A study of the mechanism of the delayed yield phenomenon

Abstract

This paper presents the results of an experimental investigation of the behavior of an annealed low carbon steel subjected to a rapidly applied constant stress and to repeated short-duration stress-pulses. The test stresses were greater than the upper yield stress. The material was aged at various temperatures between stress-pulses, and the effect of the time of aging on the number of stress-pulses to induce yielding was determined. Plastic and anelastic microstrain of the order of 30 X 10^(-6) in./in. is observed prior to the onset of yielding in rapidly applied constant stress tests and in repeated stress-pulse tests. Aging of the specimens for a sufficient length of time at a given temperature between stress-pulses induces recovery in the material such that yielding does not occur in repeated stress-pulse and aging cycles. The activation energy of the recovery process corresponds, within the limits of the experimental accuracy, to the activation energies of carbon and nitrogen diffusion in iron. These effects are discussed in terms of the dislocation theory of yielding. The, delayed yield and the microstrain are attributed to the action of dislocations within the crystals of the material. The recovery process is attributed to the diffusion of carbon and nitrogen to the dislocations which have been displaced, thus stabilizing the array of dislocations for the particular stress condition.

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