Experiment on Work-Hardening by Repeated Stress Relaxation Method

Abstract

Repeated stress-relaxation of NaCl single crystals has been studied at room temperature. It has been found that the work-hardening during repeated stress relaxation can be analyzed in terms of two processes. One appears at the early part of each cycle and corresponds to the work-hardening that proceeds fast. This process depends on the time interval for relaxation, Δt, and the initially applied stress, σ₀, suggesting that the process is related to the relaxation. This process may be interpreted as follows: The immobilization and grouping of the dislocations proceed during the relaxation and the grouped dislocations are dispersed during the reloading process, causing the hardening at the early stage of the cycle experiment. The other process is independent of Δt and corresponds to the work-hardening that proceeds slowly with the cyclic stress. This type of hardening is related to the active straining rather than the relaxation. During this process the dislocation density and inter-dislocation reaction increase with straining. This process may be explained by using the assumption that the hardening coefficient, θ_r, during the relaxation and the so-called rising coefficient, K, at the beginning of the restraining are constant.