Abstract
It is said that there exists an intimate relation between relaxation and creep although loading conditions of both tests are quite different. It is practically desirable to estimate relaxation properties of materials from creep data. In this paper, the relationship between relaxation and creep which is a basis for doing this estimation is described.
In relaxation, rate of plastic strain is an important factor for knowing it's characteristics as well as for calculating residual stress based on the stress dependency of strain rate. Previous studies have revealed that rate of plastic strain holds a linear relation with stress in log scale.However, the results of test in the present study illustrate the relation of strain rate to stress as consisting of a break line in the same diagram. The primary region (the 1st stage of relaxation), earlier stage after loading, shows that the rate of plastic strain depends on stress and total strain, consequently on plastic strain. The secondary region (the 2nd stage of relaxation), which dominates in latter stage, shows that the rate of plastic strain depends on stress, and the effect of total strain on it is considerably small. The 1st stage seems to correspond to the transient creep and the 2nd stage to the steady creep.
The residual stress-time relation is also divided into two regions similarly to the case of strain rate-stress relation. Up to the present, the interests have been placed mainly on the latter region of the second stage and consequently the stress-time relation has been presented by a straight line on log-log coordinates.
Analysis was made to predict the residual stress and the rate of strain for 1st stage from transient creep data, based on strain hardening theory, and also to predict the residual stress in 2nd stage from the data of steady creep rate in creep test. The calcurated results were not coincide satisfactorily with the experimental values. It is considered as the reason for this incoincidence that in the analysis the effects of stress history and creep recovery phenomenon which would presumably influence to relaxation process are not taken into account.
Relaxation process is taken as the creep under successively decreasing stress. Stress decrease would inevitably accompany the creep recovery. Although creep recovery that is considered to occur in relaxation has not been fully understood in the present stage of study in this field, the analysis was corrected by taking this phenomenon into account. The corrected value became nearly equal to the experimental result.
