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Mechanical Properties Of Solids
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Chapter 9Mechanical Properties Of Solids
1. Introduction
A rigid body refers to a hard solid object having a definite shape and size. However, in reality, bodies can be
stretched, compressed and bent. Even the strongest rigid steel bar can be deformed when a sufficiently large
external force is applied on it. This suggests that solid bodies are not perfectly rigid. Solids have a definite
shape and size. In order to make a change (or deform) their shapes or sizes, a force is always required.
2. Deforming Force
A deforming force is can be defined as a force that produces a change in configuration (size or shape) of the
object on applying it.
3. Elasticity
Elasticity refers to the property of an object by virtue of which it regains its original configuration after having
the deforming force removed. For instance, when we stretch a rubber band and release it, it snaps back to its
original shape and length.
4. Perfectly Elastic Body
The bodies which have the capability to regain its original configuration immediately and completely after
having the deforming force removed are termed as perfectly elastic bodies. Quartz fiber can be considered as
a perfectly elastic body.
5. Plasticity
When a body does not have the capability to regain its original size and shape completely and immediately
after having the deforming force removed, it is called as a plastic body and this property is termed as plasticity.
6. Perfectly plastic body
A body that does not regain its original configuration at all on the removal of deforming force is known as a
perfectly plastic body. Putty and paraffin wax can be considered nearly perfectly plastic bodies.
7. Stress
When an object gets deformed under the action of an external force, then at each section of the object, a
stress (an internal reaction force) is produced, which tends to restore the body into its original state.
, 7.1 Definition
The internal restoring force produced per unit area of cross section of the deformed object is termed as stress.
7.2 Mathematical Form
Applied Force Stress Area Its unit is 2 N/m or pascal (Pa).
Its dimensional formula is 1 2 ML T .
7.3 Types of stress
Three different types of stress are known. They are:
1. Longitudinal Stress
When a deforming force is applied normal to the area of cross section, then the stress is termed as
longitudinal stress or normal stress. It is further differentiated in two kinds:
a) Tensile stress: When there is an increase in length of the object under the effect of applied force, then the
stress is termed as tensile stress.
b) Compressional stress:
When there is a decrease in length of the object under the effect of applied force, then the stress is termed as
compression stress.
2. Tangential or Shearing Stress
When the deforming force acts tangentially to the surface of a body, it generates a change in the shape of the
body. This tangential force applied per unit area is termed as tangential stress or shearing stress.
3. Hydraulic Stress
When the applied force is due to a liquid uniformly from all sides, then the corresponding stress is termed as
hydrostatic stress.
8. Strain
When a deforming force gets applied on an object, the object undergoes a change in its shape and size. The
fractional change in their setup is termed as strain.
8.1 Mathematical Equation
Strain = change in dimension / original dimension
It is a dimensionless quantity and has no unit. According to the change in setup, the strain is differentiated into
three types:
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