Stress Strain Relations in Engineering Materials

A metal, in its generally used form, is a compact aggregate of crystal grains with varying shapes and orientations, each grain having grown from separate nucleus in the original melt. The properties of this aggregate are not just the statistical averages of the properties of single crystals taken over all orientations as might apparently be expected, because the cementing layer of an amorphously distributed number of atoms at the crystal grain boundaries have properties quite different from those of the crystals themselves. The grain boundary properties combined with the crystal properties determine the properties of the metal. Thus to study the stress strain relation of a polycrystaline metal we cannot proceed from the study of perfect crystals of the metal directly, but have to depend on independant study of the aggregate itself.