have been removed so that the remaining liquid is of eutectic composition. This
composition is achieved at the point where the temperature crosses the eutectic line (location 4).
At this point the primary alpha phase stops forming. The remaining liquid starts to solidify into
the lamellar (alternating layers of alpha and beta phases) structure of a eutectic composition. The
eutectic structure will grow; adding alpha to the layers of alpha and beta to the layers of beta in
the eutectic regions, and new solidification sites will continue to form. Remember that
solidification occurs rapidly and without the need for a further decrease in temperature once the
liquid reaches the eutectic line. At this point, the entire alloy has solidified into a mixture
comprised of grains of alpha and grains of eutectic mixture (alpha and beta). The microstructure
from this point at the eutectic line down to ambient temperature will look something like that
shown in micro 5.
Diffusion occurs as the alloy cools since the amount of element B in the alpha phase changes
with temperature. This occurs exactly like it did for the eutectic alloy. Diffusion must also occur
in the grains of pure alpha, as the composition of alpha phase also changes with temperature.
Hypereutectic
Finally, consider an alloy of A and B that has an
overall composition that places it to the right of
the eutectic point. When an alloy falls to the right
of the eutectic point it is called a hypereutectic
alloy. This alloy will solidify like the
hypoeutectic alloy did except it will pass through
the “beta + liquid” region of the phase diagram
rather than the “alpha + liquid” region. This will
result in a microstructure comprised of grains of beta and grains of eutectic mixture (alpha and
beta) rather than grains of alpha and grains of eutectic mixture (alpha and beta) as the
hypoeutectic alloy had.
At location 1, the alloy is at a temperature that is high enough to put it in a fully liquid phase.
When the alloy is cooled, it remains in the liquid state until it reaches the temperature where it
crosses the liquidus line (location 2). At this temperature, the beta phase starts to solidify at any
favorable nucleation sites. The beta solidifies as dendrites which grow to become grains of beta.
The first solid phase to form is called the primary phase so, in this case, primary beta is formed.
As the alloy continues to cool (location 3) the existing nucleation sites will grow as dendrites and
further nucleation sites will form within the liquid part of the mixture. Since the beta phase is
mostly element B (with a small amount of A atoms in solid solution), the remaining liquid
becomes richer in A as the liquid cools, which is indicated by the liquidus line. The composition
of the solid beta phase also becomes slightly richer in A atoms as the solid solution line shows.
This primary beta phase growth and the accompanying phase composition shifts continue until
enough B atoms have been removed so that the remaining liquid is of eutectic composition. This
composition is achieved at the point where the temperature crosses the eutectic line (location 4).
At this point the primary beta phase stops forming. The remaining liquid starts to solidify into
the lamellar (alternating layers of alpha and beta phases) structure of a eutectic composition. The
eutectic structure will grow; adding alpha to the layers of alpha and beta to the layers of beta in
the eutectic regions, and new solidification sites will continue to form. At this point, the entire
alloy quickly solidifies into a mixture of beta grains and eutectic mixture (alpha and beta) grains.
The microstructure from this point at the eutectic line down to ambient temperature will look
something like that shown in micro 5.
Diffusion occurs as the alloy cools since the amount of element B in the alpha phase changes
with temperature. This occurs exactly like it did for the eutectic alloy. Diffusion must also occur
in the grains of pure alpha, as the composition of alpha phase also changes with temperature.
Thermal Treatments (Heat-Treating)
In the previous pages on the subjects of alloying and the binary phase diagram, the
microstructures of alloys that were allowed to solidify by slow cooling were considered. It
should also be known, however, that it is possible to modify the microstructure of an alloy by
subjecting it to various thermal treatments. Heat-treating is a term used to describe all of the
controlled heating and cooling operations performed on a material in the solid state for the
purpose of altering its microstructure and/or properties. The focus of this discussion will be on
metals but is should be noted that heat-treatment is also used on ceramics and composites to
modify their properties.
The major objectives of the different kinds of thermal treatments are:
1.
2.
3.
4.
Soften the material for improved workability.
Increase the strength or hardness of the material.
Increase the toughness or resistance to fracture of the material.
Stabilize mechanical or physical properties against changes that might occur during
exposure to service environments.
5. Insure part dimensional stability.
6. Relieve undesirable residual stresses induced during part fabrication.
Different metals respond to treatment at different temperatures. Each metal has a specific
chemical composition, so changes in physical and structural properties take place at different,
critical temperatures. Even small percentages of elements in the metal composition, such as
carbon, will greatly determine the temperature, time, method and rate of cooling that needs to be
used in the heat treating process. Depending on the thermal treatment used, the atomic structure
and/or microstructure of a material may change due to movement of dislocations, an increase or
decrease in solubility of atoms, an increase in grain size, the formation of new grains of the same
or different phase, a change in the crystal structure, and others mechanisms.
Since there are so many ways in which metals are heat treated, it is not practical to discuss them
all. But, as an example, let’s look at how heat treatment is used to strengthen a copper aluminum
alloy.
Precipitation Hardening
In designing alloys for strength, an approach often taken is to develop an alloy with a structure
that consists of particles (which impede dislocation movement) dispersed in a ductile matrix.
Such a dispersion can be obtained by choosing an alloy that is a single phase at elevated
temperature but on cooling will precipitate another phase in the matrix. A thermal process is then
developed to produce the desired distribution of precipitate in the matrix. When the alloy is
strengthened by this thermal treatment, it is called precipitation strengthening or hardening.
Precipitation hardening consists of three main steps: solution treatment, quenching, and aging.
Solution treatment involves heating the alloy to a temperature that allows the alloying atoms
(called the solute) to dissolve into the solution. This results in a homogeneous solid solution of
one phase. Quenching rapidly cools the solution and freezes the atoms in solution. In more
technical terms, the quenching cools the material so fast that the atoms of the alloying elements
do not have time to diffuse out of the solution. In the as-quenched condition, the solute is
supersaturated meaning that the lattice is overly stressed by the alloying atoms. Aging is the
process where the solute particles diffuse out of solution and into clusters that distort and
strengthen the material.
The precipitation hardening process for a copper-aluminum alloy is shown graphically in the
image below. On the right is phase diagram, which is a very useful tool for understanding and
controlling polyphase structures. The phase diagram is simply a map showing the structure of
phases present as the temperature and overall composition of the alloy are varied. The images on
the right in the image show the resulting microstructure at each step in the process.
Common Heat Treating Processes
A few of the more common terms used in heat treating are introduced below. It should be noted
that not all of the term are applicable to all alloys.
Age Hardening is a relatively low-temperature heat treatment process that strengthens a material
by causing the precipitation of components or phases of alloy from a super-saturated solid
solution condition.
Annealing is a softening process in which metals are heated and then allowed to cool slowly.
The purpose of annealing is to soften the material for improve machinability, formability, and
sometimes to control magnetic properties.
Normallizing is much like annealing, but the cooling process is much faster. This results in
increased strength but less ductility in the metal. Its purpose is to refine grain structure, produce
more uniform mechanical properties, and sometimes to relieve internal and surface stresses.
Precipitation Heat Treatment is the three step process of solution treating, quenching, and age
hardening to increase the strength or hardness of an alloy.
Solution Heat Treatment involves heating the material to a temperature that puts all the
elements in solid solution and then cooling very rapidly to freeze the atoms in place.
Stress Relieving is a low temperature heat treat process that is used to reduce the level of
residual stresses in a material.
Tempering involves gently heating a hardened metal and allowing it to cool slowly will produce
a metal that is still hard but also less brittle. This process is known as tempering.
Quenching is the rapid cooling of a hot material. The medium used to quench the material can
vary from forced air, oil, water and others. Many steels are hardened by heating and quenching.
Quenching results in a metal that is very hard but also brittle.
More information on heat treatment can be found in the material (ie aluminum, steel, titanium,
etc.) sections
Ceramic Structures
As discussed in the introduction, ceramics and related materials cover a wide range of objects.
Ceramics are a little more complex than metallic structures, which is why metals were covered
first. A ceramic has traditionally been defined as “an inorganic, nonmetallic solid that is prepared
from powdered materials and is fabricated into products through the application of heat. Most
ceramics are made up of two or more elements. This is called a compound. For example, alumina
(Al2O3) is a compound made up of aluminum atoms and oxygen atoms.
The two most common chemical bonds for ceramic materials are covalent and ionic. The
bonding of atoms together is much stronger in covalent and ionic bonding than in metallic. This
is why ceramics generally have the following properties: high hardness, high compressive
strength, and chemical inertness. This strong bonding also accounts for the less attractive
properties of ceramics, such as low ductility and low tensile strength. The absence of free
electrons is responsible for making most ceramics poor conductors of electricity and heat.
However, it should be noted that the crystal structures of ceramics are many and varied and this
results in a very wide range of properties. For example, while ceramics are perceived as
electrical and thermal insulators, ceramic oxide (initially based on Y-Ba-Cu-O) is the basis for
high temperature superconductivity. Diamond and silicon carbide have a higher thermal
conductivity than aluminum or copper. Control of the microstructure can overcome inherent
stiffness to allow the production of ceramic springs, and ceramic composites which have been
produced with a fracture toughness about half that of steel. Also, the atomic structures are often
of low symmetry that gives some ceramics interesting electromechanical properties like
piezoelectricity, which is used in sensors and transducers.
The structure of most ceramics varies from relatively simple to very complex. The
microstructure can be entirely glassy (glasses only); entirely crystalline; or a combination of
crystalline and glassy. In the latter case, the glassy phase usually surrounds small crystals,
bonding them together. The main compositional classes of engineering ceramics are the oxides,
nitrides and carbides.
Ceramic Structures (continued)
Ceramic Glass
Ceramics with an entirely glassy structure have certain properties that are quite different from
those of metals. Recall that when metal in the liquid state is cooled, a crystalline solid
precipitates when the melting freezing point is reached. However, with a glassy material, as the
liquid is cooled it becomes more and more viscous. There is no sharp melting or freezing point.
It goes from liquid to a soft plastic solid and finally becomes hard and brittle. Because of this
unique property, it can be blown into shapes, in addition to being cast, rolled, drawn and
otherwise processed like a metal.
Glassy behavior is related to the atomic structure of the material. If pure silica (SiO2) is fused
together, a glass called vitreous silica is formed on cooling. The basic unit structure of this glass
is the silica tetrahedron, which is composed of a single silicon atom surrounded by four
equidistant oxygen atoms. The silicon atoms occupy the openings (interstitials) between the
oxygen atoms and share four valence electrons with the oxygen atoms through covalent bonding.
The silica atom has four valence electrons and each of the oxygen atoms has two valence
electrons so the silica tetrahedron has four extra valence electrons to share with adjacent
tetrahedral. The silicate structures can link together by sharing the atoms in two corners of the
SiO2 tetrahedrons, forming chain or ring structures. A network of silica tetrahedral chains form,
and at high temperatures these chains easily slide past each other. As the melt cools, thermal
vibrational energy decreases and the chains can not move as easily so the structure becomes
more rigid. Silica is the most important constituent of glass, but other oxides are added to change
certain physical characteristics or to lower the melting point.
Ceramic Crystalline or Partially Crystalline Material
Most ceramics usually contain both metallic and nonmetallic elements with ionic or covalent
bonds. Therefore, the structure the metallic atoms, the structure of the nonmetallic atoms, and the
balance of charges produced by the valence electrons must be considered. As with metals, the
unit cell is used in describing the atomic structure of ceramics. The cubic and the hexagonal cells
are most common. Additionally, the difference in radii between the metallic and nonmetallic ions
plays an important role in the arrangement of the unit cell.
In metals, the regular arrangement of atoms into densely packed planes led to the occurrence of
slip under stress, which gives metal their characteristic ductility. In ceramics, brittle fracture
rather than slip is common because both the arrangement of the atoms and the type of bonding is
different. The fracture or cleavage planes of ceramics are the result of planes of regularly
arranged atoms.
The building criteria for the crystal structure are:



maintain neutrality
charge balance dictates chemical formula
achieve closest packing
A few of the different types of ceramic materials outside of the glass family are described below.
Silicate Ceramics
As mentioned previously, the silica structure is the basic
structure for many ceramics, as well as glass. It has an
internal arrangement consisting of pyramid (tetrahedral or
four-sided) units. Four large oxygen (0) atoms surround each
smaller silicon (Si) atom. When silica tetrahedrons share
three corner atoms, they produce layered silicates (talc,
kaolinite clay, mica). Clay is the basic raw material for many
building products such as brick and tile. When silica
tetrahedrons share four comer atoms, they produce
framework silicates (quartz, tridymite). Quartz is formed
when the tetrahedra in this material are arranged in a regular,
orderly fashion. If silica in the molten state is cooled very
slowly it crystallizes at the freezing point. But if molten silica is cooled more rapidly, the
resulting solid is a disorderly arrangement which is glass.
Cement
Cement (Portland cement) is one of the main ingredients of concrete. There are a number of
different grades of cement but a typical Portland cement will contain 19 to 25% SiO2 , 5 to 9%
Al2O3, 60 to 64% CaO and 2 to 4% FeO. Cements are prepared by grinding the clays and
limestone in proper proportion, firing in a kiln, and regrinding. When water is added, the
minerals either decompose or combine with water, and a new phase grows throughout the mass.
The reaction is solution, recrystallization, and precipitation of a silicate structure. It is usually
important to control the amount of water to prevent an excess that would not be part of the
structure and would weaken it. The heat of hydration (heat of reaction in the adsorption of water)
in setting of the cement can be large and can cause damage
in large structures.
Nitride Ceramics
Nitrides combine the superior hardness of ceramics with
high thermal and mechanical stability, making them suitable
for applications as cutting tools, wear-resistant parts and
structural components at high temperatures. TiN has a cubic
structure which is perhaps the simplest and best known of
structure types. Cations and anions both lie at the nodes of
separate fcc lattices. The structure is unchanged if the Ti and
N atoms (lattices) are interchanged.
Ferroelectric Ceramics
Depending on the crystal structure, in some crystal lattices,
the centers of the positive and negative charges do not
coincide even without the application of external electric
field. In this case, it is said that there exists spontaneous
polarization in the crystal. When the polarization of the
dielectric can be altered by an electric field, it is called
ferroelectric. A typical ceramic ferroelectric is barium
titanate, BaTiO3. Ferroelectric materials, especially polycrystalline ceramics, are very promising
for varieties of application fields such as piezoelectric/electrostrictive transducers, and
electrooptic.
Phase Diagram
The phase diagram is important in understanding the formation and control of the microstructure
of the microstructure of polyphase ceramics, just as it is with polyphase metallic materials. Also,
nonequilibrium structures are even more prevalent in ceramics because the more complex crystal
structures are more difficult to nucleate and to grow from the melt.
Imperfections in Ceramics
Imperfections in ceramic crystals include point
defects and impurities like in metals. However,
in ceramics defect formation is strongly affected
by the condition of charge neutrality because the
creation of areas of unbalanced charges requires
an expenditure of a large amount of energy. In
ionic crystals, charge neutrality often results in
defects that come as pairs of ions with opposite
charge or several nearby point defects in which
the sum of all charges is zero. Charge neutral
defects include the Frenkel and Schottky
defects. A Frenkel-defect occurs when a host
atom moves into a nearby interstitial position to
create a vacancy-interstitial pair of cations. A
Schottky-defect is a pair of nearby cation and anion vacancies. Schottky defect occurs when a
host atom leaves its position and moves to the surface creating a vacancy-vacancy pair.
Sometimes, the composition may alter slightly to arrive at a more balanced atomic charge. Solids
such as SiO2, which have a well-defined chemical formula, are called stoichiometric compounds.
When the composition of a solid deviates from the standard chemical formula, the resulting solid
is said to be nonstoichiometric. Nonstoichiometry and the existence of point defects in a solid are
often closely related. Anion vacancies are the source of the nonstoichiometry in SiO2-x,
Introduction of impurity atoms in the lattice is likely in conditions where the charge is
maintained. This is the case of electronegative impurities that substitute a lattice anion or
electropositive substitutional impurities. This is more likely for similar ionic radii since this
minimizes the energy required for lattice distortion. Defects will appear if the charge of the
impurities is not balanced
Polymer Structure
Engineering polymers include natural materials such as rubber and synthetic materials such as
plastics and elastomers. Polymers are very useful materials because their structures can be
altered and tailored to produce materials 1) with a range of mechanical properties 2) in a wide
spectrum of colors and 3) with different transparent properties.
Mers
A polymer is composed of many
Mer –
simple molecules that are repeating
The repeating unit in a polymer chain
structural units called monomers. A Monomer –
single polymer molecule may consist A single mer unit (n=1)
of hundreds to a million monomers
Polymer –
and may have a linear, branched, or
Many mer-units along a chain (n=103 or more)
network structure. Covalent bonds
Degree of Polymerization –
hold the atoms in the polymer
The average number of mer-units in a chain.
molecules together and secondary
bonds then hold groups of polymer chains together to form the polymeric material. Copolymers
are polymers composed of two or more different types of monomers.
Polymer Chains (Thermoplastics and Thermosets)
A polymer is an organic material and the backbone of every organic material is a chain of carbon
atoms. The carbon atom has four electrons in the outer shell. Each of these valence electrons can
form a covalent bond to another carbon atom or to a foreign atom. The key to the polymer
structure is that two carbon atoms can have up to three common bonds and still bond with other
atoms. The elements found most frequently in polymers and their valence numbers are: H, F, Cl,
Bf, and I with 1 valence electron; O and S with 2 valence electrons; n with 3 valence electrons
and C and Si with 4 valence electrons.
The ability for molecules to form long chains is
a vital to producing polymers. Consider the
material polyethylene, which is made from
ethane gas, C2H6. Ethane gas has a two carbon
atoms in the chain and each of the two carbon
atoms share two valence electrons with the
other. If two molecules of ethane are brought
together, one of the carbon bonds in each
molecule can be broken and the two molecules
can be joined with a carbon to carbon bond.
After the two mers are joined, there are still two
free valence electrons at each end of the chain
for joining other mers or polymer chains. The
process can continue liking more mers and polymers together until it is stopped by the addition
of anther chemical (a terminator), that fills the available bond at each end of the molecule. This
is called a linear polymer and is building block for thermoplastic polymers.
The polymer chain is often shown in two dimensions, but it should be noted that they have a
three dimensional structure. Each bond is at 109° to the next and, therefore, the carbon backbone
extends through space like a twisted chain of TinkerToys. When stress is applied, these chains
stretch and the elongation of polymers can be thousands of times greater than it is in crystalline
structures.
The length of the polymer chain is very important. As the number of carbon atoms in the chain is
increased to beyond several hundred, the material will pass through the liquid state and become a
waxy solid. When the number of carbon atoms in the chain is over 1,000, the solid material
polyethylene, with its characteristics of strength, flexibility and toughness, is obtained. The
change in state occurs because as the length of the molecules increases, the total binding forces
between molecules also increases.
It should also be noted that the molecules are not generally straight but are a tangled mass.
Thermoplastic materials, such as polyethylene, can be pictured as a mass of intertwined worms
randomly thrown into a pail. The binding forces are the result of van der Waals forces between
molecules and mechanical entanglement between the chains. When thermoplastics are heated,
there is more molecular movement and the bonds between molecules can be easily broken. This
is why thermoplastic materials can be remelted.
There is another group of polymers in which a single
large network, instead of many molecules is formed
during polymerization. Since polymerization is initially
accomplished by heating the raw materials and brining
them together, this group is called thermosetting
polymers or plastics. For this type of network structure to
form, the mers must have more than two places for boning to occur; otherwise, only a linear
structure is possible. These chains form jointed structures and rings, and may fold back and forth
to take on a partially crystalline structure.
Since these materials are essentially comprised of one giant molecule, there is no movement
between molecules once the mass has set. Thermosetting polymers are more rigid and generally
have higher strength than thermoplastic polymers. Also, since there is no opportunity for motion
between molecules in a thermosetting polymer, they will not become plastic when heated.

