Recent developments in tensile testing

Abstract

The study of the behaviour of metals on removal and re-application of load has been facilitated by the Dalby Autographic Load-Extension Recorders. Prof. Dalby has described these instruments and the specially designed, hydraulic press in which the tensile tests are carried out, and has shown that metal possesses elastic properties right up to fracture. Up to a certain limiting stress the extension is proportional to the applied load; when this stress is exceeded the metal loses its proportional elasticity but retains the property of non proportional elasticity. When the material has passed into the condition of non-proportional elasticity, the removal and re application of the load causes a loop to be traced in the recorded load-extension diagram. Looping is a general property of metals, and the loops obtained from different metals have distinguishing characteristics. Prof. Dalby has also shown that the loop area increases with permanent set, tending towards a maximum value, and that loop area plotted against permanent set gives a smooth curve. This will be referred to as the “normal curve” for a metal.