Stress and Strain
Theory at a Glance (for IES, GATE, PSU)
1.1 Stress (ı)
When a material is subjected to an external force, a resisting force is set up within the component.
The internal resistance force per unit area acting on a material or intensity of the forces distributed
over a given section is called the stress at a point.
It uses original cross section area of the specimen and also known as engineering stress or
P is expressed in Newton (N) and A, original area, in square meters (m2), the stress ǔ will be
expresses in N/ m2. This unit is called Pascal (Pa).
As Pascal is a small quantity, in practice, multiples of this unit is used.
1 kPa = 103 Pa = 103 N/ m2
1 MPa =
(kPa = Kilo Pascal)
1 GPa = 109 Pa = 109 N/ m2
(MPa = Mega Pascal)
(GPa = Giga Pascal)
Let us take an example: A rod 10 mm q 10 mm cross-section is carrying an axial tensile load 10
kN. In this rod the tensile stress developed is given by
A 10 mm q10 mm
The resultant of the internal forces for an axially loaded member is
normal to a section cut perpendicular to the member axis.
The force intensity on the shown section is defined as the normal stress.
%Al 0 %A
Tensile stress (ıt)
If ǔ > 0 the stress is tensile. i.e. The fibres of the component
tend to elongate due to the external force. A member
subjected to an external force tensile P and tensile stress
distribution due to the force is shown in the given figure.
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