Types of polymers
o Commodity plastics
 PE = Polyethylene
 PS = Polystyrene
 PP = Polypropylene
 PVC = Poly(vinyl chloride)
 PET = Poly(ethylene terephthalate)
o Specialty or Engineering Plastics
 Teflon (PTFE) = Poly(tetrafluoroethylene)
 PC = Polycarbonate (Lexan)
 Polyesters and Polyamides (Nylon)


Composite
Structures
Components of Composite Materials
• Matrix phase: bulk materials such as:
A composite material is
Ceramics
Polymers
basically a combination of two Metals
• Reinforcement: fibers and particulates such as:
or more materials, each of
Glass
Carbon
Kevlar
which retains it own
Silicon Carbide Boron
Ceramic
distinctive properties.
Ceramic
Metallic
Aggregate
Multiphase metals are
• Interface: area of mechanical
composite materials on a
micro scale, but generally the term composite is applied to materials that are created by
mechanically bonding two or more different materials together. The resulting material
has characteristics that are not characteristic of the components in isolation. The concept
of composite materials is ancient. An example is adding straw to mud for building
stronger mud walls. Most commonly, composite materials have a bulk phase, which is
continuous, called the matrix; and a dispersed, non-continuous, phase called the


reinforcement. Some other examples of basic composites include concrete (cement mixed
with sand and aggregate), reinforced concrete (steel rebar in concrete), and fiberglass
(glass strands in a resin matrix).
In about the mid 1960’s, a new
group of composite materials,
called advanced engineered
composite materials (aka advanced
composites), began to emerge.
Advanced composites utilize a
combination of resins and fibers,
customarily carbon/graphite, kevlar,
or fiberglass with an epoxy resin.
The fibers provide the high
stiffness, while the surrounding
polymer resin matrix holds the structure together. The fundamental design concept of
composites is that the bulk phase accepts the load over a large surface area, and transfers
it to the reinforcement material, which can carry a greater load. The significance here lies
in that there are numerous matrix materials and as many fiber types, which can be
combined in countless ways to produce just the desired properties. These materials were
first developed for use in the aerospace industry because for certain application they have
a higher stiffness to weight or strength-to-weight ratio than metals. This means metal
parts can be replaced with lighter weight parts manufactured from advanced composites.
Generally, carbon-epoxy composites are two thirds the weight of aluminum, and two and
a half times as stiff. Composites are resistant to fatigue damage and harsh environments,
and are repairable.
Composites meeting the criteria of having mechanical bonding can also be produced on a
micro scale. For example, when tungsten carbide powder is mixed with cobalt powder,
and then pressed and sintered together, the tungsten carbide retains its identity. The
resulting material has a soft cobalt matrix with tough tungsten carbide particles inside.
This material is used to produce carbide drill bits and is called a metal-matrix composite.
A metal matrix composite is a type of metal that is reinforced with another material to
improve strength, wear or some other characteristics.
Composite Structures (continued)
Classification of Composite Materials
Since the reinforcement material is of primary importance in the strengthening mechanism of a
composite, it is convenient to classify composites according to the characteristics of the
reinforcement. The following three categories are commonly used.
1. Fiber Reinforced – In this group of composites, the fiber is the primary load-bearing
component.
2. Dispersion Strengthened – In this group, the matrix is the major load-bearing component.
3. Particle Reinforced – In this group, the load is shared by the matrix and the particles.
Fiber Reinforced Composites
Fiberglass is likely the best know fiber reinforced composite but carbon-epoxy and other
advanced composites all fall into this category. The fibers can be in the form of long continuous
fibers, or they can be discontinuous fibers, particles, whiskers and even weaved sheets. Fibers are
usually combined with ductile matrix materials, such as metals and polymers, to make them
stiffer, while fibers are added to brittle matrix materials like ceramics to increase toughness. The
length-to diameter ratio of the fiber, the strength of the bond between the fiber and the matrix,
and the amount of fiber are variables that affect the mechanical properties. It is important to have
a high length-to-diameter aspect ratio so that the applied load is effectively transferred form the
matrix to the fiber.
Fiber materials include:
Glass – glass is the most common and inexpensive fiber and is usually use for the reinforcement
of polymer matrices. Glass has a high tensile strength and fairly low density (2.5 g/cc).
Carbon-graphite - in advance composites, carbon fibers are the material of choice. Carbon is a
very light element, with a density of about 2.3 g/cc and its stiffness is considerable higher than
glass. Carbon fibers can have up to 3 times the stiffness of steel and up to 15 times the strength
of construction steel. The graphitic structure is preferred over the diamond-like crystalline forms
for making carbon fiber because the graphitic structure is made of densely packed hexagonal
layers, stacked in a lamellar style. This structure results in mechanical and thermal properties are
highly anisotropic and this gives component designers the ability to control the strength and
stiffness of components by varying the orientation of the fiber.
Polymer – the strong covalent bonds of polymers can lead to impressive properties when aligned
along the fiber axis of high molecular weight chains. Kevlar is an aramid (aromatic polyamide)
composed of oriented aromatic chains, which makes them rigid rod-like polymers. Its stiffness
can be as high as 125 GPa and although very strong in tension, it has very poor compression
properties. Kevlar fibers are mostly used to increase toughness in otherwise brittle matrices.
Ceramic – fibers made from materials such as Alumina and SiC (Silicon carbide) are
advantageous in very high temperature applications, and also where environmental attack is an
issue. Ceramics have poor properties in tension and shear, so most applications as reinforcement
are in the particulate form.
Metallic - some metallic fibers have high strengths but since there density is very high they are
of little use in weight critical applications. Drawing very thin metallic fibers (less than 100
micron) is also very expensive.
Dispersion Strengthen Composites
In dispersion strengthened composites, small particles on the order of 10-5 mm to 2.5 x 10-4 mm
in diameter are added to the matrix material. These particles act to help the matrix resist
deformation. This makes the material harder and stronger. Consider a metal matrix composite
with a fine distribution of very hard and small secondary particles. The matrix material is
carrying most of the load and deformation is accomplished by slip and dislocation movement.
The secondary particles impede slip and dislocation and, thereby, strengthen the material. The
mechanism is that same as precipitation hardening but effect is not quite as strong. However,
particles like oxides do not react with the matrix or go into solution at high temperatures so the
strengthening action is retained at elevated temperatures.
Particle Reinforced Composites
The particles in these composite are larger than in dispersion strengthened composites. The
particle diameter is typically on the order of a few microns. In this case, the particles carry a
major portion of the load. The particles are used to increase the modulus and decrease the
ductility of the matrix. An example of particle reinforced composites is an automobile tire which
has carbon black particles in a matrix of polyisobutylene elastomeric polymer. Particle
reinforced composites are much easier and less costly than making fiber reinforced composites.
With polymeric matrices, the particles are simply added to the polymer melt in an extruder or
injection molder during polymer processing. Similarly, reinforcing particles are added to a
molten metal before it is cast.
Interface
1. The interface is a bounding surface or zone where a discontinuity occurs, whether
physical, mechanical, chemical etc.
2. The matrix material must "wet" the fiber. Coupling agents are frequently used to improve
wettability. Well "wetted" fibers increase the interface surface area.
3. To obtain desirable properties in a composite, the applied load should be effectively
transferred from the matrix to the fibers via the interface. This means that the interface
must be large and exhibit strong adhesion between fibers and matrix. Failure at the
interface (called debonding) may or may not be desirable. This will be explained later in
fracture propagation modes.
4. Bonding with the matrix can be either weak van der Walls forces or strong covalent
bonds.
5. The internal surface area of the interface can go as high as 3000 cm2/cm3.
6. Interfacial strength is measured by simple tests that induce adhesive failure between the
fibers and the matrix. The most common is the Three-point bend test or ILSS
(interlaminar shear stress test)
We will consider the results of incorporating fibers in a matrix. The matrix, besides holding the
fibers together, has the important function of transferring the applied load to the fibers. It is of
great importance to be able to predict the properties of a composite, given the component
properties and their geometric arrangement.
Isotropy and Anisotropy in Composites
1. Fiber reinforced composite materials typically exhibit anisotropy. That is, some
properties vary depending upon which geometric axis or plane they are measured along.
2. For a composite to be isotropic in a specific property, such as CTE or Young’s modulus,
all reinforcing elements, whether fibers or particles, have to be randomly oriented. This is
not easily achieved for discontinuous fibers, since most processing methods tend to
impart a certain orientation to the fibers.
3. Continuous fibers in the form of sheets are usually used to deliberately make the
composite anisotropic in a particular direction that is known to be the principally loaded
axis or plane.
Physical and Chemical Properties
Physical properties are those that can be observed without changing the identity of the substance.
The general properties of matter such as color, density, hardness, are examples of physical
properties. Properties that describe how a substance changes into a completely different
substance are called chemical properties. Flammability and corrosion/oxidation resistance are
examples of chemical properties.
The difference between a physical and chemical property is straightforward until the phase of the
material is considered. When a material changes from a solid to a liquid to a vapor it seems like
them become a difference substance. However, when a material melts, solidifies, vaporizes,
condenses or sublimes, only the state of the substance changes. Consider ice, liquid water, and
water vapor, they are all simply H2O. Phase is a physical property of matter and matter can exist
in four phases – solid, liquid, gas and plasma.
Some of the more important physical and chemical properties from an engineering material
standpoint will be discussed in the following sections.








Phase Transformation Temperatures
Density
Specific Gravity
Thermal Conductivity
Linear Coefficient of Thermal Expansion
Electrical Conductivity and Resistivity
Magnetic Permeability
Corrosion Resistance




Density
Mass can be thinly distributed as in a pillow, or tightly packed as in a block of lead. The
space the mass occupies is its volume, and the mass per unit of volume is its density.
Mass (m) is a fundamental measure of the amount of matter. Weight (w) is a measure of
the force exerted by a mass and this force is force is produced by the acceleration of
gravity. Therefore, on the surface of the earth, the mass of an object is determined by
dividing the weight of an object by 9.8 m/s2 (the acceleration of gravity on the surface of
the earth). Since we are typically comparing things on the surface of the earth, the weight
of an object is commonly used rather than calculating its mass.
The density (r) of a material depends on the phase it is in and the temperature. (The
density of liquids and gases is very temperature dependent.) Water in the liquid state has
a density of 1 g/cm3 = 1000kg/m3 at 4o C. Ice has a density of 0.917 g/cm3 at 0oc, and it
should be noted that this decrease in density for the solid phase is unusual. For almost all

