Date post: | 21-Jan-2018 |
Category: |
Engineering |
Upload: | selfhelp-citizen-dream-merchant |
View: | 292 times |
Download: | 1 times |
C H A P T E R 1
BASICS OF MATERIALS SCIENCE & ENGG
ELEMENTS OF MATERIALS SCIENCE AND
ENGINEERING
MECHANICAL PROPERTIES
• The mechanical behavior of a material reflects the relationship
between its response or deformation to an applied load or
force.
• Important mechanical properties are
• strength,
• hardness,
• ductility, and
• stiffness.
The mechanical properties of materials are ascertained by
performing carefully designed laboratory experiments that
replicate as nearly as possible the service conditions.
Factors to be considered include the nature of the applied
load and its duration, as well as the environmental
conditions. It is possible for the load to be tensile,
compressive, or shear, and its magnitude may be constant
with time, or it may fluctuate continuously. Application time
may be only a fraction of a second, or it may extend over a
period of many years. Service temperature may be an
important factor.
Stress–strain test of a material:
If a load is static or changes relatively slowly with time
and is applied uniformly over a cross section or surface
of a member, the mechanical behavior may be
ascertained by a simple stress–strain test; these are
conducted for metals at room temperature. There are
three principal ways in which a load may be applied:
namely, tension, compression, and shear. In engineering
practice many loads are torsional rather than pure shear.
There are four test types:
tension,
compression,
torsion, and
shear.
A material that is stressed first undergoes elastic
deformation, wherein stress and strain are proportional.
The constant of proportionality is the modulus of elasticity
for tension / compression, It is the shear modulus when
the stress is shear. Poisson’s ratio represents the negative
ratio of transverse and longitudinal strains
For metals, the phenomenon of yielding occurs at the
onset of plastic or permanent deformation; yield strength
is determined by a strain offset method from the stress–
strain behavior, which is indicative of the stress at which
plastic deformation begins.
Tensile strength corresponds to the maximum tensile
stress that may be sustained by a specimen, whereas
percent elongation and reduction in area are measures
of ductility—the amount of plastic deformation that has
occurred at fracture.
Resilience is the capacity of a material to absorb energy
during elastic deformation; modulus of resilience is the area
beneath the engineering stress–strain curve up to the yield
point.
Also, static toughness represents the energy absorbed during
the fracture of a material, and is taken as the area under the
entire engineering stress–strain curve.
Ductile materials are normally tougher than brittle ones.
For the brittle ceramic materials, flexural strengths are
determined by performing transverse bending tests to
fracture.
Many ceramic bodies contain residual porosity, which is
deleterious to both their moduli of elasticity and flexural
strengths.
On the basis of stress–strain behavior, polymers fall within
three general classifications: brittle, plastic, and highly
elastic.
These materials are neither as strong nor as stiff as
metals, and their mechanical properties are sensitive to
changes in temperature and strain rate.
Viscoelastic mechanical behavior, being intermediate
between totally elastic and totally viscous, is displayed by
a number of polymeric materials. It is characterized by the
relaxation modulus, a time-dependent modulus of
elasticity. The relaxation modulus is sensitive to temp;
critical to the in-service temperature range for elastomers
is this temperature dependence.
STUDY OF PHASE DIAGRAMS
• An understanding of phase diagrams is important as it
relates to the design and control of heat treating procedures.
• Some properties of materials are functions of their
microstructures, and, consequently, of their thermal histories.
• Even though most phase diagrams represent stable (or
equilibrium) states and microstructures, they are,
nevertheless useful in understanding the development and
preservation of nonequilibrium structures and their attendant
properties.
• It is often the case that these properties are more desirable
than those associated with the equilibrium state.
Most metallic alloys, and, for that matter, ceramic, polymeric,
and composite systems are heterogeneous. Ordinarily, the
phases interact in such a way that the property combination
of the multiphase system is different from, and more
attractive than, either of the individual phases. In metal
alloys, microstructure is characterized by the number of
phases present, their proportions, and the manner in which
they are distributed or arranged. The microstructure of an
alloy depends on such variables as the alloying elements
present, their concentrations, and the heat treatment of the
alloy.
ELECTRICAL PROPERTIES
CONDUCTOR-SEMICONDUCTOR- INSULATOR