Properties of Engineering Materials
The practical application of engineering materials in manufacturing engineering depends upon through knowledge
of their particular properties under wide range of conditions. The term “property” is a qualitative or quantitative
measure of response of materials to externally imposed conditions like forces and temperature. However, the range
of properties found in different classes of materials is very large. The following properties of materials will be
briefly discussed:
1.
2.
3.
4.
5.
Physical Properties of Materials
Mechanical Properties of Materials
Electrical Properties of Materials
Magnetic Properties of Materials
Chemical Properties of Materials.
1. Physical Properties:
A. The Melting or Freezing Point:
I.
II.
III.
The melting or freezing point of pure metal is defined as the temperature at which the solid and liquid
phases can exist in stable equilibrium. When a metal is heated to melting point, the liquid phase appears,
and if more heat is supplied, the solid melts completely at constant temperature.
The freezing of a pure liquid on the other hand, may exhibit the phenomena of supercoiling, the liquid
in some cases can be lowered appreciably beyond the melting point without the appearance of crystals.
However, when crystals do not appear, the mass rapidly assumes the normal temperature of the melting
point.
The use of mercury in thermometers, manometers and other instruments arises from its low melting point;
the use of tungsten filaments in incandescent high bulbs is possible because of its extremely high melting
point.
B. Boiling Point:
The boiling point of a liquid is the temperature at which its vapor pressure equals to one atmosphere. The boiling
points of the metals except mercury are high. The boiling point of zinc (907°C) and cadmium (865°C) are
sufficiently low so that in recovery of these metals from their ores the metals are vaporized and condensed.
C. Density:
Mass per unit volume is termed as “density.” In metric system it is stated in kg/m3. The low densities of aluminum
and magnesium and of their alloys make them particularly valuable in aeronautic and transportation fields.
D. Linear Co-Efficient of Expansion:
The linear coefficient of expansion of a solid is defined as the increase in length, for each degree rise in
temperature. These coefficients are important when metals are to be exposed to a considerable range of
temperatures as in engine pistons, and other accurately fitting mechanisms.
E. Thermal Conductivity:
I.
II.
The thermal conductivity of a metal is defined as the number of kilojoules of heat that would flow per
second through a specimen one sq. meter in cross-section and 1 meter in length when the temperature
gradient is 1°C. Silver and copper show the highest thermal conductivities of all metals. Some metals like
German silver exhibit very low conductivity and hence find applications where heat losses by metallic
conduction should be kept to a minimum.
All metals are conductors of electricity; silver is the best conductor and copper is next. It should be noted
that while volume aluminum has only 61% of the conductivity of copper, nevertheless weight for weight
aluminum because of its low density, shows a conductivity nearly twice that of copper.
2. Mechanical Properties of Metals:
A. Strength:
The strength of metal is its ability to withstand various forces to which it is subjected during a test or in service.
It is usually defined as tensile strength, compressive strength, proof stress, shear strength, etc. Strength of
materials is a general expression for the measure of capacity of resistance possessed by solid masses or pieces of
various kinds to any cause tending to produce in them a permanent and disabling change of form or positive fracture.
Materials of all kinds owe their strength to the action of the forces residing in and about the molecules of the bodies
(the molecular forces) but mainly to that one’s of these known as cohesion; certain modified results of cohesion as
toughness or tenacity, hardness, stiffness and elasticity are also important elements, and strength is in relation of
the toughness and stiffness combined.
B. Elasticity:
A material is said to be perfectly elastic if the whole of the stress produced by a load disappears completely on
the removal of the load, the modulus of elasticity of Young’s modulus (E) is the proportionally constant between
stress and strain for elastic materials.
Young’s modulus is the indicative of the property called stiffness; small values of E indicate flexible materials and
large value of E reflect stiffness and rigidity. The property of spring back is a function of modulus of elasticity and
refers to the extent to which metal springs back when an elastic deforming load is removed. In metal cutting,
modulus of elasticity of the cutting tools and tool holder affects their rigidity.
C. Plasticity:
I.
II.
III.
Plasticity is the property that enables the formation of permanent deformation in a material. It is reverse
of elasticity; a plastic material will retain exactly the shape it takes under load, even after the load is
removed. Gold and lead are the highly plastic materials. Plasticity is used in stumping images on coins and
ornamental work.
During plastic deformation there is the displacement of atoms within metallic grains and consequently the
shapes of the metallic components change. It is because of this property that certain synthetic materials are
given the name “plastics”. These materials can be changed into required shape easily.
