# What is Stress – Strength of Materials?

## What is Stress?

Stress is defined as the resistance force acing per unit
cross section area of the body. It is also defined as the ratio of applied load
to the cross section area of the body.
Let’s understand this while taking an example. Considered a
body which is subjected to an external load. Due to this applied load, internal
forces induce within the body. This internal force is called as resisting force
and the direction of resisting force is opposite to the direction of applied
load. The resisting force induced resists the deformation in the body. When this
resisting force is taken on unit basis, we get a quantity called stress. Within
certain limit the resisting force induced in the body is proportional to the
deformation produced. This certain limit is called elastic limit. Within this
elastic limit the resistance force is equals to the applied load.
So finally the stress is defined as the ratio of applied
load to the cross section area. It is denoted by the symbol σ (sigma).
Mathematically,
Since resisting force is equals to the applied load i.e. P = R. So
Where,                 R = Resisting
force induced in the body.
P = Load applied on the body.
A
= Cross Section area of the body.

### Units of stress

Since the unit of load is N and the unit of cross section area is m2
or cm2 or mm2. So the SI unit of stress is N/m2
or N/cm2 or N/mm2. Generally in the numerical problems we
use N/m2 or N/mm2.
Relation between units of stress:
1 MN/m2      = 106 N/m2
= 102 N/cm2
= 1 N/mm2

### Types of Stress

The various types of stress in strength of materials are:
1. Normal Stress: the stress that acts perpendicular
to the cross section area is called normal stress.
Normal stress is further sub divided into two types
(i). Tensile stress
(ii). Compressive stress
(i). Tensile Stress: When a body is subjected to two equal and opposite
pulls, than the stress induced in the body is called tensile stress. Tensile stress
results in the increase in length and decrease in the cross section of the area
of the body.
(ii). Compressive Stress: When a body is subjected to two equal and
opposite pushes, than the stress induced in the body is called compressive
stress. Compressive stress results in the increase in the cross section area and
decrease in length of the body.
2. Shear Stress: When a body is subjected to two
equal and opposite forces acting tangential to the resisting section, than the
stress induced in the body is called shear stress. The shear stress tries to
shear off the resisting section. The shear stress acts tangential to the area. It
is denoted by symbol ‘τ’ (tau).
For better understanding of what is stress let’s see this
numerical problem
Supposed we have given a bar of cross section area 20 mm2 and
it subjected to a tensile load of 60 N. than calculate the stress induced in
the body.
Solution:
Given:  P = 60 N; A = 20.
Than from the definition of stress, the stress is given by
This is all about what is Stress, units of stress, types of stress in strength of materials. If you find anything missing or incorrect than comment us. And if you find this article informative, than like us on Facebook. #### Pankaj Mishra

Pankaj Mishra is a blogger by passion and Mechanical Engineer by profession. He has completed his B.Tech degree in mechanical engineering in the year 2015. He loves to share his knowledge and help others.

### One thought on “What is Stress – Strength of Materials?”

• August 2, 2017 at 4:15 am