other substances, the density of the solid phase is greater than that of the liquid phase.
Water vapor (vapor saturated air) has a density of 0.051 g/cm3.
Some common units used for expressing density are grams/cubic centimeter,
kilograms/cubic meter, grams/milliliter, grams/liter, pounds for cubic inch and pounds
per cubic foot; but it should be obvious that any unit of mass per any unit of volume can
be used.
Substance
Air
Gasoline
Wood
Water (ice)
Water (liquid)
Aluminum
Steel
Silver
Lead
Mercury
Gold
Density
(g/cm3)
0.0013
0.7
0.85
0.92
1.0
2.7
7.8
10.5
11.3
13.5
19.3
Specific Gravity
Specific gravity is the ratio of density of a substance compared to the density of fresh water at
4°C (39° F). At this temperature the density of water is at its greatest value and equal 1 g/mL.
Since specific gravity is a ratio, so it has no units. An object will float in water if its density is
less than the density of water and sink if its density is greater that that of water. Similarly, an
object with specific gravity less than 1 will float and those with a specific gravity greater than
one will sink. Specific gravity values for a few common substances are: Au, 19.3; mercury, 13.6;
alcohol, 0.7893; benzene, 0.8786. Note that since water has a density of 1 g/cm3, the specific
gravity is the same as the density of the material measured in g/cm3.
The Discovery of Specific Gravity
The discovery of specific gravity makes for an interesting story. Sometime around 250 B.C., the
Greek mathematician Archimedes was given the task of determining whether a craftsman had
defrauded King Heiro II of Syracuse. The king had provided a metal smith with gold to make a
crown. The king suspected that the metal smith had added less valuable silver to crown and kept
some of the gold for himself. The crown weighed the same as other crowns but due to its
intricate designs it was impossible to measure the exact volume of the crown so its density could
be determined. The king challenged Archimedes to determine if the crown was pure gold.
Archimedes had no immediate answer and pondered this question for sometime.
One day while entering a bath, he noticed that water spilled over the sides of the pool, and
realized that the amount of water that spilled out was equal in volume to the space that his body
occupied. He realized that a given mass of silver would occupy more space than an equivalent
mass of gold. Archimedes first weighed the crown and weighed out an equal mass of pure gold.
Then he placed the crown in a full container of water and the pure gold in a container of water.
He found that more water spilled over the sides of the tub when the craftsman’s crown was
submerged. It turned out that the craftsman had been defrauding the King! Legend has it that
Archimedes was so excited about his discovery that he ran naked through the streets of Sicily
shouting Eureka! Eureka! (Which is Greek for “I have found it!”).
Thermal Conductivity
Thermal conductivity (λ) is the intrinsic property of a material which relates its ability to conduct
heat. Heat transfer by conduction involves transfer of energy within a material without any
motion of the material as a whole. Conduction takes place when a temperature gradient exists in
a solid (or stationary fluid) medium. Conductive heat flow occurs in the direction of decreasing
temperature because higher temperature equates to higher molecular energy or more molecular
movement. Energy is transferred from the more energetic to the less energetic molecules when
neighboring molecules collide.
Thermal conductivity is defined as the quantity of heat (Q) transmitted through a unit thickness
(L) in a direction normal to a surface of unit area (A) due to a unit temperature gradient (ΔT)
under steady state conditions and when the heat transfer is dependent only on the temperature
gradient. In equation form this becomes the following:
Thermal Conductivity = heat × distance / (area × temperature gradient)
λ = Q × L / (A × ΔT)
Approximate values of thermal conductivity for some common materials are presented in the
table below.
Material
Thermal Conductivity Thermal Conductivity
W/m, oK
(cal/sec)/(cm2, oC/cm)
Air at 0 C
0.024
0.000057
Aluminum
205.0
0.50
Brass
109.0
-
0.8
0.002
385.0
0.99
Glass, ordinary
0.8
0.0025
Gold
310
-
Ice
1.6
0.005
Iron
-
0.163
Lead
34.7
0.083
Polyethylene HD
0.5
-
Concrete
Copper
Polystyrene expanded
0.03
-
Silver
406.0
1.01
Styrofoam
0.01
-
Steel
50.2
-
-
0.0014
0.12-0.04
0.0001
Water at 20 C
Wood
Linear Coefficient of Thermal Expansion
When heat is added to most materials, the average amplitude of the atoms' vibrating within the
material increases. This, in turn, increases the separation between the atoms causing the material
to expand. If the material does not go through a phase change, the expansion can be easily
related to the temperature change. The linear coefficient of thermal expansion ( a) describes the
relative change in length of a material per degree temperature change. As shown in the following
equation, a is the ratio of change in length ( Dl) to the total starting length (li) and change in
temperature ( DT).
By rearranging this equation, it can be seen that if the linear coefficient of thermal expansion is
known, the change in components length can be calculated for each degree of temperature
change. This effect also works in reverse. That is to say, if energy is removed from a material
then the object's temperature will decrease causing the object to contract.
Thermal expansion (and contraction) must be taken into account when designing products with
close tolerance fits as these tolerances will change as temperature changes if the materials used
in the design have different coefficients of thermal expansion. It should also be understood that
thermal expansion can cause significant stress in a component if the design does not allow for
expansion and contraction of components. The phenomena of thermal expansion can be
challenging when designing bridges, buildings, aircraft and spacecraft, but it can be put to
beneficial uses. For example, thermostats and other heat-sensitive sensors make use of the
property of linear expansion.
Linear Coefficient of Thermal Expansion for a Few Common Materials
Material
a
(m/m/oK)
a (mm/m/oK)
Aluminum
23.8 x 10-6
0.0238
Concrete
12.0 x 10 -6
0.011
Copper
17.6 x 10 -6
0.0176
Brass
18.5 x 10
-6
0.0185
Steel
12.0 x 10 -6
0.0115
-6
Timber
40.0 x 10
Quartz Glass
0.5 x 10 -6
Polymeric Materials 40-200 x 10
Acrylic
75.0 x 10
-6
0.04
0.0005
-6
0.040-0.200
0.075
Electrical Conductivity and Resistivity
It is well known that one of the subatomic particles of an atom is the electron. The electrons
carry a negative electrostatic charge and under certain conditions can move from atom to atom.
The direction of movement between atoms is random unless a force causes the electrons to move
in one direction. This directional movement of electrons due to an electromotive force is what is
known as electricity.
Electrical Conductivity
Electrical conductivity is a measure of how well a material accommodates the movement of an
electric charge. It is the ratio of the current density to the electric field strength. Its SI derived
unit is the Siemens per meter, but conductivity values are often reported as percent IACS. IACS
is an acronym for International Annealed Copper Standard, which was established by the 1913
International Electrochemical Commission. (More Information on the IACS.) The conductivity
of the annealed copper (5.8001 x 107S/m) is defined to be 100% IACS at 20°C . All other
conductivity values are related back to this conductivity of annealed copper. Therefore, iron with
a conductivity value of 1.04 x 107 S/m, has a conductivity of approximately 18% of that of
annealed copper and this is reported as 18% IACS. An interesting side note is that commercially
pure copper products now often have IACS conductivity values greater than 100% IACS because
processing techniques have improved since the adoption of the standard in 1913 and more
impurities can now be removed from the metal.
Conductivity values in Siemens/meter can be converted to % IACS by multiplying the
conductivity value by 1.7241 x10-6. When conductivity values are reported in
microSiemens/centimeter, the conductivity value is multiplied by 172.41 to convert to the %
IACS value.
Electrical conductivity is a very useful property since values are affected by such things as a
substances chemical composition and the stress state of crystalline structures. Therefore,
electrical conductivity information can be used for measuring the purity of water, sorting
materials, checking for proper heat treatment of metals, and inspecting for heat damage in some
materials.
Electrical Resistivity
Electrical resistivity is the reciprocal of conductivity. It is the is the opposition of a body or
substance to the flow of electrical current through it, resulting in a change of electrical energy
into heat, light, or other forms of energy. The amount of resistance depends on the type of
material. Materials with low resistivity are good conductors of electricity and materials with high
resistivity are good insulators.
The SI unit for electrical resistivity is the ohm meter. Resistivity values are more commonly
reported in micro ohm centimeters units. As mentioned above resistivity values are simply the
reciprocal of conductivity so conversion between the two is straightforward. For example, a
material with two micro ohm centimeter of resistivity will have ½ microSiemens/centimeter of
conductivity. Resistivity values in microhm centimeters units can be converted to % IACS
conductivity values with the following formula:
172.41 / resistivity = % IACS
Temperature Coefficient of Resistivity
As noted above, electrical conductivity values (and resistivity values) are typically reported at 20
o
C. This is done because the conductivity and resistivity of material is temperature dependant.
The conductivity of most materials decreases as temperature increases. Alternately, the
resistivity of most material increases with increasing temperature. The amount of change is
material dependant but has been established for many elements and engineering materials.
The reason that resistivity increases with increasing temperature is that the number of
imperfection in the atomic lattice structure increases with temperature and this hampers electron
movement. These imperfections include dislocations, vacancies, interstitial defects and impurity
atoms. Additionally, above absolute zero, even the lattice atoms participate in the interference of
directional electron movement as they are not always found at their ideal lattice sites. Thermal
energy causes the atoms to vibrate about their equilibrium positions. At any moment in time
many individual lattice atoms will be away from their perfect lattice sites and this interferes with
electron movement.
When the temperature coefficient is known, an adjusted resistivity value can be computed using
the following formula:
R1 = R2 * [1 + a * (T1–T2)]
Where: R1 = resistivity value adjusted to T1
R2 = resistivity value known or measured at temperature T2
a = Temperature Coefficient
T1 = Temperature at which resistivity value needs to be known
T2 = Temperature at which known or measured value was obtained
For example, suppose that resistivity measurements were being made on a hot piece of
aluminum. Normally when measuring resistivity or conductivity, the instrument is calibrated
using standards that are at the same temperature as the material being measured, and then no
correction for temperature will be required. However, if the calibration standard and the test
material are at different temperatures, a correction to the measured value must be made. Presume
that the instrument was calibrated at 20oC (68oF) but the measurement was made at 25oC (77oF)
and the resistivity value obtained was 2.706 x 10-8 ohm meters. Using the above equation and the
following temperature coefficient value, the resistivity value corrected for temperature can be
calculated.
R1 = R2 * [1 + a * (T1–T2)]
Where: R1 = ?
R2 = 2.706 x 10-8 ohm meters (measured resistivity at 25 oC)
a = 0.0043/ oC
T1 = 20 oC
T2 = 25 oC
R1 = 2.706 x 10-8ohm meters * [1 + 0.0043/ oC * (20 oC – 25 oC)]
R1 = 2.648 x 10-8ohm meters
Note that the resistivity value was adjusted downward since this example involved calculating
the resistivity for a lower temperature.
Since conductivity is simply the inverse of resistivity, the temperature coefficient is the same for
conductivity and the equation requires only slight modification. The equation becomes:
s1 = s2 / [1 + a * (T1–T2)]
Where: s1 = conductivity value adjusted to T1
s2 = conductivity value known or measured at temperature T2
a = Temperature Coefficient
T1 = Temperature at which conductivity value needs to be known
T2 = Temperature at which known or measured value was obtained
In this example let’s consider the same aluminum alloy with a temperature coefficient of 0.0043
per degree centigrade and a conductivity of 63.6% IACS at 25 oC. What will the conductivity be
when adjusted to 20 oC?
s1= 63.6% IACS / [1 + 0.0043 * (20 oC – 25 oC)]
s1= 65.0% IASC
The temperature coefficient for a few metallic elements is shown below.
Material
Temperature Coefficient (/ oC)
Nickel
0.0059
Iron
0.0060
Molybdenum
0.0046
Tungsten
0.0044
Aluminum
0.0043
Copper
0.0040
Silver
0.0038
Platinum
0.0038
Gold
0.0037
Zinc
0.0038
Magnetic Permeability
Magnetic permeability or simply permeability is the ease with which a material can be
magnetized. It is a constant of proportionality that exists between magnetic induction and
magnetic field intensity. This constant is equal to approximately 1.257 x 10-6 Henry per meter
(H/m) in free space (a vacuum). In other materials it can be much different, often substantially
greater than the free-space value, which is symbolized µ0.
Materials that cause the lines of flux to move farther apart, resulting in a decrease in magnetic
flux density compared with a vacuum, are called diamagnetic. Materials that concentrate
magnetic flux by a factor of more than one but less than or equal to ten are called paramagnetic;
materials that concentrate the flux by a factor of more than ten are called ferromagnetic. The
permeability factors of some substances change with rising or falling temperature, or with the
intensity of the applied magnetic field.
In engineering applications, permeability is often expressed in relative, rather than in absolute,
terms. If µ o represents the permeability of free space (that is, 4p X10-7H/m or 1.257 x 10-6 H/m)
and µ represents the permeability of the substance in question (also specified in henrys per
meter), then the relative permeability, µr, is given by:
µr = µ / µ 0
For non-ferrous metals such as copper, brass, aluminum etc., the permeability is the same as that
of "free space", i.e. the relative permeability is one. For ferrous metals however the value of µ r
may be several hundred. Certain ferromagnetic materials, especially powdered or laminated iron,
steel, or nickel alloys, have µr that can range up to about 1,000,000. Diamagnetic materials have
µr less than one, but no known substance has relative permeability much less than one. In
addition, permeability can vary greatly within a metal part due to localized stresses, heating
effects, etc.
When a paramagnetic or ferromagnetic core is inserted into a coil, the inductance is multiplied
by µr compared with the inductance of the same coil with an air core. This effect is useful in the
design of transformers and eddy current probes.
Corrosion
Corrosion involves the deterioration of a material as it reacts with its environment. Corrosion is
the primary means by which metals deteriorate. Corrosion literally consumes the material
reducing load carrying capability and causing stress concentrations. Corrosion is often a major
part of maintenance cost and corrosion prevention is vital in many designs. Corrosion is not
expressed in terms of a design property value like other properties but rather in more qualitative
terms such as a material is immune, resistant, susceptible or very
susceptible to corrosion.
Partial Electromotive Force Series
The corrosion process is usually electrochemical in nature, having the
essential features of a battery. Corrosion is a natural process that
commonly occurs because unstable materials, such as refined metals
want to return to a more stable compound. For example, some metals,
such as gold and silver, can be found in the earth in their natural,
metallic state and they have little tendency to corrode. Iron is a
moderately active metal and corrodes readily in the presence of water.
The natural state of iron is iron oxide and the most common iron ore is
Hematite with a chemical composition of Fe203. Rust, the most
common corrosion product of iron, also has a chemical composition of
Fe2O3.
The difficulty in terms of energy required to extract metals from their
ores is directly related to the ensuing tendency to corrode and release
this energy. The electromotive force series (See table) is a ranking of
metals with respect to their inherent reactivity. The most noble metal is
at the top and has the highest positive electrochemical potential. The
most active metal is at the bottom and has the most negative
electrochemical potential.
Standard Potential
Electrode Reaction
(at 25oC), V-SHE
Au3+ + 3e- -> Au
1.498
Pd2+
2e-
-> Pd
0.987
Hg2+ + 2e- -> Hg
+
0.854
Ag+ + e- -> Au
0.799
Cu+
0.521
+
e-
+
-
0.337
2H + 2e -> H2
0.000 (Ref.)
Pb2+ + 2e- -> Pb
-0.126
Sn2+
+
2e-
-> Sn
-0.136
Ni2+ + 2e- -> Ni
-0.250
Co2+
+
2e-
-> Co
-0.277
Cd2+ + 2e- -> Cd
-0.403
Fe2+
+
2e-
-> Fe
-0.440
Cr3+ + 3e- -> Cr
-0.744
Cr2+ + 2e- -> Cr
-0.910
2+
-
Zn + 2e -> Zn
-0.763
Mn2+ + 2e- -> Mn
-1.180
2+
Note that aluminum, as indicated by its position in the series, is a relatively reactive
metal; among structural metals, only beryllium and magnesium are more reactive.
Aluminum owes its excellent corrosion resistance to the barrier oxide film that is
bonded strongly to the surface and if damaged reforms immediately in most
environments. On a surface freshly abraded and exposed to air, the protective film is
only 10 Angstroms thick but highly effective at protecting the metal from corrosion.
-> Cu
Cu2+ + 2e- -> Cu
-
Ti + 2e -> Ti
-1.630
Al3+ + 3e- -> Al
-1.662
Be2+
-1.850
+
2e-
-> Be
Mg2+ + 2e- -> Mg
-2.363
Li+
-3.050
+
e-
-> Li
Corrosion involve two chemical processes…oxidation and reduction. Oxidation is the process of
stripping electrons from an atom and reduction occurs when an electron is added to an atom. The
oxidation process takes place at an area known as the anode. At the anode, positively charged
atoms leave the solid surface and enter into an electrolyte as ions. The ions leave their
corresponding negative charge in the form of electrons in the metal which travel to the location
of the cathode through a conductive path. At the cathode, the corresponding reduction reaction
takes place and consumes the free electrons. The electrical balance of the circuit is restored at the
cathode when the electrons react with neutralizing positive ions, such as hydrogen ions, in the
electrolyte. From this description, it can be seen that there are four essential components that are
needed for a corrosion reaction to proceed. These components are an anode, a cathode, an
electrolyte with oxidizing species, and some direct electrical connection between the anode and
cathode. Although atmospheric air is the most common environmental electrolyte, natural
waters, such as seawater rain, as well as man-made solutions, are the environments most
frequently associated with corrosion problems.
A typical situation might involve a piece of metal
that has anodic and cathodic regions on the same
surface. If the surface becomes wet, corrosion
may take place through ionic exchange in the
surface water layer between the anode and
cathode. Electron exchange will take place
through the bulk metal. Corrosion will proceed
at the anodic site according to a reaction such as
M → M++ + 2ewhere M is a metal atom. The resulting metal
cations (M++) are available at the metal surface to
become corrosion products such as oxides,
hydroxides, etc. The liberated electrons travel
through the bulk metal (or another low resistance
electrical connection) to the cathode, where they
are consumed by cathodic reactions such as
2H+ + 2e- → H 2
The basic principles of corrosion that were just covered, generally apply to all corrosion situation
except certain types of high temperature corrosion. However, the process of corrosion can be
very straightforward but is often very complex due to variety of variable that can contribute to
the process. A few of these variable are the composition of the material acting in the corrosion
cell, the heat treatment and stress state of the materials, the composition of the electrolyte, the
distance between the anode and the cathode, temperature, protective oxides and coating, etc.
Types of Corrosion
Corrosion is commonly classified based on the appearance of the corroded material. The
classifications used vary slightly from reference to reference but there is generally considered to
be eight different forms of corrosion. There forms are:
Uniform or general – corrosion that is distributed more or less uniformly over a surface.
Localized – corrosion that is confined to small area. Localized corrosion often occurs due to a
concentrated cell. A concentrated cell is an electrolytic cell in which the electromotive force is
caused by a concentration of some components in the electrolyte. This difference leads to the
formation of distinct anode and cathode regions.



