Local approach methods are becoming increasingly popular as practical tools for cleavage fracture toughness prediction. Their application involves two distinct elements: calculation of "individual" probabilities of failure, dictated by the local mechanical fields; and summation of these failure probabilities to predict the probability of component failure. In this work, we demonstrate that development of the local approach methods to date has been essentially focused on improving the criterion for predicting local failure as a function of the local mechanical fields. Yet, the existing methods fail to predict with sufficient accuracy the effects of irradiation and defect geometry on fracture toughness when the calculations are based on a common set of model parameters. A possible reason for this, common to all methods, is found in the calculation of the cumulative failure probability, which is based on the weakestlink argument. We discuss the implications of the weakest-link assumption, identify those situations where it needs to be reconsidered, and propose future work that will increase our understanding for improving the calculation of global failure probability.
INTRODUCTIONThe ability to predict changes in cleavage fracture toughness behaviour of ferritic RPV steels accounting for the effects of irradiation and defect geometry is important to safety assessment and life extension decisions in nuclear plant. Local approaches to cleavage fracture offer a promising methodology to help achieve this. In principle, these are based on current understanding of the micro-mechanisms involved in the cleavage failure phenomenon, such as the nucleation of microcracks at second-phase brittle particles, the propagation of such micro-cracks within grains and the propagation of a critical micro-crack leading to component failure. The scope of this work does not include local approaches that involve changes in
It is important for safety case justification of the continued use of nuclear power plant that any changes in the ferritic RPV steels' fracture toughness with temperature, irradiation and geometry can be accounted for, particularly with regards to plant life extension. It has been demonstrated that local approach methods have the potential to provide such estimates by assessing the likelihood of cleavage rupture.Here a micro-structurally informed, post-processing, local approach methodology with a new rupture criterion is presented. This has been applied to a base and weld material pair, made available under the EU FP7 PERFORM 60 Programme. This material has been selected as tensile and fracture data are available along with results of recently performed analyses to characterise and quantify the size distribution of fracture initiators for the weld material (predominantly found to be alumino-silicates) which is used as an input to the model.A series of finite element analyses have been performed for two-dimensional three-point bend specimens over a range of temperatures, constraint and irradiation states. Application of the local approach model to these results has then been favourably compared to the experimentally determined toughness. This has been achieved for a range of conditions when using the experimentally determined initiator particle distribution and maintaining the same calibration terms throughout for each material.
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