force per unit area is called stress.

Stress = Force / cross-sectional area

Stress= N / m^2 = pascal = Pa


extension per unit length is called strain

Strain = extension / original length

Strain has no units because both are lengths and cancel each other.

To understand this basic concept of stress-strain we suppose a metal piece placed in tension compression testing machine and we obtain a following graph from which we conclude following things.

Proportional limit (hooke’s law)

From origin O to proportional limit (P) the straight line first observed by Sir Robert Hooke called Hooke’s law that within proportional limit stress-strain are directly proportional to each other.

proportionality is replaced by proportionality constant called Modulus of Elasticity¬†k¬†or Young’s modulus.

Elastic Limit

Elastic limit is limit beyond which material will never gain it’s original shape when load is removed.

elastic and plastic ranges

Area from O to P is called elastic range and area from P to E is called plastic range.

yield point

This is the point at which material will have an appreciable elongation without increase in load.

ultimate strength

The maximum ordinate in stress-strain diagram is ultimate strength.

rupture strength

is also called breaking point where the material is break due to maximum stress and strain.


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