<P> The relationship between the stress and strain that a particular material displays is known as that particular material's stress--strain curve . It is unique for each material and is found by recording the amount of deformation (strain) at distinct intervals of tensile or compressive loading (stress). These curves reveal many of the properties of a material (including data to establish the Modulus of Elasticity, E). </P> <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>

The strain in the aluminum portion is ____ the steel portion