From the diagram one can see the different mark points on the curve when a ductile material like mild steel is subjected to tensile test, then it passes various stages before fracture. They are:
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.
Hook’s law is applicable up to this point.
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.
UPPER YIELD 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.
LOWER YIELD POINT
After the yield point, the curve typically decreases slightly because of dislocations escaping from Cottrell atmospheres.
ULTIMATE TENSILE 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.
Breaking point or breaking stress is point where strength of material breaks. The stress associates with this point is known as breaking strength or rupture strength. On the stress strain curve, point B is the breaking stress point. As brittle materials fail due to shear, it will be equal to the ultimate shear stress.
Toughness can be determined by integrating the stress-strain curve. It is the energy of mechanical deformation per unit volume prior to fracture.
The area under the stress-strain curve upto the elastic limit depicts the Modulus of resilience(MR) which signifies the ability of material to store or absorb energy without permanent deformation.
Young's modulus is the ratio of stress to strain(slope of the curve) up to P.