برآورد ∗C شامل تأثیر تنش آستانه
Abstract
1- Introduction
2- Estimation method of C∗
3- Finite element analysis
4- Accuracy of the modified methods and discussion
5- Conclusions
References
Abstract
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.
Introduction
C∗ is one of the parameters used for characterizing the creep crack growth and the stress field at the crack tip region. Accurate estimation of C⁄ plays an important role in the analysis of fracture mechanics and lifetime prediction for structures at elevated temperatures. Many studies have been conducted to research estimation methods of C⁄ . The General Electric/ Electric Power Research Institute (GE/EPRI) method is widely used to estimate C⁄ for homogeneous materials [1,2]. The reference stress method is another widely used method for estimating C⁄ [3,4]. Based on the reference stress method, Xuan [5] proposed a method to estimate C⁄ for mismatched weld creep cracks and Kim [6,7] proposed an enhanced reference stress method to estimate C⁄ . Other methods have also been proposed to estimate C⁄ for mismatched weld creep cracks [8,9] in addition to cracks in thin T-sections [10] and annular discs [11]. Further, the calculation equation of C⁄ in the American Society for Testing Materials (ASTM) E 1457 was modified for mismatched weld creep cracks by Xuan [12]. However, there has been no research on the estimation of C⁄ for materials with power-law creep constitutive equations that include a term of threshold stress. Since threshold stress exists in some alloys due to the presence of second-phase or nanometer-sized particles, such as dispersion hardened alloys [13–16] and nanocomposites [17,18], it is necessary to establish estimation methods of C⁄ with the effect of threshold stress included.