Pitting – corrosion that is confined to small areas and take the form of cavities on a
surface.
Crevice – corrosion occurring at locations where easy access to the bulk environment is
prevented, such as the mating surfaces of two components.
Filiform – Corrosion that occurs under some coatings in the form of randomly distributed
threadlike filaments.
Intergranular – preferential corrosion at or along the grain boundaries of a metal.

Exfoliation – a specific form of
corrosion that travels along grain
boundaries parallel to the surface of
the part causing lifting and flaking at
the surface. The corrosion products
expand between the uncorroded
layers of metal to produce a look that
resembles pages of a book.
Exfoliation corrosion is associated
with sheet, plate and extruded
products and usually initiates at
unpainted or unsealed edges or holes
of susceptible metals.
Galvanic – corrosion associated primarily with the electrical coupling of materials with
significantly different electrochemical potentials.
Environmental Cracking – brittle fracture of a normally ductile material that occurs partially
due to the corrosive effect of an environment.





Corrosion fatigue – fatigue cracking that is characterized by uncharacteristically short
initiation time and/or growth rate due to the damage of corrosion or buildup of corrosion
products.
High temperature hydrogen attack – the loss of strength and ductility of steel due to a
high temperature reaction of absorbed hydrogen with carbides. The result of the reaction
is decarburization and internal fissuring.
Hydrogen Embrittlement – the loss of ductility of a metal resulting from absorption of
hydrogen.
Liquid metal cracking – cracking caused by contact with a liquid metal.
Stress corrosion – cracking of a metal due to the combined action of corrosion and a
residual or applied tensile stress.
Erosion corrosion – a corrosion reaction accelerated by the relative movement of a corrosive
fluid and a metal surface.
Fretting corrosion – damage at the interface of two contacting surfaces under load but capable
of some relative motion. The damage is accelerated by movement at the interface that
mechanically abraded the surface and exposes fresh material to corrosive attack.
Dealloying – the selective corrosion of one or more components of a solid solution alloy.

Dezincification – corrosion resulting in the selective removal of zinc from copper-zinc
alloys.

The mechanical properties of a material are those properties that involve a reaction to an
applied load. The mechanical properties of metals determine the range of usefulness of a
material and establish the service life that can be expected. Mechanical properties are
also used to help classify and identify material. The most common properties considered
are strength, ductility, hardness, impact resistance, and fracture toughness.
Most structural materials are anisotropic, which means that their material properties vary
with orientation. The variation in properties can be due to directionality in the
microstructure (texture) from forming or cold working operation, the controlled
alignment of fiber reinforcement and a variety of other causes. Mechanical properties are
generally specific to product form such as sheet, plate, extrusion, casting, forging, and
etc. Additionally, it is common to see mechanical property listed by the directional grain
structure of the material. In products such as sheet and plate, the rolling direction is called
the longitudinal direction, the width of the product is called the transverse direction, and
the thickness is called the short transverse direction. The grain orientations in standard
wrought forms of metallic products are shown the image.


Mechanical Properties


The mechanical properties of a material are not constants and often change as a function
of temperature, rate of loading, and other conditions. For example, temperatures below
room temperature generally cause an increase in strength properties of metallic alloys;
while ductility, fracture toughness, and elongation usually decrease. Temperatures above
room temperature usually cause a decrease in the strength properties of metallic alloys.
Ductility may increase or decrease with increasing temperature depending on the same
variables
It should also be noted that there is often significant variability in the values obtained
when measuring mechanical properties. Seemingly identical test specimen from the same
lot of material will often produce considerable different results. Therefore, multiple tests
are commonly conducted to determine mechanical properties and values reported can be
an average value or calculated statistical minimum value. Also, a range of values are
sometimes reported in order to show variability.





Stress and Strain
Stress
The term stress (s) is used to express the loading in terms of force applied to a certain
cross-sectional area of an object. From the perspective of loading, stress is the applied
force or system of forces that tends to deform a body. From the perspective of what is
happening within a material, stress is the internal distribution of forces within a body that
balance and react to the loads applied to it. The stress distribution may or may not be
uniform, depending on the nature of the loading condition. For example, a bar loaded in
pure tension will essentially have a uniform tensile stress distribution. However, a bar
loaded in bending will have a stress distribution that changes with distance perpendicular
to the normal axis.
Simplifying assumptions are often used to represent stress as a vector quantity for many
engineering calculations and for material property determination. The word "vector"
typically refers to a quantity that has a "magnitude" and a "direction". For example, the
stress in an axially loaded bar is simply equal to the applied force divided by the bar's
cross-sectional area.

Some common measurements of stress are:
Psi = lbs/in2 (pounds per square inch)
ksi or kpsi = kilopounds/in2 (one thousand or 103 pounds per square inch)
Pa = N/m 2 (Pascals or Newtons per square meter)
kPa = Kilopascals (one thousand or 103 Newtons per square meter)
GPa = Gigapascals (one million or 106 Newtons per square meter)
*Any metric prefix can be added in front of psi or Pa to indicate the multiplication factor
It must be noted that the stresses
in most 2-D or 3-D solids are
actually more complex and need
be defined more methodically.
The internal force acting on a
small area of a plane can be
resolved into three components:
one normal to the plane and two
parallel to the plane. The normal
force component divided by the
area gives the normal stress (s),
and parallel force components
divided by the area give the shear
stress (t). These stresses are
average stresses as the area is
finite, but when the area is




allowed to approach zero, the stresses become stresses at a point. Since stresses are
defined in relation to the plane that passes through the point under consideration, and the
number of such planes is infinite, there appear an infinite set of stresses at a point.
Fortunately, it can be proven that the stresses on any plane can be computed from the
stresses on three orthogonal planes passing through the point. As each plane has three
stresses, the stress tensor has nine stress components, which completely describe the state
of stress at a point.
Strain
Strain is the response of a system to an applied stress. When a material is loaded with a
force, it produces a stress, which then causes a material to deform. Engineering strain is
defined as the amount of deformation in the direction of the applied force divided by the
initial length of the material. This results in a unitless number, although it is often left in
the unsimplified form, such as inches per inch or meters per meter. For example, the
strain in a bar that is being stretched in tension is the amount of elongation or change in
length divided by its original length. As in the case of stress, the strain distribution may
or may not be uniform in a complex structural element, depending on the nature of the
loading condition.

If the stress is small, the material may only strain a small amount and the material will
return to its original size after the stress is released. This is called elastic deformation,
because like elastic it returns to its unstressed state. Elastic deformation only occurs in a
material when stresses are lower than a critical stress called the yield strength. If a
material is loaded beyond it elastic limit, the material will remain in a deformed condition
after the load is removed. This is called plastic deformation.
Engineering and True Stress and Strain
The discussion above focused on engineering stress and strain, which use the fixed,
undeformed cross-sectional area in the calculations. True stress and strain measures
account for changes in cross-sectional area by using the instantaneous values for the area.
The engineering stress-strain curve does not give a true indication of the deformation
characteristics of a metal because it is based entirely on the original dimensions of the
specimen, and these dimensions change continuously during the testing used to generate
the data.
Engineering stress and strain data is commonly used because it is easier to generate the
data and the tensile properties are adequate for engineering calculations. When
considering the stress-strain curves in the next section, however, it should be understood
that metals and other materials continues to strain-harden until they fracture and the stress
required to produce further deformation also increase.

Stress Concentration
When an axial load is applied to a piece of
material with a uniform cross-section, the
norm al stress will be uniformly distributed
over the cross-section. However, if a hole is
drilled in the material, the stress distribution
will no longer be uniform. Since the
material that has been removed from the
hole is no longer available to carry any load,
the load must be redistributed over the
remaining material. It is not redistributed
evenly over the entire remaining crosssectional area but instead will be
redistributed in an uneven pattern that is
highest at the edges of the hole as shown in
the image. This phenomenon is known as
stress concentration.
Tensile Properties
Tensile properties indicate how the material will react to forces being applied in tension. A
tensile test is a fundamental mechanical test where a carefully prepared specimen is loaded in a
very controlled manner while measuring the applied load and the elongation of the specimen
over some distance. Tensile tests are used to determine the modulus of elasticity, elastic limit,
elongation, proportional limit, reduction in area, tensile strength, yield point, yield strength and
other tensile properties.
The main product of a tensile test is a load versus elongation curve which is then converted into a
stress versus strain curve. Since both the engineering stress and the engineering strain are
obtained by dividing the load and elongation by constant values (specimen geometry
information), the load-elongation curve will have the same shape as the engineering stress-strain
curve. The stress-strain curve relates the applied stress to the resulting strain and each material
has its own unique stress-strain curve. A typical engineering stress-strain curve is shown below.
If the true stress, based on the actual cross-sectional area of the specimen, is used, it is found that
the stress-strain curve increases continuously up to fracture.
Linear-Elastic Region and Elastic Constants
As can be seen in the figure, the stress and strain initially increase with a linear relationship. This
is the linear-elastic portion of the curve and it indicates that no plastic deformation has occurred.
In this region of the curve, when the stress is reduced, the material will return to its original
shape. In this linear region, the line obeys the relationship defined as Hooke's Law where the
ratio of stress to strain is a constant.
The slope of the line in this region where stress is proportional to strain and is called the
modulus of elasticity or Young's modulus. The modulus of elasticity (E) defines the properties
of a material as it undergoes stress, deforms, and then returns to its original shape after the stress
is removed. It is a measure of the stiffness of a given material. To compute the modulus of
elastic , simply divide the stress by the strain in the material. Since strain is unitless, the modulus
will have the same units as the stress, such as kpi or MPa. The modulus of elasticity applies
specifically to the situation of a component being stretched with a tensile force. This modulus is
of interest when it is necessary to compute how much a rod or wire stretches under a tensile load.
There are several different kinds of moduli depending on the way the material is being stretched,
bent, or otherwise distorted. When a component is subjected to pure shear, for instance, a
cylindrical bar under torsion, the shear modulus describes the linear-elastic stress-strain
relationship.
Axial strain is always accompanied by lateral strains of opposite sign in the two directions
mutually perpendicular to the axial strain. Strains that result from an increase in length are
designated as positive (+) and those that result in a decrease in length are designated as negative
(-). Poisson's ratio is defined as the negative of the ratio of the lateral strain to the axial strain
for a uniaxial stress state.
Poisson's ratio is sometimes also defined as the ratio of the absolute values of lateral and axial
strain. This ratio, like strain, is unitless since both strains are unitless. For stresses within the
elastic range, this ratio is approximately constant. For a perfectly isotropic elastic material,
Poisson's Ratio is 0.25, but for most materials the value lies in the range of 0.28 to 0.33.
Generally for steels, Poisson’s ratio will have a value of approximately 0.3. This means that if
there is one inch per inch of deformation in the direction that stress is applied, there will be 0.3
inches per inch of deformation perpendicular to the direction that force is applied.
Only two of the elastic constants are independent so if two constants are known, the third can be
calculated using the following formula:
E = 2 (1 + n) G.
Where:
E = modulus of elasticity (Young's modulus)
n = Poisson's ratio
G = modulus of rigidity (shear modulus).
A couple of additional elastic constants that may be encountered include the bulk modulus (K),
and Lame's constants (m and l). The bulk modulus is used describe the situation where a piece of
material is subjected to a pressure increase on all sides. The relationship between the change in
pressure and the resulting strain produced is the bulk modulus. Lame's constants are derived
from modulus of elasticity and Poisson's ratio.
Yield Point
In ductile materials, at some point, the stress-strain curve deviates from the straight-line
relationship and Law no longer applies as the strain increases faster than the stress. From this
point on in the tensile test, some permanent deformation occurs in the specimen and the material
is said to react plastically to any further increase in load or stress. The material will not return to
its original, unstressed condition when the load is removed. In brittle materials, little or no plastic
deformation occurs and the material fractures near the end of the linear-elastic portion of the
curve.
With most materials there is a gradual transition from elastic to plastic behavior, and the exact
point at which plastic deformation begins to occur is hard to determine. Therefore, various
criteria for the initiation of yielding are used depending on the sensitivity of the strain
measurements and the intended use of the data. (See Table) For most engineering design and
specification applications, the yield strength is used. The yield strength is defined as the stress
required to produce a small, amount of plastic deformation. The offset yield strength is the stress
corresponding to the intersection of the stress-strain curve and a line parallel to the elastic part of
the curve offset by a specified strain (in the US the offset is typically 0.2% for metals and 2% for
plastics).
To determine the yield strength using this offset, the point is found
In Great Britain, the yield
on the strain axis (x-axis) of 0.002, and then a line parallel to the
strength is often referred
stress-strain line is drawn. This line will intersect the stress-strain
to as the proof stress. The
line slightly after it begins to curve, and that intersection is defined
offset value is either 0.1%
as the yield strength with a 0.2% offset. A good way of looking at
or 0.5%
offset yield strength is that after a specimen has been loaded to its
0.2 percent offset yield strength and then unloaded it will be 0.2
percent longer than before the test. Even though the yield strength is meant to represent the exact
point at which the material becomes permanently deformed, 0.2% elongation is considered to be
a tolerable amount of sacrifice for the ease it creates in defining the yield strength.
Some materials such as gray cast iron or soft copper exhibit essentially no linear-elastic
behavior. For these materials the usual practice is to define the yield strength as the stress
required to produce some total amount of strain.




