Estimation of C∗ including the effect of threshold stress

In some alloys such as 9%Cr heat resistant steels and magnesium alloys, the creep constitutive equation of the power-law requires a term of threshold stress due to the presence of second phase particles. It is necessary to establish an estimation method of C∗ for such alloys to predict the life of their components. In this paper, the General Electric/Electric Power Research Institute (GE/EPRI) method and the reference stress method were modified to estimate C∗ for power-law creep materials with threshold stress. The finite element method was used to verify the accuracy of the modified methods. The accuracy of the calculation equation of C∗ in the American Society for Testing Materials (ASTM) E 1457 was also assessed. The results indicated that the modified GE/EPRI method was sufficiently exact as an engineering method. h1 was slightly affected by the applied load and significantly affected by the threshold stress. The accuracy of the modified reference stress method increased with increased applied load and was within ±40%. The accuracy of the calculation equation of C∗ in ASTM E 1457 was not affected by the threshold stress and the equation could be directly used for power-law creep materials with threshold stress.