AN ANALYSIS OF PHYSICAL SIGNIFICANCE OF CONSTITUTIVE EQUATIONS FOR CREEP UNDER CONSTANT LOAD AND CONSTANT STRESS

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Creep life of low alloy heat resistant steels can be successfully predicted by use of two parameters, namely, imaginary initial strain rate and strain rate acceleration factor, OMEGA. In spite of this practical convenience, the physical significance of these parameters remain unclear. The present study was undertaken to accumulate and analyze the data on stress and temperature dependence of these parameters not only for heat resistant steels but also for single phase materials such as aluminum and aluminum-3at.%magnesium alloy. It was made clear that the magnitude of omega for single phase materials was almost equal to that of stress exponent for imaginary initial strain rate at lower stresses and basically independent of stress. In case of aluminaum-3at.% magnesium alloy at higher stresses, the values of omega decreased with increasing stress and they became less than the stress exponent for their imaginary initial strain rate. In contrary to the case of single phase materials, the magnitude of omega in heat resistant steels which were strengthened with precipitates was much larger than that of stress exponent and they decreased with increasing stress. To understand the physical significance of the omega, the ratio of effective stress (sigma^<**>) to total flow stress (sigma) was determined from the analysis of stress relaxation curve of single phase aluminum-3at.% magnesium alloy. From the analysis based on the effective stress, it was demonstrated that OMEGA was expressed as below ;OMEGA=n_p+m^<**> (* ln sigma**/* In ln sigma),where n_p is the stress exponent for mobile dislocation density, m^<**> is the effective stress exponent for mobile dislocation velocity. Thus we inferred from the present results that the presence of precipitates in heat resistant steels affect the magnitude of (* ln sigma**/* In ln sigma).