True elastic limit is a very low value and is related to the motion of a few hundred
dislocations. Micro strain measurements are required to detect strain on order of 2 x 10 -6
in/in.
Proportional limit is the highest stress at which stress is directly proportional to strain. It
is obtained by observing the deviation from the straight-line portion of the stress-strain
curve.
Elastic limit is the greatest stress the material can withstand without any measurable
permanent strain remaining on the complete release of load. It is determined using a
tedious incremental loading-unloading test procedure. With the sensitivity of strain
measurements usually employed in engineering studies (10 -4in/in), the elastic limit is
greater than the proportional limit. With increasing sensitivity of strain measurement, the
value of the elastic limit decreases until it eventually equals the true elastic limit
determined from micro strain measurements.
Yield strength is the stress required to produce a small-specified amount of plastic
deformation. The yield strength obtained by an offset method is commonly used for
engineering purposes because it avoids the practical difficulties of measuring the elastic
limit or proportional limit.
Ultimate Tensile Strength
The ultimate tensile strength (UTS) or, more simply, the tensile strength, is the maximum
engineering stress level reached in a tension test. The strength of a material is its ability to
withstand external forces without breaking. In brittle materials, the UTS will at the end of the
linear-elastic portion of the stress-strain curve or close to the elastic limit. In ductile materials,
the UTS will be well outside of the elastic portion into the plastic portion of the stress-strain
curve.
On the stress-strain curve above, the UTS is the highest point where the line is momentarily flat.
Since the UTS is based on the engineering stress, it is often not the same as the breaking
strength. In ductile materials strain hardening occurs and the stress will continue to increase until
fracture occurs, but the engineering stress-strain curve may show a decline in the stress level
before fracture occurs. This is the result of engineering stress being based on the original crosssection area and not accounting for the necking that commonly occurs in the test specimen. The
UTS may not be completely representative of the highest level of stress that a material can
support, but the value is not typically used in the design of components anyway. For ductile
metals the current design practice is to use the yield strength for sizing static components.
However, since the UTS is easy to determine and quite reproducible, it is useful for the purposes
of specifying a material and for quality control purposes. On the other hand, for brittle materials
the design of a component may be based on the tensile strength of the material.
Measures of Ductility (Elongation and Reduction of Area)
The ductility of a material is a measure of the extent to which a material will deform before
fracture. The amount of ductility is an important factor when considering forming operations
such as rolling and extrusion. It also provides an indication of how visible overload damage to a
component might become before the component fractures. Ductility is also used a quality control
measure to assess the level of impurities and proper processing of a material.
The conventional measures of ductility are the
engineering strain at fracture (usually called the
elongation ) and the reduction of area at
fracture. Both of these properties are obtained
by fitting the specimen back together after
fracture and measuring the change in length and
cross-sectional area. Elongation is the change in
axial length divided by the original length of the
specimen or portion of the specimen. It is
expressed as a percentage. Because an
appreciable fraction of the plastic deformation
will be concentrated in the necked region of the
tensile specimen, the value of elongation will
depend on the gage length over which the
measurement is taken. The smaller the gage length the greater the large localized strain in the
necked region will factor into the calculation. Therefore, when reporting values of elongation ,
the gage length should be given.
One way to avoid the complication from necking is to base the elongation measurement on the
uniform strain out to the point at which necking begins. This works well at times but some
engineering stress-strain curve are often quite flat in the vicinity of maximum loading and it is
difficult to precisely establish the strain when necking starts to occur.
Reduction of area is the change in cross-sectional area divided by the original cross-sectional
area. This change is measured in the necked down region of the specimen. Like elongation, it is
usually expressed as a percentage.
As previously discussed, tension is just one of the way that a material can be loaded. Other ways
of loading a material include compression, bending, shear and torsion, and there are a number of
standard tests that have been established to characterize how a material performs under these
other loading conditions. A very cursory introduction to some of these other material properties
will be provided on the next page.
Compressive, Bearing, & Shear Properties
Compressive Properties
In theory, the compression test is simply the opposite of the tension test
with respect to the direction of loading. In compression testing the sample
is squeezed while the load and the displacement are recorded.
Compression tests result in mechanical properties that include the
compressive yield stress, compressive ultimate stress, and compressive
modulus of elasticity.
Compressive yield stress is measured in a manner identical to that done for
tensile yield strength. When testing metals, it is defined as the stress
corresponding to 0.002 in./in. plastic strain. For plastics, the compressive
yield stress is measured at the point of permanent yield on the stress-strain
curve. Moduli are generally greater in compression for most of the
commonly used structural materials.
Ultimate compressive strength is the stress required to rupture a specimen.
This value is much harder to determine for a compression test than it is for a tensile test since
many material do not exhibit rapid fracture in compression. Materials such as most plastics that
do not rupture can have their results reported as the compressive strength at a specific
deformation such as 1%, 5%, or 10% of the sample's original height.
For some materials, such as concrete, the compressive strength is the most important material
property that engineers use when designing and building a structure. Compressive strength is
also commonly used to determine whether a concrete mixture meets the requirements of the job
specifications.
Bearing Properties
Bearing properties are used when designing mechanically fastened joints. The purpose of a
bearing test is to determine the the deformation of a hole as a function of the applied bearing
stress. The test specimen is basically a piece of sheet or plate with a carefully prepared hole
some standard distance from the edge. Edge-to-hole diameter ratios of 1.5 and 2.0 are common.
A hardened pin is inserted through the hole and an axial load applied to the specimen and the
pin. The bearing stress is computed by dividing the load applied to the pin, which bears against
the edge of the hole, by the bearing area (the product of the pin diameter and the sheet or plate
thickness). Bearing yield and ultimate stresses are obtained from bearing tests. BYS is computed
from a bearing stress deformation curve by drawing a line parallel to the initial slope at an offset
of 0.02 times the pin diameter. BUS is the maximum stress withstood by a bearing specimen.
Shear Properties
A shearing stress acts parallel to the stress plane, whereas a tensile or compressive stress acts
normal to the stress plane. Shear properties are primarily used in the design of mechanically
fastened components, webs, and torsion members, and other components subject to parallel,
opposing loads. Shear properties are dependant on the type of shear test and their is a variety of
different standard shear tests that can be performed including the single-shear test, double-shear
test, blanking-shear test, torsion-shear test and others. The shear modulus of elasticity is
considered a basic shear property. Other properties, such as the proportional limit stress and
shear ultimate stress, cannot be treated as basic shear properties because of “form factor” effects.
Hardness
Hardness is the resistance of a material to localized deformation. The term can apply to
deformation from indentation, scratching, cutting or bending. In metals, ceramics and most
polymers, the deformation considered is plastic deformation of the surface. For elastomers and
some polymers, hardness is defined at the resistance to elastic deformation of the surface. The
lack of a fundamental definition indicates that hardness is not be a basic property of a material,
but rather a composite one with contributions from the yield strength, work hardening, true
tensile strength, modulus, and others factors. Hardness measurements are widely used for the
quality control of materials because they are quick and considered to be nondestructive tests
when the marks or indentations produced by the test are in low stress areas.
There are a large variety of methods used for determining the hardness of a substance. A few of
the more common methods are introduced below.
Mohs Hardness Test
One of the oldest ways of measuring hardness was devised by the German mineralogist Friedrich
Mohs in 1812. The Mohs hardness test involves observing whether a materials surface is
scratched by a substance of known or defined hardness. To give numerical values to this physical
property, minerals are ranked along the Mohs scale, which is composed of 10 minerals that have
been given arbitrary hardness values. Mohs hardness test, while greatly facilitating the
identification of minerals in the field, is not suitable for accurately gauging the hardness of
industrial materials such as steel or ceramics. For engineering materials, a variety of instruments
have been developed over the years to provide a precise measure of hardness. Many apply a load
and measure the depth or size of the resulting indentation. Hardness can be measured on the
macro-, micro- or nano- scale.
Brinell Hardness Test
The oldest of the hardness test methods in common use on engineering materials today is the
Brinell hardness test. Dr. J. A. Brinell invented the Brinell test in Sweden in 1900. The Brinell
test uses a desktop machine to applying a specified load to a hardened sphere of a specified
diameter. The Brinell hardness number, or simply the Brinell number, is obtained by dividing the
load used, in kilograms, by the measured surface area of the indentation, in square millimeters,
left on the test surface. The Brinell test is frequently used to determine the hardness metal
forgings and castings that have a large grain structures. The Brinell test provides a measurement
over a fairly large area that is less affected by the course grain structure of these materials than
are Rockwell or Vickers tests.
A wide range of materials can be tested using a Brinell test simply by varying the test load and
indenter ball size. In the USA, Brinell testing is typically done on iron and steel castings using a
3000Kg test force and a 10mm diameter ball. A 1500 kilogram load is usually used for
aluminum castings. Copper, brass and thin stock are frequently tested using a 500Kg test force
and a 10 or 5mm ball. In Europe Brinell testing is done using a much wider range of forces and
ball sizes and it is common to perform Brinell tests on small parts using a 1mm carbide ball and
a test force as low as 1kg. These low load tests are commonly referred to as baby Brinell tests.
The test conditions should be reported along with the Brinell hardness number. A value reported
as "60 HB 10/1500/30" means that a Brinell Hardness of 60 was obtained using a 10mm
diameter ball with a 1500 kilogram load applied for 30 seconds.
Rockwell Hardness Test
The Rockwell Hardness test also uses a machine to apply a specific load and then measure the
depth of the resulting impression. The indenter may either be a steel ball of some specified
diameter or a spherical diamond-tipped cone of 120° angle and 0.2 mm tip radius, called a brale.
A minor load of 10 kg is first applied, which causes a small initial penetration to seat the indenter
and remove the effects of any surface irregularities. Then, the dial is set to zero and the major
load is applied. Upon removal of the major load, the depth reading is taken while the minor load
is still on. The hardness number may then be read directly from the scale. The indenter and the
test load used determine the hardness scale that is used (A, B, C, etc).
For soft materials such as copper alloys, soft steel, and aluminum alloys a 1/16" diameter steel
ball is used with a 100-kilogram load and the hardness is read on the "B" scale. In testing harder
materials, hard cast iron and many steel alloys, a 120 degrees diamond cone is used with up to a
150 kilogram load and the hardness is read on the "C" scale. There are several Rockwell scales
other than the "B" & "C" scales, (which are called the common scales). A properly reported
Rockwell value will have the hardness number followed by "HR" (Hardness Rockwell) and the
scale letter. For example, 50 HRB indicates that the material has a hardness reading of 50 on the
B scale.
A -Cemented carbides, thin steel and shallow case hardened steel
B -Copper alloys, soft steels, aluminum alloys, malleable iron, etc.
C -Steel, hard cast irons, pearlitic malleable iron, titanium, deep case hardened
steel and other materials harder than B 100
D -Thin steel and medium case hardened steel and pearlitic malleable iron
E -Cast iron, aluminum and magnesium alloys, bearing metals
F -Annealed copper alloys, thin soft sheet metals
G -Phosphor bronze, beryllium copper, malleable irons
H -Aluminum, zinc, lead
K, L, M, P, R, S, V -Bearing metals and other very soft or thin materials,
including plastics.
Rockwell Superficial Hardness Test
The Rockwell Superficial Hardness Tester is used to test thin materials, lightly carburized steel
surfaces, or parts that might bend or crush under the conditions of the regular test. This tester
uses the same indenters as the standard Rockwell tester but the loads are reduced. A minor load
of 3 kilograms is used and the major load is either 15 or 45 kilograms depending on the indenter
used. Using the 1/16" diameter, steel ball indenter, a "T" is added (meaning thin sheet testing) to
the superficial hardness designation. An example of a superficial Rockwell hardness is 23
HR15T, which indicates the superficial hardness as 23, with a load of 15 kilograms using the
steel ball.
Vickers and Knoop Microhardness Tests
The Vickers and Knoop Hardness Tests are a modification of the Brinell test and are used to
measure the hardness of thin film coatings or the surface hardness of case-hardened parts. With
these tests, a small diamond pyramid is pressed into the sample under loads that are much less
than those used in the Brinell test. The difference between the Vickers and the Knoop Tests is
simply the shape of the diamond pyramid indenter. The Vickers test uses a square pyramidal
indenter which is prone to crack brittle materials. Consequently, the Knoop test using a rhombicbased (diagonal ratio 7.114:1) pyramidal indenter was developed which produces longer but
shallower indentations. For the same load, Knoop indentations are about 2.8 times longer than
Vickers indentations.
An applied load ranging from 10g to 1,000g is used. This low amount of load creates a small
indent that must be measured under a microscope. The measurements for hard coatings like TiN
must be taken at very high magnification (i.e. 1000X), because the indents are so small. The
surface usually needs to be polished. The diagonals of the impression are measured, and these
values are used to obtain a hardness number (VHN), usually from a lookup table or chart. The
Vickers test can be used to characterize very hard materials but the hardness is measured over a
very small region.
The values are expressed like 2500 HK25 (or HV25) meaning 2500 Hardness Knoop at 25 gram
force load. The Knoop and Vickers hardness values differ slightly, but for hard coatings, the
values are close enough to be within the measurement error and can be used interchangeably.
Scleroscope and Rebound Hardness Tests
The Scleroscope test is a very old test that involves dropping a diamond tipped hammer, which
falls inside a glass tube under the force of its own weight from a fixed height, onto the test
specimen. The height of the rebound travel of the hammer is measured on a graduated scale. The
scale of the rebound is arbitrarily chosen and consists on Shore units, divided into 100 parts,
which represent the average rebound from pure hardened high-carbon steel. The scale is
continued higher than 100 to include metals having greater hardness. The Shore Scleroscope
measures hardness in terms of the elasticity of the material and the hardness number depends on
the height to which the hammer rebounds, the harder the material, the higher the rebound.
The Rebound Hardness Test Method is a recent advancement that builds on the Scleroscope.
There are a variety of electronic instruments on the market that measure the loss of energy of the
impact body. These instruments typically use a spring to accelerate a spherical, tungsten carbide
tipped mass towards the surface of the test object. When the mass contacts the surface it has a
specific kinetic energy and the impact produces an indentation (plastic deformation) on the
surface which takes some of this energy from the impact body. The impact body will lose more
energy and it rebound velocity will be less when a larger indentation is produced on softer
material. The velocities of the impact body before and after impact are measured and the loss of
velocity is related to Brinell, Rockwell, or other common hardness value.
Durometer Hardness Test
A Durometer is an instrument that is commonly used for measuring the indentation hardness of
rubbers/elastomers and soft plastics such as polyolefin, fluoropolymer, and vinyl. A Durometer
simply uses a calibrated spring to apply a specific pressure to an indenter foot. The indenter foot
can be either cone or sphere shaped. An indicating device measures the depth of indentation.
Durometers are available in a variety of models and the most popular testers are the Model A
used for measuring softer materials and the Model D for harder materials.
Barcol Hardness Test
The Barcol hardness test obtains a hardness value by measuring the penetration of a sharp steel
point under a spring load. The specimen is placed under the indenter of the Barcol hardness
tester and a uniform pressure is applied until the dial indication reaches a maximum. The Barcol
hardness test method is used to determine the hardness of both reinforced and non-reinforced
rigid plastics and to determine the degree of cure of resins and plastics.
Creep and Stress Rupture Properties
Creep Properties
Creep is a time-dependent deformation of a
material while under an applied load that is
below its yield strength. It is most often occurs
at elevated temperature, but some materials
creep at room temperature. Creep terminates in
rupture if steps are not taken to bring to a halt.
Creep data for general design use are usually
obtained under conditions of constant uniaxial
loading and constant temperature. Results of
tests are usually plotted as strain versus time up
to rupture. As indicated in the image, creep
often takes place in three stages. In the initial
stage, strain occurs at a relatively rapid rate but the rate gradually decreases until it becomes
approximately constant during the second stage. This constant creep rate is called the minimum
creep rate or steady-state creep rate since it is the slowest creep rate during the test. In the third
stage, the strain rate increases until failure occurs.
Creep in service is usually affected by changing conditions of loading and temperature and the
number of possible stress-temperature-time combinations is infinite. While most materials are
subject to creep, the creep mechanisms is often different between metals, plastics, rubber,
concrete.
Stress Rupture Properties
Stress rupture testing is similar to creep testing except that the stresses are higher than those used
in a creep testing. Stress rupture tests are used to determine the time necessary to produce failure
so stress rupture testing is always done until failure. Data is plotted log-log as in the chart
above. A straight line or best fit curve is usually obtained at each temperature of interest. This
information can then be used to extrapolate time to failure for longer times. A typical set of stress
rupture curves is shown below.
Toughness
The ability of a metal to deform plastically and to absorb energy in the process before fracture is
termed toughness. The emphasis of this definition should be placed on the ability to absorb
energy before fracture. Recall that ductility is a measure of how much something deforms
plastically before fracture, but just because a material is ductile does not make it tough. The key
to toughness is a good combination of strength and ductility. A material with high strength and
high ductility will have more toughness than a material with low strength and high ductility.
Therefore, one way to measure toughness is by calculating the area under the stress strain curve
from a tensile test. This value is simply called “material toughness” and it has units of energy per
volume. Material toughness equates to a slow absorption of energy by the material.
There are several variables that have a profound influence on the toughness of a material. These
variables are:



