Estimates of J and COD for Complex-Cracked Pipes Based on FE Analyses and ERS Method

Abstract

A complex crack can occur at the dissimilar metal weld parts of nuclear pipes due to stress corrosion cracking. The fracture behavior of such crack is different with those of a simple through-wall crack. Thus, the engineering estimation equations for predicting the fracture mechanics parameters, i.e. crack opening displacement and J-integral, should be newly provided in order to evaluate the leak-before-break of pipe including complex cracks. In the previous research, an engineering scheme using reduced thickness analogy was suggested to calculate the crack opening displacement and J-integral for complex cracked pipes. However, this method has limitations in the sense that a through-wall crack is assumed to be developed only in the circumferential direction, and the effect of crack closure cannot be considered. Another scheme for complex cracked pipes is the enhanced reference stress method considering crack closure effect by suggesting the optimized reference load for complex crack geometry. It presented the more accurate results compared to previous engineering estimation scheme. However, an elastic component of crack opening displacement and J-integral were calculated by assuming the crack shape as the simple through-wall crack due to the absence of engineering solutions for complex-cracked pipes. In this context, finite element based evaluation of fracture parameters was performed to confirm the validation of previous estimation schemes. Moreover, the variations of crack opening displacement and J-integral according to a change of surface crack depth ratio of complex crack are systematically investigated based on detailed 3-D finite element analyses. Furthermore, enhanced reference stress method considering finite element based elastic values from the complex-cracked pipe models is assessed to overcome the limitation of previous enhanced reference stress approach.