Q.The point on stress-strain curve at which strain begins to increase even without increase in stress is called .... (A) elastic point (B) yield point (C) breaking point (D) neck point.

The stress-strain curve of a material is obtained by loading a wire in small steps, from zero up to the point of fracture, and plotting the resulting stress against strain at each step. The curve begins as a straight line through the origin (region $OA$), where Hooke's law holds and stress is proportional to strain; the stress at $A$ is the proportional limit. Beyond $A$, up to a point $B$ (the yield point), the curve bends but the material is still elastic overall — unloading anywhere in $OB$ retraces the same path back to zero strain, so this recovery is fully reversible; the corresponding stress is the elastic limit. Beyond $B$, strain increases rapidly for very little further increase in stress, and unloading no longer retraces the loading path — the material keeps a permanent set (residual strain) once unloaded, so this region ($B$ to the fracture point $D$) is called the plastic-flow or plastic-deformation region.

Different materials trace out different portions of this general curve. Brittle materials (glass, ceramics) fracture while still within the elastic limit, with almost no plastic region. Ductile materials (copper, aluminium, wrought iron) have a large plastic range and lengthen considerably before finally breaking. Malleable materials (gold, silver) can be hammered into thin sheets. Rubber shows an unusually large elastic region — stretching to many times its original length and still recovering fully — but its stress-strain curve within that region is markedly non-linear rather than a straight Hooke's-law line; a material capable of such large elastic strain is called an elastomer.

A related phenomenon, elastic hysteresis, occurs in materials such as vulcanised rubber: as the applied stress is reduced back to zero, the strain lags behind and does not fall to zero immediately. Plotting a full loading-then-unloading cycle produces two different curves rather than one retraced line, enclosing a loop; the area of this loop equals the energy dissipated (as heat) during one complete cycle of deformation.