The existence of a friction stress for high-temperature creep

Abstract

Recent experimental work indicating the existence of a friction stress, σ₀ during high temperature creep is reviewed. The friction stress concept has been used to alter the normal power law creep equation to the relationship ε=B(σ−σ₀)⁴. It is contended that linear treatment of the decremental unloading data used to determine ao is inadequate in view of the inherent nonlinearity of the stress/strain rate relationship. A modified analysis which accounts for the power law nature of the creep process is proposed in which these data are treated in a manner similar to stress relaxation data. On this basis, the evidence for a friction stress in a pure metal and in a precipitation-hardened alloy is shown to be inconclusive. However, the existence of a threshold is confirmed for a dispersion-strengthened alloy. In addition, further experimental results are presented for pure lead in which delay times of zero creep are not observed for small unloadings. Since these delay times are a necessary part of the decremental unloading technique and the model which has been suggested to account for σ₀, the applicability of this approach to other materials is questioned.