Although materials generally become more ductile with increase in temperature and decrease in strain rate, many metals have a local minimum in ductility at specific temperatures and strain rates. There is thus a region where a decrease in stress level results in a decrease of elongation before fracture. Failure is observed to occur at interfaces and particularly at grain boundaries nearly perpendicular to the maximum principal tensile stress. The nucleation of cavities and cracks, however, appears to arise initially from shear processes at these interfaces at temperatures and stresses where grain boundary sliding is significant but transverse boundary movement is negligible. Thus, tensile and shear stresses play different roles. Since cavities have a finite volume and link together to cause failure, the hydrostatic component of stress is important and so, in evaluating criteria for fracture, a precise definition of the stress system is necessary. Further, chemical composition, segregation and microstructural features play a vital role and materials that are only slightly different may show quite dissimilar behaviour.