When a ductile material like mild steel is subjected to tensile force, it undergoes different stages before failure. Stress strain curve is the graphical representation of this stages. 

Different stages when ductile material subjected to force till its failure are

• PROPORTIONAL LIMIT (A): As shown in the stress strain curve of mild steel, up to the point A, stress and strain follow a relationship. This is known as Hook’s law. Up to the limit of proportionality, stress directly followed the strain. This means ratio of stress and strain remains constant.

• ELASTIC LIMIT (B): Upto this limit, material will regain its original shape is unloaded. Point B is known as elastic point

• YIELD LIMIT: When material is loaded beyond its elastic limit, it will not regain its original shape. There will be always some deformation.

• UPPER YIELD POINT(C): A point at which maximum load or stress required to initiate the plastic deformation of material such point is called upper yield point.
• LOWER YIELD POINT(D): A point at which minimum load or stress required to maintain the plastic behaviour of the material such point is known as lower yield point.

• ULTIMATE STRESS: This is the maximum stress a material can bear. Value of stress corresponds to peak point on stress strain curve for mild steel is the ultimate stress.

• BREAKING STRESS: Point on the stress strain curve where material fails is known as breaking point. Stress corresponds to this point is known as breaking stress.