Stress strain relationship
Stress
It is defined as force per unit area within materials that arises from externally applied forces.
Stress is given by the following formula:
𝜎=
𝐹
𝐴
where, 𝜎 is the stress applied, F is the force applied and A is the area of the force application.
Strain
It is the amount of deformation experienced by the body in the direction of force applied, divided by the initial
dimensions of the body.
The following equation gives the relation for deformation in terms of the length of a solid:
𝜖=
𝛿𝑙
𝐿
where, 𝜖 is the strain due to the stress applied, l is the change in length and L is the original length of the material.
The strain is a dimensionless quantity as it just defines the relative change in shape.
Hooke’s Law
This principle of physics talks about elasticity and how the force required to extend or compress an elastic object
by a certain distance is proportional to that distance. More force produces more distance.
Hook’s law defines the linear relationship between stress and strain for a material within elastic limit.
𝜎 ∝ 𝜖
𝜎=𝐸𝜖
Where, E is the Young’s modulus or modulus of elasticity.
In the case of metals, Hooke’s law dictates that for most metals, greater changes in length will create greater
internal forces. That means stress is directly proportional to strain. This is because metals exhibit elasticity up to a
certain limit.
In simple words, if the tensile/compressive load is doubled, the increase/decrease in length will also double as long
as the metal is within the proportional limit.
Stress-strain curve
The stress-strain curve is a graph that shows the change in stress as strain increases. It is a widely used reference
graph for metals in material science and manufacturing. There are various sections on the stress and strain curve
that describe different behavior of a ductile material depending on the amount of stress induced. Stress and strain
curves for brittle, hard (but not ductile) and plastic materials are different.
Proportional Limit
Almost all metals behave like an elastic object over a specific range. This range varies for different metals and is
affected by factors such as mechanical properties, atmospheric exposure (corrosion), grain size, heat treatment, and
working temperature.
When the testing machine starts pulling on the test piece, it undergoes tensile stress. Initially, the material follows
Hooke’s law.
The strain will be proportional to stress. It means that the ratio of stress to strain will is a constant. In material
science, this constant is known as Young’s modulus of elasticity and is one of the most important mechanical
properties to consider when choosing the right material for an application.
There is no permanent deformation either. The metal will behave like a spring and return to its original dimension
on the removal of load.
The point up to which this proportional behavior is observed is known as the proportional limit. With increasing
stress, strain increases linearly. In the diagram above, this rule applies up until the yield’s strength indicator.
Young’s Modulus of Elasticity
It is defined as the ratio of longitudinal stress to strain within the proportional limit of a material. Also known as
modulus of resilience, it is analogous to the stiffness of a spring. That’s also why the Hooke’s law includes a spring
constant.
Let’s say we have 2 materials with the same length and cross-section. To change the dimensions in equal measure,
the material with a higher Young’s modulus value requires greater force.
Elastic Point & Yield Point
As the test piece is subjected to increasing amounts of tensile force, stresses increase beyond the proportional limit.
The stress-strain relationship deviates from Hooke’s law. The strain increases at a faster rate than stress which
manifests itself as a mild flattening of the curve in the stress and strain graph.
This is the part of the graph where the first curve starts but has not yet taken a turn downwards. Although the
proportionality of stress to strain is lost, the property of elasticity isn’t, and on the removal of load, the metal will
still return to its original dimensions.
The change in dimension within the elastic limit is thus temporary and reversible. The elastic limit of a material
ascertains its stability under stress.
Here lies the reason why engineering calculations use a material’s yield strength for determining its ability to resist
a load. If the load is greater than the yield strength, the result will be unwanted plastic deformation.
Plastic Behavior
When the test piece is pulled further on the testing machine, the property of elasticity is lost. This aligns with the
start of the strain hardening region in the stress-strain graph.
The yield strength point is where the plastic deformation of the material is first observed. If the material is
unclamped from the testing machine beyond this point, it will not return to its original length.
Strain hardening is said to occur when the number of dislocations in the material becomes too high and they start
to obstruct each other’s movement. The material constantly rearranges itself and tends to harden.
Necking
The plastic deformation continues to occur with increasing stress. In due time, a narrowing of cross-section will be
observed at a point on the rod. This phenomenon is known as necking. The stress is so high that it leads to the
formation of a neck at the weakest point of the rod. You can see this happen in the video above.
The stress-strain curve also shown the region where necking occurs. Its starting-point also gives us the ultimate
tensile strength of a material.
Ultimate tensile strength shows the maximum amount of stress a material can handle. Reaching this value pushes
the material towards failure and breaking.
