When stress is plotted against the corresponding strain. shape of the graph depends on the kind of material Even among meta there are wide variations.
The graph shown is a typical stress-strain graph for a wire of d metal. The stress is a simple tensile stress and the strain is the percenelougation. During the portion of the eurve, up to strain `lt1%,` the strees is proportional to strain until the pirind a, the proportional limit, is reached. The proportiornal relation botween strees aid strain in in this region is called Hooke,s law.
From a to b stress and strain are not proportional. However, if the load is removed at any point between O and b, the wire regains its original length. The region Ob is called the elastic region and the point b is called the elastic limit or yield point.
If the wire is loaded further, the strain increases rapidly, but when the load is suddenly removed at some point beyond b say at c. the wire does notregain its original length but follows the dashed line. The length at zero stress is now greater than the original length and the wire is said to have a permanent set.
Further increase of load beyond c produces a large increase in strainuntil a point e is reached at which fracture takes place. From b to e the metal is said to undergo plastic flow or plastic deformation, during which slipping takes place within the metal along the planes of maximum shearing stress. Point d indicates the beginning of severe nonuniform plastic deformation as a 'neck' begins to form in the wire. The actual area of cross section may be only a fraction of the original cross sectional area so that the true stress at the neck continues to increase until the wire breaks at e (fracture point).
If large plastic deformation takes plare between the elastic limit and the fracture point, the metal is said to be ductile. If fracture occurs soon after the elastic limit, the metal in said to be brittle.
