Plastic and viscous deformation of metals

Deformation characteristics of tensile specimens of several alloys, including electrolytic copper, α-brass, and 304 stainless steel, have been studied by application of stress and measurement of change of length in a soft tensile machine. By means of experiments in which the stress rate is reduced suddenly from a positive value to zero and the strain rate measured, both during loading and during creep, it is found that permanent deformation consists of two components, a plastic component for which the strain rate is a function of stress and stress rate, and a viscous component which is functionally dependent on stress and temperature. Plastic deformation is relatively more evident at increasing stress rate but declines in importance through the series copper, a-brass, and stainless steel. As a consequence, for a fixed strain rate during loading, the initial creep rate is low in copper and little creep occurs; in stainless steel, however, the initial creep rate is nearly equal to the loading strain rate and creep is pronounced. The theory is not fully developed but is based on a competition between thermal and mechanical release of dislocation segments from obstacles or sources. Release produces a strain increment which may be small or large depending on the relative values of stress and structural resistance. Plastic deformation occurs when the applied stress is close to the mechanical threshold, mechanical release is relatively easy, and the strain consists, at a given strain rate, of a few large strain increments per unit time. For viscous flow the relative stress is low, thermal release easy, and the strain rate is composed of many small strain increments in each unit of time.