Stress Strain Curve

Explanation of Curve :

(a) Proportional limit : The portion OP of the stress-strain graph is a straight line showing that Hook's law is obeyed i.e. stress is directly proportional to the strain. The point P is called proportional limit.

(b) Elastic region : PE portion of graph is not a straight line. If the wire is unloaded at point E, the graph between stress and strain is obtained in reverse direction along EPO. The point E is called elastic limit. The portion of graph berween O and E is called elastic region. For this portion of graph Hook's law is not obeyed by wire.

                   Stress Strain diagram

(c) Permanent set : If wire is loaded beyond the point E i.e. elastic limit, the strain increases much more rapidly than the stress, which is indicated by portion EA of the curve. If the wire is unloaded at A, the graph between stress and strain will not be along AEPO but will be along AO'. Hence, if the wire is completely unloaded even then its length will increase permanently by some amount corresponding to OO' portion of graph. It is called permanent set.

(d) Plastic behaviour : Beyond the point A, the length of the wire starts increasing for no increase in stress i.e. we say that wire begins to flow after point A and continues up to point C of the stress-strain graph. The point A is called yield point. Increase in length of wire for virtually no increase in stress is called plastic behaviour of the wire.

(e) Breaking stress : Beyond the point C, the graph shows that the length of wire will increase even if wire is unloaded. In this region, necks and waists are formed along the length of wire as a result of which wire breaks ultimately corresponding to point B. Point B is called breaking point of the wire and the portion EB of the graph is called plastic region and stress corresponding to point B is called breaking stress or ultimate stress.