Strain rate (rate of loading)
Temperature
Notch effect
A metal may possess satisfactory toughness under static loads but may fail under dynamic loads
or impact. As a rule ductility and, therefore, toughness decrease as the rate of loading increases.
Temperature is the second variable to have a major influence on its toughness. As temperature is
lowered, the ductility and toughness also decrease. The third variable is termed notch effect, has
to due with the distribution of stress. A material might display good toughness when the applied
stress is uniaxial; but when a multiaxial stress state is produced due to the presence of a notch,
the material might not withstand the simultaneous elastic and plastic deformation in the various
directions.
There are several standard types of toughness test that generate data for specific loading
conditions and/or component design approaches. Three of the toughness properties that will be
discussed in more detail are 1) impact toughness, 2) notch toughness and 3) fracture toughness.
Impact Toughness
The impact toughness (AKA Impact strength) of a material can be determined with a Charpy or
Izod test. These tests are named after their inventors and were developed in the early 1900’s
before fracture mechanics theory was available. Impact properties are not directly used in
fracture mechanics calculations, but the economical impact tests continue to be used as a quality
control method to assess notch sensitivity and for comparing the relative toughness of
engineering materials.
The two tests use different specimens and
methods of holding the specimens, but both tests
make use of a pendulum-testing machine. For
both tests, the specimen is broken by a single
overload event due to the impact of the
pendulum. A stop pointer is used to record how
far the pendulum swings back up after fracturing
the specimen. The impact toughness of a metal is
determined by measuring the energy absorbed in
the fracture of the specimen. This is simply
obtained by noting the height at which the
pendulum is released and the height to which the
pendulum swings after it has struck the specimen
. The height of the pendulum times the weight of
the pendulum produces the potential energy and
the difference in potential energy of the pendulum
at the start and the end of the test is equal to the absorbed
energy.
Since toughness is greatly affected by temperature, a
Charpy or Izod test is often repeated numerous times with
each specimen tested at a different temperature. This
produces a graph of impact toughness for the material as a
function of temperature. An impact toughness versus
temperature graph for a steel is shown in the image. It can
be seen that at low temperatures the material is more
brittle and impact toughness is low. At high temperatures
the material is more ductile and impact toughness is
higher. The transition temperature is the boundary
between brittle and ductile behavior and this temperature
is often an extremely important consideration in the
selection of a material.
Fatigue Properties
Fatigue cracking is one of the primary damage mechanisms of structural components. Fatigue
cracking results from cyclic stresses that are below the ultimate tensile stress, or even the yield
stress of the material. The name “fatigue” is based on the concept that a material becomes “tired”
and fails at a stress level below the nominal strength of the material. The facts that the original
bulk design strengths are not exceeded and the only warning sign of an impending fracture is an
often hard to see crack, makes fatigue damage especially dangerous.
The fatigue life of a component can be expressed as the number of loading cycles required to
initiate a fatigue crack and to propagate the crack to critical size. Therefore, it can be said that
fatigue failure occurs in three stages – crack initiation; slow, stable crack growth; and rapid
fracture.
As discussed previously, dislocations play a major role in the fatigue
crack initiation phase. In the first stage, dislocations accumulate near
surface stress concentrations and form structures called persistent
slip bands (PSB) after a large number of loading cycles. PSBs are
areas that rise above (extrusion) or fall below (intrusion) the surface
of the component due to movement of material along slip planes.
This leaves tiny steps in the surface that serve as stress risers where
tiny cracks can initiate. These tiny crack (called microcracks)
nucleate along planes of high shear stress which is often 45o to the
loading direction.
In the second stage of fatigue, some of the tiny microcracks join
together and begin to propagate through the material in a direction that is perpendicular to the
maximum tensile stress. Eventually, the growth of one or a few crack of the larger cracks will
dominate over the rest of the cracks. With continued cyclic loading, the growth of the dominate
crack or cracks will continue until the remaining uncracked section of the component can no
longer support the load. At this point, the fracture toughness is exceeded and the remaining
cross-section of the material experiences rapid fracture. This rapid overload fracture is the third
stage of fatigue failure.
Factors Affecting Fatigue Life
In order for fatigue cracks to initiate, three basic factors are necessary. First, the loading pattern
must contain minimum and maximum peak values with large enough variation or fluctuation.
The peak values may be in tension or compression and may change over time but the reverse
loading cycle must be sufficiently great for fatigue crack initiation. Secondly, the peak stress
levels must be of sufficiently high value. If the peak stresses are too low, no crack initiation will
occur. Thirdly, the material must experience a sufficiently large number of cycles of the applied
stress. The number of cycles required to initiate and grow a crack is largely dependant on the
first to factors.
In addition to these three basic factors, there are a host of other variables, such as stress
concentration, corrosion, temperature, overload, metallurgical structure, and residual stresses
which can affect the propensity for fatigue. Since fatigue cracks generally initiate at a surface,
the surface condition of the component being loaded will have an effect on its fatigue life.
Surface roughness is important because it is directly related to the level and number of stress
concentrations on the surface. The higher the stress concentration the more likely a crack is to
nucleate. Smooth surfaces increase the time to nucleation. Notches, scratches, and other stress
risers decrease fatigue life. Surface residual stress will also have a significant effect on fatigue
life. Compressive residual stresses from machining, cold working, heat treating will oppose a
tensile load and thus lower the amplitude of cyclic loading
The figure shows several types of loading that could initiate a fatigue crack. The upper left figure
shows sinusoidal loading going from a tensile stress to a compressive stress. For this type of
stress cycle the maximum and minimum stresses are equal. Tensile stress is considered positive,
and compressive stress is negative. The figure in the upper right shows sinusoidal loading with
the minimum and maximum stresses both in the tensile realm. Cyclic compression loading can
also cause fatigue. The lower figure shows variable-amplitude loading, which might be
experienced by a bridge or airplane wing or any other component that experiences changing
loading patterns. In variable-amplitude loading, only those cycles exceeding some peak threshold
will contribute to fatigue cracking.
Polymer:
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Britannica Concise Encyclopedia:
polymer
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Any of a class of natural or synthetic substances composed of macromolecules that are multiples of
monomers. The monomers need not all be the same or have the same structure. Polymers may consist
of long chains of unbranched or branched monomers or may be cross-linked networks of monomers in
two or three dimensions. Their backbones may be flexible or rigid. Some natural inorganic materials
(e.g., the minerals diamond, graphite, and feldspar) and certain man-made inorganic materials (e.g.,
glass) have polymer-like structures. Many important natural materials are organic polymers, including
cellulose (from sugar monomers; polysaccharide), lignin, rubber, proteins (from amino acids), and
nucleic acids (from nucleotides). Synthetic organic polymers include many plastics, including
polyethylene, the nylons, polyurethanes, polyesters, vinyls (e.g., PVC), and synthetic rubbers. The
silicone polymers, with an inorganic backbone of silicon and oxygen atoms and organic side groups, are
among the most important mixed organic-inorganic compounds.
For more information on polymer, visit Britannica.com.
McGraw-Hill Science & Technology Encyclopedia:
Polymer
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Polymers, macromolecules, high polymers, and giant molecules are high-molecular-weight
materials composed of repeating subunits. These materials may be organic, inorganic, or
organometallic, and synthetic or natural in origin. Polymers are essential materials for almost
every industry as adhesives, building materials, paper, cloths, fibers, coatings, plastics, ceramics,
concretes, liquid crystals, photoresists, and coatings. They are also major components in soils
and plant and animal life. They are important in nutrition, engineering, biology, medicine,
computers, space exploration, health, and the environment.
Natural inorganic polymers include diamonds, graphite, sand, asbestos, agates, chert, feldspars,
mica, quartz, and talc. Natural organic polymers include polysaccharides (or polycarbohydrates)
such as starch and cellulose, nucleic acids, and proteins. Synthetic inorganic polymers include
boron nitride, concrete, many high-temperature superconductors, and a number of glasses.
Siloxanes or polysiloxanes represent synthetic organometallic polymers. See also Silicone resins.
Synthetic polymers used for structural components weigh considerably less than metals, helping
to reduce the consumption of fuel in vehicles and aircraft. They even outperform most metals
when measured on a strength-per-weight basis. Polymers have been developed which can also be
used for engineering purposes such as gears, bearings, and structural members.
Nomenclature
Many polymers have both a common name and a structure-based name specified by the
International Union of Pure and Applied Chemistry (IUPAC). Some polymers are commonly
known by their acronyms. Some companies use trade names to identify the specific polymeric
products they manufacture. For example, Fortrel® polyester is a poly(ethylene terephthalate)
(PET) fiber. Polymers are often generically named, such as rayon, polyester, and nylon. See also
Organic nomenclature; Polyamide resins; Polyester resins.
Composition
Polymer structures can be represented by similar or identical repeat units. These are derived from
smaller molecules, called monomers, which react to form the polymer. Propylene monomer and
the repeat unit it forms in polypropylene are shown below. 1
With the exception of its end groups, polypropylene is composed entirely of this repeat unit. The
number of units (n) in a polymer chain is called the degree of polymerization (DP). Other polymers, such
as proteins, can be described in terms of the approximate repeat unit where the nature of R (a
substituted atom or group of atoms) varies. See also Polyvinyl resins; Protein.
Primary structure
The sequence of repeat units within a polymer is called its primary structure. Unsymmetrical
reactants, such as substituted vinyl monomers, react almost exclusively to give a “head-to-tail”
product, in which the R substituents occur on alternate carbon atoms. A variety of head-to-head
structures are also possible.
Each R-substituted carbon atom is a chiral center (an atom in a molecule attached to four
different groups) with different geometries possible. Arrangements where the substitutes on the
chiral carbon are random are referred to as atactic structures. Arrangements where the geometry
about the chiral carbon alternates are said to be syndiotactic. Structures where the geometry
about the chiral atom has the same geometry are said to be isotactic or stereoregular.
Stereoregular polymers are produced using special stereoregulating catalyst systems. A series of
soluble catalysts have been developed that yield products with high stereoregularity and low
chain-size disparity. As expected, polymers with regular structures—that is, isotactic and
syndiotactic structures—tend to be more crystalline and stronger.
Polymers can be linear or branched with varying amounts and lengths of branching. Most
polymers contain some branching.
Copolymers are derived from two different monomers, which may be represented as A and B.
There exists a large variety of possible structures and, with each structure, specific properties.
These varieties include alternating, random, block, and graft see (illustration). See also
Copolymer.
Copolymer structures: (a) alternating, (b) random, (c) block, (d) graft.
Secondary structure
This refers to the localized shape of the polymer, which is often the consequence of hydrogen
bonding. Most flexible to semiflexible linear polymer chains tend toward two structures—helical
and pleated sheet/skirtlike. The pleated skirt arrangement is most prevalent for polar materials
where hydrogen bonding can occur. In nature, protein tissue is often of a pleated skirt
arrangement. For both polar and nonpolar polymer chains, there is a tendency toward helical
formation with the inner core having “like” secondary bonding forces. See also Hydrogen bond.
Tertiary structure
This refers to the overall shape of a polymer, such as in polypeptide folding. Globular proteins
approximate rough spheres because of a complex combination of environmental and molecular
constraints, and bonding opportunities. Many natural and synthetic polymers have
“superstructures,” such as the globular proteins and aggregates of polymer chains, forming
bundles and groupings.
Quaternary structure
This refers to the arrangement in space of two or more polymer subunits, often a grouping of
tertiary structures. For example, hemoglobin (quaternary structure) is essentially the combination
of four myoglobin (tertiary structure) units. Many crystalline synthetic polymers form
spherulites.
Synthesis
For polymerization to occur, monomers must have at least two reaction points or functional
groups. There are two main reaction routes to synthetic polymer formation—addition and
condensation. In chain-type kinetics, initiation starts a series of monomer additions that result in
the reaction mixture consisting mostly of unreacted monomer and polymer. Vinyl polymers,
derived from vinyl monomers and containing only carbon in their backbone, are formed in this
way. Examples of vinyl polymers include polystyrene, polyethylene, polybutadiene,
polypropylene (see structure), and poly(vinyl chloride). 2
The second main route is a step-wise polymerization. Polymerization occurs in a step-wise
fashion so that the average chain size within the reaction mixture may have an overall degree of
polymerization of 2, then 5, then 10, and so on, until the entire mixture contains largely polymer
with little or no monomer left. Polymers typically produced using the step-wise process are
called condensation polymers, and include polyamides, polycarbonates, polyesters, and
polyurethanes (see structures). 3
Condensation polymer chains are characterized as having a noncarbon atom in their backbone. For
polyamides the noncarbon is nitrogen (N), while for polycarbonates it is oxygen (O). Condensation
polymers are synthesized using melt (the reactants are heated causing them to melt), solution (the
reactants are dissolved), and interfacial (the reactants are dissolved in immiscible solvents) techniques.
See also Polymerization; Polyolefin resins; Polyurethane resins.
Molecular properties
These are used to help determine the structure and behavior of the polymer. The molecular
weight of a particular polymer chain is the product of the number of units times the molecular
weight of the repeating unit. Two statistical averages describe polymers, the number-average
molecular weight and the weight-average molecular weight. See also Molecular weight.
Size is the most important property of polymers allowing for storage of information (nucleic
acids and proteins). Polymeric materials remember any action that distorts or moves polymer
chains or segments (such as bending, stretching, and melting). Size also accounts for an
accumulation of the interchain and intrachain secondary attractive forces called van der Waals
forces. For nonpolar polymers, such as polyethylene, the attractive forces for each repeating unit
are less than that for polar polymers. Polyvinyl chloride, a polar polymer, has attractive forces
that include both dispersion and dipole-dipole forces so that the total attractive forces are
proportionally larger than those for polyethylene. Polymers with hydrogen bonding (such as
proteins, polysaccharides, nucleic acids, and nylons) have attractive forces that are even greater.
Hydrogen bonding is so strong in cellulose that cellulose is not soluble in water until the interand intrachain hydrogen bonds are broken.
Polymers often have a combination of ordered regions, called crystalline regions, and disordered
or amorphous regions. Crystalline regions are more rigid, contributing to strength and resistance
to external forces. The amorphous regions contribute to polymers' flexibility. Most commercial
polymers have a balance between amorphous and crystalline regions, allowing a balance
between flexibility and strength.
Polymers are viscoelastic materials. Ductile polymers, such as polyethylene and polypropylene,
“give” or “yield,” and at high elongations some strengthening and orientation occur. A brittle
polymer, such as polystryene, does not give much and breaks at a low elongation. A fiber, a
polymer material that is much longer than it is wide, exhibits high strength, high stiffness, and
little elongation.
Materials
Fibers are polymer materials that are strong in one direction, and they are much longer (>100
times) than they are wide. Elastomers (or rubbers) are polymeric materials that can be distorted
through the application of force, and when the force is removed, the material returns to its
original shape. Plastics are materials that have properties between fibers and elastomers—they
are hard and flexible. Coatings and adhesives are generally derived from polymers that are
members of other groupings (for example, polysiloxanes are elastomers, but also are used as
adhesives). Industrially important adhesives and coatings include laminates, sealants and caulks,
composites, films, polyblends, liquid crystals, ceramics, cements, and smart materials. See also
Adhesive; Liquid crystals; Polymeric composite; Rubber.
Additives
Processed polymeric materials are generally a combination of the polymer and the materials that
are added to modify its properties, assist in processing, and introduce new properties. Additives
can be solids, liquids, or gases. Typical additives are plasticizers, antioxidants, colorants, fillers,
and reinforcements. See also Antioxidant; Inhibitor (chemistry).
Recycling
Many polymers are thermoplastics, that is, they can be reshaped through application of heat and
pressure and used in the production of other thermoplastic materials. The recycling of
thermosets, polymers that do not melt but degrade prior to softening, is more difficult. These
materials are often ground into a fine powder, are blended with additives (often adhesives or
binders), and then are reformed.
Computer Desktop Encyclopedia:
polymer
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Meaning "many parts," it is a material constructed of smaller molecules of the same substance
that form larger molecules. For example, plastic is a synthetic polymer, while protein is a natural
polymer. See polymer semiconductor.
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McGraw-Hill Dictionary of Architecture & Construction:
polymer
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One of a group of high-molecular-weight resin-like, organic compounds whose structures usually
can be represented by repeated small units. Some polymers are elastomers, some are plastics, and
some are fibers.
Oxford Dictionary of Sports Science & Medicine:
polymer
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A large molecule formed by the linkage between a large number of smaller molecules. For
example, proteins are polymers made from amino acid molecules, and glycogen is a polymer
made from glucose molecules.
Columbia Encyclopedia:
polymer
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polymer (pŏl'əmər), chemical compound with high molecular weight consisting of a number of
structural units linked together by covalent bonds (see chemical bond). The simple molecules that may
become structural units are themselves called monomers; two monomers combine to form a dimer, and
three monomers, a trimer. A structural unit is a group having two or more bonding sites. A bonding site
may be created by the loss of an atom or group, such as H or OH, or by the breaking up of a double or
triple bond, as when ethylene, H2C-CH2, is converted into a structural unit for polyethylene, -H2C-CH2-. In
a linear polymer, the structural units are connected in a chain arrangement and thus need only be
bifunctional, i.e., have two bonding sites. When the structural unit is trifunctional (has three bonding
sites), a nonlinear, or branched, polymer results. Ethylene, styrene, and ethylene glycol are examples of
bifunctional monomers, while glycerin and divinyl benzene are both polyfunctional. Polymers containing
a single repeating unit, such as polyethylene, are called homopolymers. Polymers containing two or
more different structural units, such as phenol-formaldehyde, are called copolymers. All polymers can
be classified as either addition polymers or condensation polymers. An addition polymer is one in which
the molecular formula of the repeating structural unit is identical to that of the monomer, e.g.,
polyethylene and polystyrene. A condensation polymer is one in which the repeating structural unit
contains fewer atoms than that of the monomer or monomers because of the splitting off of water or
some other substance, e.g., polyesters and polycarbonates (see illustration). Many polymers occur in
nature, such as silk, cellulose, natural rubber, and proteins. In addition, a large number of polymers have
been synthesized in the laboratory, leading to such commercially important products as plastics,
synthetic fibers, and synthetic rubber. Polymerization, the chemical process of forming polymers from
their component monomers, is often a complex process that may be initiated or sustained by heat,
pressure, or the presence of one or more catalysts.
Dictionary of Cultural Literacy: Science:
polymer
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(POL-uh-muhr)
In chemistry, a long molecule made up of a chain of smaller, simpler molecules.
 Proteins and many carbohydrates, such as cellulose, are polymers. Plastics are also polymers.
Oxford Dictionary of Biochemistry:
polymer
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1. (in chemistry and biochemistry) any substance that is composed of molecules containing a large
number of constitutional units (or 'mers') that are in repetitive covalent linkage and that may be
of one or more than one species. Polymers are generally considered to comprise at least ten
mers, although sometimes the term is taken to imply simply 'more than one' mer. Hence
'polymer' may or may not embrace oligomer, depending on the branch of chemistry or
biochemistry concerned.
2. (in molecular biology and enzymology) (sometimes) an alternative term for multimer. See also
heteropolymer, homopolymer.
—polymeric adj.; polymerize or polymerise vb.; polymerization or polymerisation n.
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Saunders Veterinary Dictionary:
polymer
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A compound, usually of high molecular weight, formed by combination of simpler molecules
(monomers).

