<P> Stress--strain curves of various materials vary widely, and different tensile tests conducted on the same material yield different results, depending upon the temperature of the specimen and the speed of the loading . It is possible, however, to distinguish some common characteristics among the stress--strain curves of various groups of materials and, on this basis, to divide materials into two broad categories; namely, the ductile materials and the brittle materials . </P> <P> Consider a bar of cross sectional area A being subjected to equal and opposite forces F pulling at the ends so the bar is under tension . The material is experiencing a stress defined to be the ratio of the force to the cross sectional area of the bar: </P> <Dl> <Dd> s t r e s s = F A (\ displaystyle \ mathrm (stress) = (\ tfrac (F) (A))) </Dd> </Dl> <Dd> s t r e s s = F A (\ displaystyle \ mathrm (stress) = (\ tfrac (F) (A))) </Dd>

What does the area under the elastic portion of the stress strain represent