PH8151 Engineering Physics Chapter 1

1.2. TESTING MECHANICAL PROPERTIES OF MATERIALS

on engineers to understand how the various mechanical properties are measured and what
these properties represent; they may be called upon to design structures/components us-
ing predetermined materials such that unacceptable levels of deformation and/or failure
will not occur. For example, in a suspension bridge as shown in Figure1.1, the weight
of the bridge deck and automobiles imposes tensile forces on the vertical suspender ca-
bles. These forces are in turn transferred to the main suspension cable, which sags in a
more-or-less parabolic shape. The metal alloy(s) from which these cables are constructed
must meet certain stiness and strength criteria. Stiness and strength of the alloy(s)
may be assessed from tests performed using a tensile-testing apparatus and the resulting
stress-strain diagrams [ 1 ] which we will discuss later in this chapter. Similar conditions
are encountered in every engineering branch: designing of printed circuit boards, inte-
grated circuits, computer components, long distance power transmission cables, electrical
transformers, turbines in thermal, nuclear and hydroelectric power stations. Considera-
tion of mechanical properties are essential even in designing and construction of common
objects of daily usage such as tables, chairs, pen, pencil, etc. Therefore, it is important
for a designer or an engineer to understand the significance of the mechanical properties,
and in addition, develop a sense of perspective about acceptable magnitudes of property
values.

In this chapter we study one aspect of mechanical properties of materials, namely,
elasticity. In physics,elasticity is the ability of a body to resist a distorting influence
and to return to its original size and shape when that influence or force is removed.
Elasticity of a material is determined mostly by a material parameter called a modulus,
which measures the amount of force per unit area needed to achieve a given amount of
deformation. A higher modulus typically indicates that the material is harder to deform
[ 2 ].

Learning Resource : Elasticity, Young’s Modulus

A lecture on elasticity by the legendary Prof. Walter Lewin,

from one of his MIT Physics courses.

https://youtu.be/ifri4c0luz8?list=

PLyQSN7X0ro203puVhQsmCj9qhlFQ-As8e

1.2 Testing Mechanical Properties of Materials

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, shear or
torsional and its magnitude may be constant with time, or it may fluctuate continuously.
Schematic illustrations of how these dierent types load produce change of dimensions
and shapes in materials are shown in Figure1.2. We now describe the tensile test of a solid
material, say medium carbon steel, as a representative example of testing of mechanical
properties.

PH8151 3 LICET