Stress and Strain


When some external system of forces or load act on a body, the internal forces (equal and opposite) are set up at a various section of the body which resist the external forces. This internal force per unit area at any section of the body is known as unit stress or simply stress.


Stress (σ) = P / A                  [P= Force or load acting on the body,  A= Cross-sectional area of the body.]

In S.I units, the stress is usually expressed in Pascal(Pa) such that 1Pa=1N/m2. In actual practice, we use a bigger unit of stress i.e., megapascal(Mpa) and gigapascal(Gpa) such that.

1Mpa = 1 x 106 N/m2 = 1 N/mm2

1Gpa = 1 x 109 N/m2 = 1 kN/mm2 


Strain :-

When a system of forces acts on a body, it undergoes some deformation. This deformation per unit length is known as unit strain or simply a strain.


Strain (ε) = δl / l                     [ δl= Change in length of the body, and      l= Original length of the body.]

The strength of materials may broadly be defined as that branch of engineering – science, which deals with the ability of various types of materials to resist its failure and their behaviour under the action of the forces.

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