p.-fume fever — see polytetrafluoroethylene.
Mosby's Dental Dictionary:
polymer
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(pol′e-mur)
n
A longchain hydrocarbon. In dentistry, the polymer is supplied as a powder to be mixed with the
monomer for fabrication of appliances and restorations.
Random House Word Menu:
categories related to 'polymer'
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For a list of words related to polymer, see:

Substances, Particles, and Atomic Architecture - polymer: large molecule made up of smaller
monomers repeated many times

Plastics, Paper, and Textiles - polymer: natural or synthetic substance formed by chaining
together many simple molecules to form giant molecules with different physical properties
Rhymes:
polymer
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See words rhyming with "polymer."
Bradford's Crossword Solver's Dictionary:
polymer
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See crossword solutions for the clue Polymer.
Wikipedia on Answers.com:
Polymer
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Appearance of real linear polymer chains as recorded using an atomic force microscope on surface
under liquid medium. Chain contour length for this polymer is ~204 nm; thickness is ~0.4 nm.[1]
A polymer is a large molecule (macromolecule) composed of repeating structural units. These
sub-units are typically connected by covalent chemical bonds. Although the term polymer is
sometimes taken to refer to plastics, it actually encompasses a large class of compounds
comprising both natural and synthetic materials with a wide variety of properties.
Because of the extraordinary range of properties of polymeric materials,[2] they play an essential
and ubiquitous role in everyday life.[3] This role ranges from familiar synthetic plastics and
elastomers to natural biopolymers such as nucleic acids and proteins that are essential for life.
Natural polymeric materials such as shellac, amber, and natural rubber have been used for
centuries. A variety of other natural polymers exist, such as cellulose, which is the main
constituent of wood and paper. The list of synthetic polymers includes synthetic rubber, Bakelite,
neoprene, nylon, PVC, polystyrene, polyethylene, polypropylene, polyacrylonitrile, PVB,
silicone, and many more.
Most commonly, the continuously linked backbone of a polymer used for the preparation of
plastics consists mainly of carbon atoms. A simple example is polyethylene ('polythene' in
British English), whose repeating unit is based on ethylene monomer. However, other structures
do exist; for example, elements such as silicon form familiar materials such as silicones,
examples being Silly Putty and waterproof plumbing sealant. Oxygen is also commonly present
in polymer backbones, such as those of polyethylene glycol, polysaccharides (in glycosidic
bonds), and DNA (in phosphodiester bonds).
Polymers are studied in the fields of polymer chemistry, polymer physics, and polymer science.
Contents
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1 Etymology
2 Polymer synthesis
o 2.1 Laboratory synthesis
o 2.2 Biological synthesis
o 2.3 Modification of natural polymers
3 Polymer properties
o 3.1 Monomers and repeat units
o 3.2 Microstructure
 3.2.1 Polymer architecture
 3.2.2 Chain length
 3.2.3 Monomer arrangement in copolymers
 3.2.4 Tacticity
o 3.3 Polymer morphology
 3.3.1 Crystallinity
 3.3.2 Chain conformation
o 3.4 Mechanical properties
 3.4.1 Tensile strength
 3.4.2 Young's modulus of elasticity
o 3.5 Transport properties
o
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3.6 Phase behavior
 3.6.1 Melting point
 3.6.2 Glass transition temperature
 3.6.3 Mixing behavior
 3.6.4 Inclusion of plasticizers
o 3.7 Chemical properties
4 Standardized polymer nomenclature
5 Polymer characterization
6 Polymer degradation
o 6.1 Product failure
7 See also
8 References
9 Bibliography
10 External links
Etymology
The word polymer is derived from the Greek words πολύ- - poly- meaning "many"; and μέρος meros meaning "part". The term was coined in 1833 by Jöns Jacob Berzelius, although his
definition of a polymer was quite different from the modern definition.
Polymer synthesis
Main article: Polymerization
The repeating unit of the polymer polypropylene
Polymerization is the process of combining many small molecules known as monomers into a
covalently bonded chain or network. During the polymerization process, some chemical groups
may be lost from each monomer. This is the case, for example, in the polymerization of PET
polyester. The monomers are terephthalic acid (HOOC-C6H4-COOH) and ethylene glycol (HOCH2-CH2-OH) but the repeating unit is -OC-C6H4-COO-CH2-CH2-O-, which corresponds to the
combination of the two monomers with the loss of two water molecules. The distinct piece of
each monomer that is incorporated into the polymer is known as a repeat unit or monomer
residue.
Laboratory synthesis
Laboratory synthetic methods are generally divided into two categories, step-growth
polymerization and chain-growth polymerization.[4] The essential difference between the two is
that in chain growth polymerization, monomers are added to the chain one at a time only,[5]
whereas in step-growth polymerization chains of monomers may combine with one another
directly.[6] However, some newer methods such as plasma polymerization do not fit neatly into
either category. Synthetic polymerization reactions may be carried out with or without a catalyst.
Laboratory synthesis of biopolymers, especially of proteins, is an area of intensive research.
Biological synthesis
Microstructure of part of a DNA double helix biopolymer
Main article: Biopolymer
There are three main classes of biopolymers: polysaccharides, polypeptides, and polynucleotides.
In living cells, they may be synthesized by enzyme-mediated processes, such as the formation of
DNA catalyzed by DNA polymerase. The synthesis of proteins involves multiple enzymemediated processes to transcribe genetic information from the DNA to RNA and subsequently
translate that information to synthesize the specified protein from amino acids. The protein may
be modified further following translation in order to provide appropriate structure and
functioning.
Modification of natural polymers
Many commercially important polymers are synthesized by chemical modification of naturally
occurring polymers. Prominent examples include the reaction of nitric acid and cellulose to form
nitrocellulose and the formation of vulcanized rubber by heating natural rubber in the presence of
sulfur. Ways in which polymers can be modified include oxidation, cross-linking and endcapping.
Polymer properties
Polymer properties are broadly divided into several classes based on the scale at which the
property is defined as well as upon its physical basis.[7] The most basic property of a polymer is
the identity of its constituent monomers. A second set of properties, known as microstructure,
essentially describe the arrangement of these monomers within the polymer at the scale of a
single chain. These basic structural properties play a major role in determining bulk physical
properties of the polymer, which describe how the polymer behaves as a continuous macroscopic
material. Chemical properties, at the nano-scale, describe how the chains interact through various
physical forces. At the macro-scale, they describe how the bulk polymer interacts with other
chemicals and solvents.
Monomers and repeat units
The identity of the monomer residues (repeat units) comprising a polymer is its first and most
important attribute. Polymer nomenclature is generally based upon the type of monomer residues
comprising the polymer. Polymers that contain only a single type of repeat unit are known as
homopolymers, while polymers containing a mixture of repeat units are known as copolymers.
Poly(styrene), for example, is composed only of styrene monomer residues, and is therefore
classified as a homopolymer. Ethylene-vinyl acetate, on the other hand, contains more than one
variety of repeat unit and is thus a copolymer. Some biological polymers are composed of a
variety of different but structurally related monomer residues; for example, polynucleotides such
as DNA are composed of a variety of nucleotide subunits.
A polymer molecule containing ionizable subunits is known as a polyelectrolyte or ionomer.
Microstructure
The microstructure of a polymer (sometimes called configuration) relates to the physical
arrangement of monomer residues along the backbone of the chain.[8] These are the elements of
polymer structure that require the breaking of a covalent bond in order to change. Structure has a
strong influence on the other properties of a polymer. For example, two samples of natural
rubber may exhibit different durability, even though their molecules comprise the same
monomers.
Polymer architecture
Main article: Polymer architecture
Branch point in a polymer
An important microstructural feature of a polymer is its architecture, which relates to the way
branch points lead to a deviation from a simple linear chain.[9] A branched polymer molecule is
composed of a main chain with one or more substituent side chains or branches. Types of
branched polymers include star polymers, comb polymers, brush polymers, dendronized
polymers, ladders, and dendrimers.[9]
A polymer's architecture affects many of its physical properties including, but not limited to,
solution viscosity, melt viscosity, solubility in various solvents, glass transition temperature and
the size of individual polymer coils in solution.
A variety of techniques may be employed for the synthesis of a polymeric material with a range
of architectures, for example Living polymerization.
Various polymer architectures.
Chain length
The physical properties[10] of a polymer are strongly dependent on the size or length of the
polymer chain.[11] For example, as chain length is increased, melting and boiling temperatures
increase quickly.[11] Impact resistance also tends to increase with chain length, as does the
viscosity, or resistance to flow, of the polymer in its melt state.[12] Chain length is related to melt
viscosity roughly as 1:103.2, so that a tenfold increase in polymer chain length results in a
viscosity increase of over 1000 times[citation needed]. Increasing chain length furthermore tends to
decrease chain mobility, increase strength and toughness, and increase the glass transition
temperature (Tg)[citation needed]. This is a result of the increase in chain interactions such as Van der
Waals attractions and entanglements that come with increased chain length[citation needed]. These
interactions tend to fix the individual chains more strongly in position and resist deformations
and matrix breakup, both at higher stresses and higher temperatures[citation needed].
A common means of expressing the length of a chain is the degree of polymerization, which
quantifies the number of monomers incorporated into the chain.[13][14] As with other molecules, a
polymer's size may also be expressed in terms of molecular weight. Since synthetic
polymerization techniques typically yield a polymer product including a range of molecular
weights, the weight is often expressed statistically to describe the distribution of chain lengths
present in the same. Common examples are the number average molecular weight and weight
average molecular weight.[15][16] The ratio of these two values is the polydispersity index,
commonly used to express the "width" of the molecular weight distribution.[17] A final
measurement is contour length, which can be understood as the length of the chain backbone in
its fully extended state.[18]
The flexibility of an unbranched chain polymer is characterized by its persistence length.
Monomer arrangement in copolymers
Main article: copolymer
Monomers within a copolymer may be organized along the backbone in a variety of ways.
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Alternating copolymers possess regularly alternating monomer residues:[19] [AB...]n (2).
Periodic copolymers have monomer residue types arranged in a repeating sequence: [AnBm...] m
being different from n .
Statistical copolymers have monomer residues arranged according to a known statistical rule. A
statistical copolymer in which the probability of finding a particular type of monomer residue at
a particular point in the chain is independent of the types of surrounding monomer residue may
be referred to as a truly random copolymer[20][21] (3).
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Block copolymers have two or more homopolymer subunits linked by covalent bonds[19] (4).
Polymers with two or three blocks of two distinct chemical species (e.g., A and B) are called
diblock copolymers and triblock copolymers, respectively. Polymers with three blocks, each of a
different chemical species (e.g., A, B, and C) are termed triblock terpolymers.
Graft or grafted copolymers contain side chains that have a different composition or
configuration than the main chain.(5)
Tacticity
Main article: Tacticity
Tacticity describes the relative stereochemistry of chiral centers in neighboring structural units
within a macromolecule. There are three types: isotactic (all substituents on the same side),
atactic (random placement of substituents), and syndiotactic (alternating placement of
substituents).
Polymer morphology
Polymer morphology generally describes the arrangement and microscale ordering of polymer
chains in space.
Crystallinity
When applied to polymers, the term crystalline has a somewhat ambiguous usage. In some cases,
the term crystalline finds identical usage to that used in conventional crystallography. For
example, the structure of a crystalline protein or polynucleotide, such as a sample prepared for xray crystallography, may be defined in terms of a conventional unit cell composed of one or
more polymer molecules with cell dimensions of hundreds of angstroms or more.
A synthetic polymer may be loosely described as crystalline if it contains regions of threedimensional ordering on atomic (rather than macromolecular) length scales, usually arising from
intramolecular folding and/or stacking of adjacent chains. Synthetic polymers may consist of
both crystalline and amorphous regions; the degree of crystallinity may be expressed in terms of
a weight fraction or volume fraction of crystalline material. Few synthetic polymers are entirely
crystalline.[22]
The crystallinity of polymers is characterized by their degree of crystallinity, ranging from zero
for a completely non-crystalline polymer to one for a theoretical completely crystalline polymer.
Polymers with microcrystalline regions are generally tougher (can be bent more without
breaking) and more impact-resistant than totally amorphous polymers.[23]
Polymers with a degree of crystallinity approaching zero or one will tend to be transparent, while
polymers with intermediate degrees of crystallinity will tend to be opaque due to light scattering
by crystalline or glassy regions. Thus for many polymers, reduced crystallinity may also be
associated with increased transparency.
Chain conformation
The space occupied by a polymer molecule is generally expressed in terms of radius of gyration,
which is an average distance from the center of mass of the chain to the chain itself.
Alternatively, it may be expressed in terms of pervaded volume, which is the volume of solution
spanned by the polymer chain and scales with the cube of the radius of gyration.[24]
Mechanical properties
A polyethylene sample necking under tension.
The bulk properties of a polymer are those most often of end-use interest. These are the
properties that dictate how the polymer actually behaves on a macroscopic scale.
Tensile strength
The tensile strength of a material quantifies how much stress the material will endure before
suffering permanent deformation.[25][26] This is very important in applications that rely upon a
polymer's physical strength or durability. For example, a rubber band with a higher tensile
strength will hold a greater weight before snapping. In general, tensile strength increases with
polymer chain length and crosslinking of polymer chains.
Young's modulus of elasticity
Young's Modulus quantifies the elasticity of the polymer. It is defined, for small strains, as the
ratio of rate of change of stress to strain. Like tensile strength, this is highly relevant in polymer
applications involving the physical properties of polymers, such as rubber bands. The modulus is
strongly dependent on temperature. Viscoelasticity describes a complex time-dependent elastic
response, which will exhibit hysteresis in the stress-strain curve when the load is removed.
Dynamic mechanical analysis or DMA measures this complex modulus by oscillating the load
and measuring the resulting strain as a function of time.
Transport properties
Transport properties such as diffusivity relate to how rapidly molecules move through the
polymer matrix. These are very important in many applications of polymers for films and
membranes.
Phase behavior
Melting point
The term melting point, when applied to polymers, suggests not a solid-liquid phase transition
but a transition from a crystalline or semi-crystalline phase to a solid amorphous phase. Though
abbreviated as simply Tm, the property in question is more properly called the crystalline melting
temperature. Among synthetic polymers, crystalline melting is only discussed with regards to
thermoplastics, as thermosetting polymers will decompose at high temperatures rather than melt.
Glass transition temperature
A parameter of particular interest in synthetic polymer manufacturing is the glass transition
temperature (Tg), which describes the temperature at which amorphous polymers undergo a
