Stress strain curve is a behaviour of material when it is subjected to load. In this diagram stresses are plotted along the vertical axis and as a result of these stresses, corresponding strains are plotted along the horizontal axis. As shown below in the stress strain curve.

From the diagram one can see the different mark points on the curve. It is because, when a ductile material like mild steel is subjected to tensile test, then it passes various stages before fracture.

These stages are:

- Proportional Limit
- Elastic Limit
- Yield Point
- Ultimate Stress Point
- Breaking Point

Proportional Limit:

Proportional limit is point on the curve up to which the value of stress and strain remains proportional. From the diagram point P is the called the proportional limit point or it can also be known as limit of proportionality. The stress up to this point can be also be known as proportional limit stress.

Hook’s law of proportionality from diagram can be defined between point OP. It is so, because OP is a straight line which shows that Hook’s law of stress strain is followed up to point P.

Elastic Limit:

Elastic limit is the limiting value of stress up to which the material is perfectly elastic. From the curve, point E is the elastic limit point. Material will return back to its original position, If it is unloaded before the crossing of point E. This is so, because material is perfectly elastic up to point E.

Yield Stress Point:

Yield stress is defined as the stress after which material extension takes place more quickly with no or little increase in load. Point Y is the yield point on the graph and stress associated with this point is known as yield stress.

Ultimate Stress Point:

Ultimate stress point is the maximum strength that material have to bear stress before breaking. It can also be defined as the ultimate stress corresponding to the peak point on the stress strain graph. On the graph point U is the ultimate stress point. After point U material have very minute or zero strength to face further stress.

Breaking Stress (Point Of Rupture):

Breaking point or breaking stress is point where strength of material breaks. The stress associates with this point known as breaking strength or rupture strength. On the stress strain curve, point B is the breaking stress point.