Fracture
Once in the necking region, we can see that the load does not have to increase for further plastic deformation. A
fracture will occur at the neck usually with a cup and cone shape formation at either end of the rod. This point is
known as the fracture or rupture point and is denoted by E on the stress and strain graph.
Why stress-strain curve is important?
The stress-strain curve provides design engineers with a long list of important parameters needed for application
design. A stress-strain graph gives us many mechanical properties such as strength, toughness, elasticity, yield
point, strain energy, resilience, and elongation during load.
It also helps in fabrication. Whether you are looking to perform extrusion, rolling, bending or some other operation,
the values stemming from this graph will help you to determine the forces necessary to induce plastic deformation.
[FIND OUT MORE DESCRIPTION OF THIS QUESTION]
D. Ductility:
It is the ability of a metal to withstand elongation or bending. Due to this property, wires are made by drawing out
through a hole. The material shows a considerable amount of plasticity during the ductile extension. This is a
valuable property in chains, ropes etc., because they do not snap off, while in service, without giving sufficient
warning by elongation.
E. Malleability:
This is the property by virtue of which a material may be hammered or rolled into thin sheets without rupture. This
property generally increases with the increase of temperature.
F. Toughness (or Tenacity):
Toughness (or tenacity) is the strength with which the material opposes rupture. It is due to the attraction which the
molecules have for each other; giving them power to resist tearing apart.
The area under the stress-strain curve indicates the toughness (i.e., energy which can be absorbed by the material
upto the point of rupture). Although the engineering stress-strain curve is often used for this computation, a more
realistic result is obtained from a true-stress curve. Toughness is expressed as energy absorbed (Nm) per unit volume
of material participating in absorption (m3) or Nm/m3. This result is obtained by multiplying the ordinate by the
abscissa (in appropriate units) of stress-strain plot.
G. Brittleness:
Lack of ductility is brittleness. When a body breaks easily when subjected to shocks it is said to be brittle.
H. Hardness:
I.
II.
III.
IV.
Hardness is usually defined as resistance of material to penetration. Hard materials resist scratches or being
worn out by friction with another body.
Hardness is primarily a function of the elastic limit (i.e., yield strength) of the material and to a lesser extent
a function of the work hardening co-efficient. The modulus of elasticity also exerts a slight effect on
hardness.
In the most generally accepted test, an indentor is pressed into the surface of the material by slowly applied
known load, and the extent of the resulting impression is measured mechanically or optically. A large
impression for a given load and indentor indicates soft material, and the opposite is true for small
impression.
The converse of hardness is known as softness.
I. Fatigue:
I.
II.
III.
IV.
When subjected to fluctuating or repeating loads (or stresses), materials tend to develop a characteristic
behavior which is different from that (or materials) under steady loads. Fatigue is the phenomenon that
leads to fracture under such conditions.
Fracture takes place under repeated or fluctuating stresses whose maximum value is less than the tensile
strength of the material (under steady load). Fatigue fracture is progressive, beginning as minute cracks
that grow under the action of the fluctuating stress.
Fatigue fracture starts at the point of highest stress. This point may be determined by the shape of the part;
for instant, by stress concentration in a groove. It can also be caused by surface finish, such as tool marks
or scratches, and by internal voids such as shrinking cracks and cooling in castings and weldments and
defects introduced during mechanical working and by defects, stresses introduced by electroplating.
It must be remembered that surface and internal defects are stress raisers, and the point of highest actual
stress may occur at these rather than at the minimum cross-section of highest normal stress. Thus,
processing methods are extremely important as they affect fatigue behavior.
J. Creep:
I.
II.
“Creep” is the slow plastic deformation of metals under constant stress or under prolonged loading
usually at high temperature. It can take place and lead to fracture at static stresses much smaller than those
which will break the specimen by loading it quickly. Creep is specially taken care of while designing I.C.
engines, boilers and turbines.
The creep at a room temperature is known as low temperature creep and occurs in load pipes, roofing, glass
as well as in white metal bearings. The creep at high temperatures is known a high temperature creep. It
mainly depends upon metal, service temperature to be encountered and the stress involved. For studying its
effects, the specimens are put under a constant load; the creep is measured during various time intervals
and results then plotted to get a creep curve.
3. Electrical Properties of Materials:
One of the important characteristics of the materials is their ability to permit or resist the flow of electricity.
Materials to be used in electrical equipment can be selected on the basis of their properties, such as:
i.
ii.
iii.
iv.
v.
vi.
