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