MICROMECHANICAL ANALYSIS OF FRACTURE BEHAVIOUR OF CIRCUMFERENTIALLY WELDED PIPES

Abstract

The Primary Heat Transport (PHT) piping of Pressurized Heavy Water Reactor (PHWR) is a critical system -- any failure of this system will lead to very grave consequences, not speaking of huge monetary losses resulting from non-utilization of the reactor setup. Hence, it is necessary to have an accurate and reliable method for predicting the fracture behaviour in such components. Fracture resistance of a ductile material is conventionally characterised by JR-curves. The original idea was that a unique fracture ' resistance curve would suffice to characterise the material. However, testing of different types of specimens revealed considerable different JR curves due to the different levels of triaxility imposed on the specimens. This raises the question of transferring fracture parameters from specimens to component level. Under such circumstances it becomes advisable to use Micromechanical Analysis (Damage Mechanics). The difficulty of geometry-dependency is largely overcome by the Damage Mechanics, which models the drop in load carrying capacity of a material with the increase in plastic strain. Such modelling is done considering nucleation, growth and coalescence of the voids in a material following large-scale plasticity. The constitutive model introduced by Gurson and modified by Tvergaard and Needleman is used in this work to predict ductile fracture of welded pipes. In this model (GTN model) the model parameters are determined using a hybrid methodology of metallographic study, different tensile tests subsequent numerical analysis and curve fitting and finally prediction of components fracture behaviour and comparison of the analytical and experimental results. The microstructural study, evaluation of basic tensile properties (stress-strain relations) and basic fracture properties (JR-curves) were earlier determined at National Material Laboratory, Jamshedpur. For component testing several welded pipes were tested at SERC, Chennai. In this current work different Notched Round Tensile Bars were tested at Fatigue and Fracture Laboratory, Hall-4, BARC to obtain the Load-OD curves. V A parametric study on different Gurson parameters was carried out to experience the effect of different parameters on the material behaviour (Load-AD curves of NRTB and JR-curves of CT specimen). Later, the model parameters were calibrated by fitting the analytical results in the experimental results. At the final stage the obtained parameters were applied in the model to predict the fracture behaviour of piping component and the predicted and experimental results were compared