Explicit Crack Modeling based Approach for Structural Integrity Assessment of Brittle and Quasi-Brittle Structures

Abstract

There is considerable variation in the fracture properties of brittle and quasi-brittle materials. Due to this large variation, probabilistic models are employed for estimating failure of brittle components/structures. However, due to limitations and shortcomings in the models, the predictions are not accurate. The shortcomings include: inability to handle stress concentrations, dependence of empirical constants on loading conditions, incorrect size-effect predictions and limited applications of the model. Although higher design margins can accommodate the inaccuracy in predictions, the cost of manufacturing increases. The work presented herein is directed towards addressing these issues. An approach based on explicit crack modeling (ECM) for accurately estimating failure in brittle/quasi-brittle components and structures is presented. Factors which govern fracture in a structure (fracture energy, strength of the material, damage behavior of the material, heterogeneity in the material microstructure) are incorporated in the ECM approach. The approach was validated by predicting the failure probability of L-shaped specimens at varying load levels followed by comparison of the predictions with published data. The study showed that the predictions from the ECM approach were not only in good agreement with the published data but were also more accurate than the Weibull model based predictions. The ECM approach can also predict size effect--the dependence of fracture properties and their statistical variation on the size of the specimen. This capability was demonstrated through failure prediction of specimens in tensile and flexural tests. Specimens of different sizes were considered and the predicted fracture properties were in good agreement with those obtained experimentally. The ECM approach for estimating failure of components/structures subjected to complex physical conditions was illustrated through the failure estimation of nuclear reactor graphite components. For modeling stresses in the graphite components subjected to high temperature and neutron irradiation, a constitutive model for evaluating the stresses was constructed and implemented through a user material (UMAT) subroutine in finite element software Abaqus. UMAT was integrated with Extended Finite Element (XFEM) technique for modeling irradiation-induced failure of the components under in-reactor conditions. Component lifetime as well as crack initiation and propagation details were predicted. This type of detailed failure information has the potential to improve design guidelines and standards of brittle components/structures.