Resistivity,
Conductivity,
Temperature coefficient of resistance,
Dielectric strength,
Thermoelectricity, and
Other electrical properties.
Resistivity:
It is a characteristic property of the material of which the conductor is made. It is that electrical property of a material
due to which, it impedes or resists the flow of electricity through it. The electrical resistivity of a metal is the
resistance of a specimen of 1 cm in length and 1 sq. cm in cross-section. Since these values for metals are very
small if expressed in ohms, they are usually given in micro- ohms, where 1 micro-ohm = 10-6 ohm.
Conductivity:
The conductivity (σ) is the reciprocal of electrical resistivity.
Temperature Coefficient of Resistance:
It is usually employed to specify the variation of resistivity, ρ with temperature.
Dielectric Strength:
It means the insulating capacity of a material against high voltages. A material having high dielectric-strength can
withstand sufficiently high voltage field across it before it will breakdown and conduct. A dielectric is an insulator.
Thermoelectricity:
If two dissimilar metals are joined and this junction is then heated, a small voltage in the millivolt range is produced,
and this is known as thermoelectric effect. Thermoelectric effect forms the basis of the thermocouple operation.
Superconductivity:
Some metals and compounds lose their electrical resistance abruptly before absolute zero is reached and become
superconductor. Superconductivity, therefore, refers to the phenomenon of abrupt drop of resistivity of some metals
at a temperature, called superconducting transition temperature, before absolute zero is reached. This transition
temperature is 0.4 K for titanium, 1.17 K for aluminum and 9.2 K for niobium, 14 K for NbH, 1.6 K for Nb4, and
18 K for Nb3S4.
Superconductivity state can be abolished by the application of an external magnetic field or produced by a
sufficiently large current flowing through the conductor.
4. Magnetic Properties of Materials:
I.
Those materials in which a state of magnetization can be induced are called “magnetic materials”. Such
materials create a magnetic field in the surrounding space.
II. The magnetic properties of materials arise from the spin of electrons and the orbital motion of electrons around
the atomic nuclei. In several atoms the opposite spins neutralize one another, but when there is an excess of
electrons spinning in one direction, magnetic field is produced. All substances except ferromagnetic material
which can form permanent magnets, exhibit magnetic effects only when subjected to an external
electromagnetic field.
III. Study of the magnetic properties is necessary because the science of magnetism explains many aspects of the
structure and behavior of the matter.
Some of the important magnetic properties are:
A. Permeability.
B. Coercive Force.
C. Magnetic hysteresis.
Permeability:
It is property of magnetic material which indicates that how easily magnetic flux is build up in material. It is
determined by ratio of magnetic flux density to magnetizing force producing this magnetic flux density. It is the
ratio of the flux density in a material to the magnetizing force producing that flux density and is denoted by μ;
μ = μ0 μr
where μ0 is the permeability of free space having a value of 4π x 10-7 H/m.
Coercive Force:
It may be defined as the magnetizing force which is necessary to neutralize completely the magnetism in an
electromagnet after the value of magnetizing force becomes zero.
Magnetic Hysteresis
It is an important material by which is firstly becomes magnetized and then de-magnetization process. Lack of
retrace ability of magnetization curve is called hysteresis and is related to existence of magnetic domains in material.
Magnetic hysteresis is rising temperature at which given material ceases to be ferromagnetic, or falling temperature
at which, it becomes magnetic.
Below Curie temperature (it is the rising temperature at which the given material ceases to be ferromagnetic, or the
falling temperature at which it becomes magnetic) all magnetic material exhibit the phenomenon called hysteresis
which is defined as the lagging of magnetization or induction flux density (B) behind the magnetizing force (H) or
it is that quality of a magnetic substance due to which energy is dissipated in it on reversal of its magnetism.
5. Chemical Properties of Materials:
A study of chemical properties of materials is necessary because most of the engineering materials, when they come
in contact with other substances with which they can react, tend to suffer from chemical deterioration.
The chemical properties describe the combining tendencies, corrosion characteristics, reactivities, solubilities, etc.,
of substances.
Some of the chemical properties are:
A. Corrosion resistance.
B. Chemical composition.
C. Acidity or alkalinity.
Corrosion
Corrosion is a gradual, chemical or electrochemical attack on a metal by its surroundings so that the metal is
converted into an oxide, salt or some other compound. It may be brought about by almost unlimited number of
factors or corrosive media such as air, industrial atmospheres, soils, acids, bases and salt solutions. It may also occur
at elevated temperature in media which are inert when near or below room temperature.