抄録
In this paper is presented analysis of stress relaxation at elevated temperature by using assumptions,
(1) that the relaxation of the applied stress (σ) consists of two relaxation processes of internal stress (σi) and effective stress (σe),
(2) that the decrement of σi is defined by the Bailey-Orowan equation, and
(3) that the plastic strain rate εp at stress relaxation is expressed by the relation εp=A exp (ασe), where A and α are constants.
From this analysis, the following conclusions have been given.
(1) Relaxation mainly depends on the decrement of σe in the earlier stage and in the later stage on the decrement of σi.
(2) The relaxation properties after re-loading are determined by the magnitude of instantaneous internal stress. If re-loading is carried out in the period where σi is larger than the initial internal stress, relaxation resistance is increased, and if re-loading in the period where σi is smaller, relaxation resistance decreases.
(3) Relaxation rate in the later stage is approximately represented in the following equation,
σ≅-r/(1+h/E)
where r is the rate of recovery, h the coefficient of work hardening, and E is Young's modulus.
(4) If r and h for creep can be substituted with those for relaxation, the following relation between the minimum creep rate εc(=r/h) and relaxation rate εr(=-σ/E) is obtained.
εc>εr
Putting the stress exponents of these strain rate as nc for creep and as nr for relaxation, the inequality nc>nr is also obtained. However there are such cases where these relations fail for the certain kind of steel accompanied by such structural changes as strain aging.
