Gina Cavallo
MANE 6940
June 3, 2014
1. Explain briefly (~ 200 words each) basic facts about the atomic, microscopic and
macroscopic structures of the following materials:
Metals and Alloys
Polymeric Materials
Metals and Alloys:
The atoms of metals and alloys are held together by forces between electrons of adjacent
atoms. The atom radius size varies from 10^-7 to 10^-6 mm. Metals vary in phases depending
on the temperature, the most common phases being the α and ϒ phases. They are mostly
created in a liquid state and form their structure upon solidifying when cooled. As the
temperature of the metal/alloy decreases, the distances between atoms decrease. Then, the
distance at which bonding occurs is reached at random sites, which produces the first nuclei of
crystal growth.
Metals and alloys typically have simple cubic, body centered cubic, face centered cubic, and
hexagonal close packed crystal structures. They can also have defects in the crystal lattice, such
as point, line, and surface defects. The 3 main types of point defects are vacancies, interstitial
atoms, and substitutional atoms. Point defects can strengthen an alloy. Surface defects are the
separation between crystals or parts of a crystal. Line defects are defects in the arrangement of
atoms in two perpendicular directions and of macroscopic size in the third direction.
Class Textbook: Mechanics of Solid Materials, J. Lemaitre, J.-L. Chaboche
Most ceramics are compounds, made up of 2 or more elements. The most common atomic
bonds are covalent and ionic bonds, which are stronger than metallic bonding. Therefore,
ceramics tend to have high hardness and high compressive strength. It’s also why they’re less
ductile and have lower tensile strength. There are no free electrons, therefore, ceramics do not
easily conduct electricity and heat.
The crystal structures of ceramics differ, resulting in a varying range of properties. For example,
some have high temperatures superconductivity.
The microstructure can vary from glassy, completely crystalline, or a combination of the two.
Silicate ceramics are the basic structure for many ceramics. It has a pyramid atomic
Ceramic crystals have imperfections such as point defects. The defect formation is mainly based
on the condition of charge neutrality of the atoms. One type of defect is the Frenkel-defect,
which happens when a host atom moves to a close interstitial location to create a vacancyinterstitial pair of cations. A Schottky-defect is a pair of nearby cation and anion vacancies.
Class Textbook: Mechanics of Solid Materials, J. Lemaitre, J.-L. Chaboche
Poylmeric Materials
Polymers are often transparent and comprise of carbon atoms in a chain arrangement. They
mainly consist of covalent bonds between carbon atoms. Hydrogen, Oxygen, and Nitrogen
atoms can also be found attached to the chain. These chains of molecules can be structured
randomly in amorphous polymers, but have some order on a smaller scale. The chains can also
become arranged in packs, forming crystallites, which form a superstructure. Depending on the
temperature, they are found in 4 different states, glass state, transition state, rubbery state, and
fluid state.
Polymers are flexible, but their chains don’t have a uniform structure on a molecular scale.
Class Textbook: Mechanics of Solid Materials, J. Lemaitre, J.-L. Chaboche
Composites are made from 2 or more materials combined to produce a material with a
different characteristic than each of the individual parts. They typically have a bulk
phase, the matrix, and a non-continous phase, the reinforcement. The bulk phase spreads
the load over a large surface area, and transfers it to the reinforcement material, which
can withstand a larger load. Reinforcements usually hinder crack propagation.
The fibres in fibre-resin composites are arranged in either unidirectional (fibers are
parallel), bidirectional (by crossing of unidirectional sheets), and tridirectional. They are
anisotropic (directionally dependent).
2.- Explain briefly (~ 100 words each) basic facts about the microscopic processes that take
place when the following materials are subjected to gradually increasing mechanical loads:
Metals and Alloys
Polymeric Materials
Metals and Alloys
Under an external load, a slip displacement may occur, in which an edge or screw
dislocation moves across a crystal and irreversible displacement occurs. Bonds break
only in the location of the dislocation line. An edge dislocation can also move
perpendicular to its slip plane, creating a climb displacement.
During uniaxial external loading, which first increases and then decreases, the
polycrystal deforms in the following sequence: First, it experiences elastic deformation.
Then, the elastic limit is the point at which the state of stress or strain causes the first
irreversible movements of dislocations. Then plastic deformation occurs. The
phenomenon of hardening can occur after this, then, viscoplastic deformation, and finally
restoration or recovery.
Fracture can also occur, which destroys the cohesion of the matter by creating
surface or volume discontinuities within the material. Brittle fractures involves only the
fracture of interatomic bonds. Lattice defects or geometrical imperfections result in
stress concentration, and begin the fracture process. Ductile fracture occurs when large
local deformations occur at the location of crystalline defects. This instability causes
decohesion at the interface or a fracture, creating a microcrack or a cavity.
Under a cyclic load, fatigue failure starts with nucleation. Cyclic plastic
microdeformations occur near defects, and further slip is prevented due to the increase of
dislocation nodes. This is followed by the initiation of microcracks, the growth of
microcracks, and the growth of a macrocack.
Class Textbook: Mechanics of Solid Materials, J. Lemaitre, J.-L. Chaboche
Ceramics are brittle. When they are under tension, preexisting cracks amplify the stress
and create a single main crack at the top of a flaw. This crack will quickly propagate and
cause the material to break. However, under compression, the mechanical properties of
ceramics are fairly good. By nature, ceramics contain cracks and defects. The higher
number of these defects a ceramic material has, the lower its modulus of elasticity.
Polymer deformations are generally elastic when the mechanical loads are below a
certain value. There is a relative movement of chain segments which is thermally
activated, but the bonds are not destroyed. Above a certain load, there is irreversible
reorientation of chain segments resulting in additional strains beside the elastic stains.
These strains remain for a while after the load is removed. A high load destroys the
substructure of the crystallite by breaking the weakest link while new bonds develop.
The combination of external loads and thermal activation breaks molecular bonds,
resulting in fractures. The initiation zones are usually those with defects, impurities, or
crystalline flaws.
Composites are resistant to fatigue damage, and are repairable. However, repeated cyclic
loads can cause the laminates to separate between layers. Individual fibers can break out
from the matrix. Failures in compression can occur at the individual reinforcing fibers in
compression. Failures in tension can occur when one or multiple layers fail in tension of
the matrix or when the bonds between the matrix and the fibers is broken.
Other Sources
Class Textbook: Mechanics of Solid Materials, J. Lemaitre, J.-L. Chaboche