A study on ductile fracture initiation in the PHT piping material of an Indian PHWR using local approach

Abstract

The ductile fracture phenomenon is a local mechanism, which consists of nucleation, growth and coalescence of microscopic voids. These microscopic voids are generally formed around the second phase particles because of the debonding of these particles from the parent matrix or the cracking of these particles themselves. In the present work, the ductile fracture of primary heat transport (PHT) system piping material of an Indian pressurised heavy water reactor (PHWR) was analysed using different models of local approach. Such local approaches use micromechanical models to predict crack initiation and stable crack growth. Two different types of models are used. The first one is based on the critical cavity growth (Rice and Tracey's cavity growth model and Budiansky and coworker's model). The other model is based on the combined effect of damage and yielding (Tai and Yang's and modified Tai and Yang's model). An in-house Elasto-Plastic finite element code thesis was modified and used for the analysis of the notched tensile specimens. In addition, the notched round tensile specimens were used to determine the true stress–strain curve. The fracture strains of the different specimens were determined from the experiment with some modifications. By integrating the cavity growth equations of the respective models up to the fracture strain, the critical values of the parameters were determined. The effect of hydrostatic stress on the critical parameters was studied by varying the notch root radius of the specimens. It was observed that the critical value of Rice and Tracey's parameter is a weak function of stress triaxiality whereas critical parameters of other models showed more dependency on the level of triaxiality. Especially Budiansky and coworker's model showed maximum variation of the parameter with respect to the notch root radius.