The basic data on the mechanical properties of a ductile metal are obtained from a tension test, in which a suitably designed specimen is subjected to increasing axial load until it fractures. The load and elongation are measured at frequent intervals during the test and are expressed as average stress and strain according to the equations in the previous section. The data obtained from the tension test are generally plotted as a stress-strain diagram.Below Figure shows a typical stress-strain curve for a metal such as aluminium or copper.
Typical tension stress-strain curve: The initial linear portion of the curve OA is the elastic region within which Hooke’s law is obeyed. Point A is the elastic limit, defined as the greatest stress that the metal can withstand without experiencing a permanent strain when the load is removed. The determination of the elastic limit is quite tedious, not at all routine, and dependent on the sensitivity of the strain-measuring instrument.
For these reasons it is often replaced by the proportional limit, point A’. The proportional limit is the stress at which the stress strain curve deviates from linearity. The slope of the stress-strain curve in this region is the modulus of elasticity. For engineering purposes the limit of usable elastic behaviour is described by the yield strength, point B. The yield strength is defined as the stress which will produce a small amount of permanent deformation, generally a strain equal to 0.2 per cent or 0.002 inches per inch. In Fig. above fig this permanent strain, or offset, is OC. Plastic deformation begins when the elastic limit is exceeded. As the plastic deformation of the specimen increases, the metal becomes stronger (strain hardening) so that the load required to extend the specimen increases with further straining. Eventually the load reaches a maximum value. The maximum load divided by the original area of the specimen is the ultimate tensile strength. For a ductile metal the diameter of the specimen begins to decrease rapidly beyond maximum load, so that the load required to continue deformation drops off until the specimen fractures. Since the average stress is based on the original area of the specimen, it also decreases from maximum load to fracture.