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

The ductile fracture phenomenon is a local mechanism, which consists of nuc leation, growth and coalescence of microscopic voids. These microscopic voi ds are generally formed around the second phase particles because of the de bonding 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 initiati on and stable crack growth. Two different types of models are used. The fir st one is based on the critical cavity growth (Rice and Tracey's cavity gro wth 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 w as modified and used for the analysis of the notched tensile specimens. In addition, the notched round tensile specimens were used to determine the tr ue stress-strain curve. The fracture strains of the different specimens wer e determined from the experiment with some modifications. By integrating th e cavity growth equations of the respective models up to the fracture strai n, the critical values of the parameters were determined. The effect of hyd rostatic stress on the critical parameters was studied by varying the notch root radius of the specimens. It was observed that the critical value of R ice 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 vari ation of the parameter with respect to the notch root radius. (C) 1999 Else vier Science Ltd. All rights reserved.