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 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.
This is the point at which material will have an appreciable elongation without increase in load.
The maximum ordinate in stress-strain diagram is ultimate strength.
is also called breaking point where the material is break due to maximum stress and strain.