The hardening and softening of metals by cyclic stressing

Abstract

Metals subjected to alternating tensile and compressive stresses give characteristic stress-strain relationships. Recent studies have shown that the Bauschinger effect in single crystals and the hardening that is observed during cycling at constant strain amplitude are closely related to the modes of deformation of single crystals extended by means of tensile stresses only. Other observations on stress-strain hysteresis loops of single crystals have suggested a specific mechanism for the reduction in loop width which occurs during continued cycling at low stresses and low frequencies. New investigations have been made of the hardening produced in annealed polycrystal­line copper and aluminium by alternating stresses applied at a frequency of 100 c/s and of such magnitude as to give complete fatigue failures in 2 x 10 6 cycles. It is shown that the hardened material, as judged by tensile testing, is quite different from that produced by unidirectional deformation and, in particular, the large temperature dependence of the flow stress suggests that an important contribution to hardening arises from the production of point defects during fatigue. Similarly, when strain-hardened polycrystalline copper is fatigued at a stress lower than its initial flow stress, there is some evidence that point defects are involved in the mechanisms which lead to the softening which is observed.