transition from a rubbery, viscous amorphous liquid, to a brittle, glassy amorphous solid. The
glass transition temperature may be engineered by altering the degree of branching or
crosslinking in the polymer or by the addition of plasticizer.[27]
Mixing behavior
Phase diagram of the typical mixing behavior of weakly interacting polymer solutions.
In general, polymeric mixtures are far less miscible than mixtures of small molecule materials.
This effect results from the fact that the driving force for mixing is usually entropy, not
interaction energy. In other words, miscible materials usually form a solution not because their
interaction with each other is more favorable than their self-interaction, but because of an
increase in entropy and hence free energy associated with increasing the amount of volume
available to each component. This increase in entropy scales with the number of particles (or
moles) being mixed. Since polymeric molecules are much larger and hence generally have much
higher specific volumes than small molecules, the number of molecules involved in a polymeric
mixture is far smaller than the number in a small molecule mixture of equal volume. The
energetics of mixing, on the other hand, is comparable on a per volume basis for polymeric and
small molecule mixtures. This tends to increase the free energy of mixing for polymer solutions
and thus make solvation less favorable. Thus, concentrated solutions of polymers are far rarer
than those of small molecules.
Furthermore, the phase behavior of polymer solutions and mixtures is more complex than that of
small molecule mixtures. Whereas most small molecule solutions exhibit only an upper critical
solution temperature phase transition, at which phase separation occurs with cooling, polymer
mixtures commonly exhibit a lower critical solution temperature phase transition, at which phase
separation occurs with heating.
In dilute solution, the properties of the polymer are characterized by the interaction between the
solvent and the polymer. In a good solvent, the polymer appears swollen and occupies a large
volume. In this scenario, intermolecular forces between the solvent and monomer subunits
dominate over intramolecular interactions. In a bad solvent or poor solvent, intramolecular forces
dominate and the chain contracts. In the theta solvent, or the state of the polymer solution where
the value of the second virial coefficient becomes 0, the intermolecular polymer-solvent
repulsion balances exactly the intramolecular monomer-monomer attraction. Under the theta
condition (also called the Flory condition), the polymer behaves like an ideal random coil. The
transition between the states is known as a coil-globule transition.
Inclusion of plasticizers
Inclusion of plasticizers tends to lower Tg and increase polymer flexibility. Plasticizers are
generally small molecules that are chemically similar to the polymer and create gaps between
polymer chains for greater mobility and reduced interchain interactions. A good example of the
action of plasticizers is related to polyvinylchlorides or PVCs. A uPVC, or unplasticized
polyvinylchloride, is used for things such as pipes. A pipe has no plasticizers in it, because it
needs to remain strong and heat-resistant. Plasticized PVC is used for clothing for a flexible
quality. Plasticizers are also put in some types of cling film to make the polymer more flexible.
Chemical properties
The attractive forces between polymer chains play a large part in determining a polymer's
properties. Because polymer chains are so long, these interchain forces are amplified far beyond
the attractions between conventional molecules. Different side groups on the polymer can lend
the polymer to ionic bonding or hydrogen bonding between its own chains. These stronger forces
typically result in higher tensile strength and higher crystalline melting points.
The intermolecular forces in polymers can be affected by dipoles in the monomer units.
Polymers containing amide or carbonyl groups can form hydrogen bonds between adjacent
chains; the partially positively charged hydrogen atoms in N-H groups of one chain are strongly
attracted to the partially negatively charged oxygen atoms in C=O groups on another. These
strong hydrogen bonds, for example, result in the high tensile strength and melting point of
polymers containing urethane or urea linkages. Polyesters have dipole-dipole bonding between
the oxygen atoms in C=O groups and the hydrogen atoms in H-C groups. Dipole bonding is not
as strong as hydrogen bonding, so a polyester's melting point and strength are lower than
Kevlar's (Twaron), but polyesters have greater flexibility.
Ethene, however, has no permanent dipole. The attractive forces between polyethylene chains
arise from weak van der Waals forces. Molecules can be thought of as being surrounded by a
cloud of negative electrons. As two polymer chains approach, their electron clouds repel one
another. This has the effect of lowering the electron density on one side of a polymer chain,
creating a slight positive dipole on this side. This charge is enough to attract the second polymer
chain. Van der Waals forces are quite weak, however, so polyethylene can have a lower melting
temperature compared to other polymers.
Standardized polymer nomenclature
There are multiple conventions for naming polymer substances. Many commonly used polymers,
such as those found in consumer products, are referred to by a common or trivial name. The
trivial name is assigned based on historical precedent or popular usage rather than a standardized
naming convention. Both the American Chemical Society (ACS)[28] and IUPAC[29] have
proposed standardized naming conventions; the ACS and IUPAC conventions are similar but not
identical.[30] Examples of the differences between the various naming conventions are given in
the table below:
Common name
ACS name
IUPAC name
Poly(ethylene oxide) or
PEO
Poly(oxyethylene)
Poly(oxyethene)
Poly(ethylene
terephthalate) or PET
Poly(oxy-1,2-ethanediyloxycarbonyl-1,4phenylenecarbonyl)
Poly(oxyetheneoxyterephthaloyl)
Nylon 6
Poly[amino(1-oxo-1,6-hexanediyl)]
Poly[amino(1-oxohexan-1,6diyl)]
In both standardized conventions, the polymers' names are intended to reflect the monomer(s)
from which they are synthesized rather than the precise nature of the repeating subunit. For
example, the polymer synthesized from the simple alkene ethene is called polyethylene, retaining
the -ene suffix even though the double bond is removed during the polymerization process:
Polymer characterization
Main article: Polymer characterization
The characterization of a polymer requires several parameters which need to be specified. This is
because a polymer actually consists of a statistical distribution of chains of varying lengths, and
each chain consists of monomer residues which affect its properties.
A variety of lab techniques are used to determine the properties of polymers. Techniques such as
wide angle X-ray scattering, small angle X-ray scattering, and small angle neutron scattering are
used to determine the crystalline structure of polymers. Gel permeation chromatography is used
to determine the number average molecular weight, weight average molecular weight, and
polydispersity. FTIR, Raman and NMR can be used to determine composition. Thermal
properties such as the glass transition temperature and melting point can be determined by
differential scanning calorimetry and dynamic mechanical analysis. Pyrolysis followed by
analysis of the fragments is one more technique for determining the possible structure of the
polymer. Thermogravimetry is a useful technique to evaluate the thermal stability of the
polymer. Detailed analyses of TG curves also allow us to know a bit of the phase segregation in
polymers. Rheological properties are also commonly used to help determine molecular
architecture (molecular weight, molecular weight distribution and branching)as well as to
understand how the polymer will process, through measurements of the polymer in the melt
phase. Another polymer characterization technique is Automatic Continuous Online Monitoring
of Polymerization Reactions (ACOMP) which provides real-time characterization of
polymerization reactions. It can be used as an analytical method in R&D, as a tool for reaction
optimization at the bench and pilot plant level and, eventually, for feedback control of full-scale
reactors. ACOMP measures in a model-independent fashion the evolution of average molar mass
and intrinsic viscosity, monomer conversion kinetics and, in the case of copolymers, also the
average composition drift and distribution. It is applicable in the areas of free radical and
controlled radical homo- and copolymerization, polyelectrolyte synthesis, heterogeneous phase
reactions, including emulsion polymerization, adaptation to batch and continuous reactors, and
modifications of polymers.[31][32][33]
Polymer degradation
Main article: Polymer degradation
A plastic item with thirty years of exposure to heat and cold, brake fluid, and sunlight. Notice the
discoloration, swollen dimensions, and crazing of the material
Polymer degradation is a change in the properties—tensile strength, color, shape, or molecular
weight—of a polymer or polymer-based product under the influence of one or more
environmental factors, such as heat, light, chemicals and, in some cases, galvanic action. It is
often due to the scission of polymer chain bonds via hydrolysis, leading to a decrease in the
molecular mass of the polymer.
Although such changes are frequently undesirable, in some cases, such as biodegradation and
recycling, they may be intended to prevent environmental pollution. Degradation can also be
useful in biomedical settings. For example, a copolymer of polylactic acid and polyglycolic acid
is employed in hydrolysable stitches that slowly degrade after they are applied to a wound.
The susceptibility of a polymer to degradation depends on its structure. Epoxies and chains
containing aromatic functionalities are especially susceptible to UV degradation while polyesters
are susceptible to degradation by hydrolysis, while polymers containing an unsaturated backbone
are especially susceptible to ozone cracking. Carbon based polymers are more susceptible to
thermal degradation than inorganic polymers such as polydimethylsiloxane and are therefore not
ideal for most high-temperature applications. High-temperature matrices such as bismaleimides
(BMI), condensation polyimides (with an O-C-N bond), triazines (with a nitrogen (N) containing
ring), and blends thereof are susceptible to polymer degradation in the form of galvanic
corrosion when bare carbon fiber reinforced polymer CFRP is in contact with an active metal
such as aluminum in salt water environments.
The degradation of polymers to form smaller molecules may proceed by random scission or
specific scission. The degradation of polyethylene occurs by random scission—a random
breakage of the bonds that hold the atoms of the polymer together. When heated above 450 °C,
polyethylene degrades to form a mixture of hydrocarbons. Other polymers, such as poly(alpha-
methylstyrene), undergo specific chain scission with breakage occurring only at the ends. They
literally unzip or depolymerize back to the constituent monomer.
The sorting of polymer waste for recycling purposes may be facilitated by the use of the Resin
identification codes developed by the Society of the Plastics Industry to identify the type of
plastic.
Product failure
Chlorine attack of acetal resin plumbing joint
In a finished product, such a change is to be prevented or delayed. Failure of safety-critical
polymer components can cause serious accidents, such as fire in the case of cracked and
degraded polymer fuel lines. Chlorine-induced cracking of acetal resin plumbing joints and
polybutylene pipes has caused many serious floods in domestic properties, especially in the USA
in the 1990s. Traces of chlorine in the water supply attacked vulnerable polymers in the plastic
plumbing, a problem which occurs faster if any of the parts have been poorly extruded or
injection molded. Attack of the acetal joint occurred because of faulty molding, leading to
cracking along the threads of the fitting which is a serious stress concentration.
Ozone-induced cracking in natural rubber tubing
Polymer oxidation has caused accidents involving medical devices. One of the oldest known
failure modes is ozone cracking caused by chain scission when ozone gas attacks susceptible
elastomers, such as natural rubber and nitrile rubber. They possess double bonds in their repeat
units which are cleaved during ozonolysis. Cracks in fuel lines can penetrate the bore of the tube
and cause fuel leakage. If cracking occurs in the engine compartment, electric sparks can ignite
the gasoline and can cause a serious fire.
Fuel lines can also be attacked by another form of degradation: hydrolysis. Nylon 6,6 is
susceptible to acid hydrolysis, and in one accident, a fractured fuel line led to a spillage of diesel
into the road. If diesel fuel leaks onto the road, accidents to following cars can be caused by the
slippery nature of the deposit, which is like black i
Read more: http://www.answers.com/topic/polymer#ixzz1sqagH9Sd
Elastomer
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This article does not cite any references or sources. Please help improve this article by adding
citations to reliable sources. Unsourced material may be challenged and removed. (September
2008)
An elastomer is a polymer with the property of viscoelasticity (colloquially "elasticity"),
generally having notably low Young's modulus and high yield strain compared with other
materials. The term, which is derived from elastic polymer, is often used interchangeably with
the term rubber, although the latter is preferred when referring to vulcanisates. Each of the
monomers which link to form the polymer is usually made of carbon, hydrogen, oxygen and/or
silicon. Elastomers are amorphous polymers existing above their glass transition temperature, so
that considerable segmental motion is possible. At ambient temperatures, rubbers are thus
relatively soft (E~3MPa) and deformable. Their primary uses are for seals, adhesives and molded
flexible parts.
[edit] Background
(A) is an unstressed polymer; (B) is the same polymer under stress. When the stress is removed, it will
return to the A configuration. (The dots represent cross-links)
Elastomers are usually thermosets (requiring vulcanization) but may also be thermoplastic (see
thermoplastic elastomer). The long polymer chains cross-link during curing, i.e., vulcanizing.
The molecular structure of elastomers can be imagined as a 'spaghetti and meatball' structure,
with the meatballs signifying cross-links. The elasticity is derived from the ability of the long
chains to reconfigure themselves to distribute an applied stress. The covalent cross-linkages
ensure that the elastomer will return to its original configuration when the stress is removed. As a
result of this extreme flexibility, elastomers can reversibly extend from 5-700%, depending on
the specific material. Without the cross-linkages or with short, uneasily reconfigured chains, the
applied stress would result in a permanent deformation.
Temperature effects are also present in the demonstrated elasticity of a polymer. Elastomers that
have cooled to a glassy or crystalline phase will have less mobile chains, and consequentially
less elasticity, than those manipulated at temperatures higher than the glass transition
temperature of the polymer.
It is also possible for a polymer to exhibit elasticity that is not due to covalent cross-links, but
instead for thermodynamic reasons.
[edit] Examples of elastomers
Unsaturated rubbers that can be cured by sulfur vulcanization:
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Natural polyisoprene: cis-1,4-polyisoprene natural rubber (NR) and trans-1,4-polyisoprene
gutta-percha
Synthetic polyisoprene (IR for Isoprene Rubber)
Polybutadiene (BR for Butadiene Rubber)
Chloroprene rubber (CR), polychloroprene, Neoprene, Baypren etc.
Butyl rubber (copolymer of isobutylene and isoprene, IIR)
o Halogenated butyl rubbers (chloro butyl rubber: CIIR; bromo butyl rubber: BIIR)
Styrene-butadiene Rubber (copolymer of styrene and butadiene, SBR)
Nitrile rubber (copolymer of butadiene and acrylonitrile, NBR), also called Buna N rubbers
o Hydrogenated Nitrile Rubbers (HNBR) Therban and Zetpol
(Unsaturated rubbers can also be cured by non-sulfur vulcanization if desired).
Saturated rubbers that cannot be cured by sulfur vulcanization:
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EPM (ethylene propylene rubber, a copolymer of ethylene and propylene) and EPDM rubber
(ethylene propylene diene rubber, a terpolymer of ethylene, propylene and a dienecomponent)
Epichlorohydrin rubber (ECO)
Polyacrylic rubber (ACM, ABR)
Silicone rubber (SI, Q, VMQ)
Fluorosilicone Rubber (FVMQ)
Fluoroelastomers (FKM, and FEPM) Viton, Tecnoflon, Fluorel, Aflas and Dai-El
Perfluoroelastomers (FFKM) Tecnoflon PFR, Kalrez, Chemraz, Perlast
Polyether block amides (PEBA)
Chlorosulfonated polyethylene (CSM), (Hypalon)
Ethylene-vinyl acetate (EVA)
"The definitions are not authentic as the Rubber which is classified in World Customs
Organisation Books in Chapter 40, where as the above definitions stating all rubber and different
polymers in same chapter which is classified in Chapter 39 of the World Custom Organisation's
Harmonised Commodity for Description and coding system. One should go through all
differentiation while editing between Plastics and articles thereof and Rubber and articles
thereof."
Various other types of elastomers:
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Thermoplastic elastomers (TPE)
The proteins resilin and elastin
Polysulfide rubber