Describe the relationship between stress applied to a body and strain deveoped in it using graphical method?

Explanation

(i) Portion OA: In this region, stress is very small such that stress is proportional to strain, which means Hooke's law is valid. The point A is called limit of proportionality because above this point Hooke's law is not valid. The slope of the line OA gives the Young's modulus of the wire.
(ii) Portion AB: This region is reached if the stress is increased by a very small amount. In this region, stress is not proportional to the strain. But once the stretching force is removed, the wire will regain its original length. This behaviour ends at point B and hence, the point B is known as yield point. The elastic behaviour of the material in stress-strain curve is OAB.
(iii) Portion BC: If the wire is stretched beyond the point B, stress is increases and the wire will not regain its original length after the removal of stretching force.
(iv) Portion CD: With further increase in stress (beyond the point C), the strain increases rapidly and reaches the point D. Beyond D, the strain increases even when the load is removed and breaks (ruptures) at the point E. Hence, the maximum stress (here D) beyond which the wire breaks is called breaking stress or tensile strength. The corresponding point D is known as fracture point. The region BDDE represents the plastic behaviour of the material of the wire.