Stress-Strain Curve

Variation of strain with strain

Stress-Strain Graph

When a wire is stretched by a load, it is seen that for a small value of load, the extension is directly proportional to the load. In other words, the stress is directly proportional to the strain. When a graph between stress and strain is plotted, then the stress is found to be directly proportional to the point A, this point knows as the Proportional Limit. When stress is increased beyond A , then for small stress, there is a large strain on the wire upto point B, When the load is gradually removed, the wire returns to its original length. The Wire Regains its original dimensions only when load applied is less than or equal to a certain limit. This limit is called  Elastic Limit. thus, elastic limit is defined as maximum stress on whose removal, the bodies regain their original dimensions. The point B on the curve is known as Elastic Limit.

If the stress or load increases beyond point B, the strain further increases.  This Increase in strain is represented by BC part of the curve. Now, if the load is removed, the wire doesnt regain its original length. But the increase in length of the wire is permanent. The permanent strain in the wire is known as Permanent Set. 

Now, as the stress beyond C is increased, there is large strain in the wire. This large increase in the strain for small stress is represented by CE part of the curve. The wire breaks at point E which is also knows as Fracture Point. The material of the wire from point C to point E shows the Plastic behaviour or Plastic Deformation.