The FEM based calculation of crack-tip strain rate for determining the crack growth rate of 304 stainless steel in BWR environments

摘要

This research calculates the crack growth rate (CGR) of the cooling pipe in the nuclear power plant. The crack is caused by the tension left in welding as well as by the coolant chemicals. There is, by far, no satisfactory deterministic model that can predict the crack growth rate. Although Macdonald's CEFM model is more close to a deterministic one, it requires accurate calculation of the crack-tip strain rate before the crack growth rate can be obtained by using Faraday's equation. The empirical equation used by the CEFM model for estimating the crack-tip strain rate is a drawback in making it a totally deterministic model. This research is set to make up this critical defect by applying non-linear finite element method to calculate the crack-tip strain rate. According to the metallic property, there is a plastic zone forming around the crack-tip. This research found when applying finite element method. the number of the elements (namely the mesh density) in the vicinity of the crack-tip has significant effects on the crack-tip strain obtained. Therefore, this research pioneered by using different mesh density in the vicinity of the crack-tip for the calculation of the crack-tip strain, the convergence of which is used for selecting the appropriate mesh density. The selected mesh density is then used to determine the dimension of the plastic zone, in which the crack-tip strain rate can be calculated. When the FEM results are compared with the experimental data, the nonlinear finite element method combined with CEFM shows a very satisfactory prediction capability.