In this region, the solid behaves as an elastic body. In the region from A to B, stress and strain are not proportional. Nevertheless, the body still returns to its original dimension when the load is removed. The point B in the curve is known as yield point (also known as elastic limit ) and the corresponding stress is known as yield strength ( σ y ) of the material.
If the load is increased further, the stress developed exceeds the yield strength and strain increases rapidly even for a small change in the stress. The portion of the curve between B and D shows this. When the load is removed, say at some point C between B and D, the body does not regain its original dimension. In this case, even when the stress is zero, the strain is not zero.
The material is said to have a permanent set . The deformation is said to be plastic deformation . The point D on the graph is the ultimate tensile strength ( σ u ) of the material. Beyond this point, additional strain is produced even by a reduced applied force and fracture occurs at point E.
If the ultimate strength and fracture points D and E are close, the material is said to be brittle . If they are far apart, the material is said to be ductile . Fig. .
A typical stress-strain curve for a metal. As stated earlier, the stress-strain behaviour varies from material to material. For example, rubber can be pulled to several times its original length and still returns to its original shape. Fig.
. shows stress-strain curve for the elastic tissue of aorta, present in the heart. Note that although elastic region is very large, the material does not obey Hooke’s law over most of the region. Secondly, there is no well defined plastic region.
Substances like tissue of aorta, rubber etc. which can be stretched to cause large strains are called elastomers . . ELASTIC MODULI The proportional region within the elastic limit of the stress-strain curve (region OA in