Atomistic Theory and Simulation of Fracture
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Robin L.B. Selinger and Diana Farkas, Guest Editors As a branch of computational materials science, simulation studies of fracture are aimed at addressing both practical problems in materials engineering and issues in the basic science of solid mechanics. One practical goal is the development of computational tools to predict the fracture toughness of materials as a function of composition, microstructure, temperature, environment, and loading conditions. Such tools are needed to speed the development of novel high-strength structural materials by identifying likely candidate formulations and reducing the number of laboratory trials needed for testing and validation. In the realm of more-basic research, computer simulation of fracture has already provided insight into the stability and limiting speed of crack propagation, the origins of brittle/ductile behavior, and the roughness of fracture surfaces. Fracture involves phenomena occurring at a range of length scales. The imposed loading is applied at relatively macroscopic distances from the crack tip, and the resulting stress is greatly magnified in the crack-tip region. The response of the crack tip is controlled by these stresses on the one hand, and on the other hand, it is a response at the atomistic level, where the crack advances by breaking individual atomic bonds. Continuum analysis thus gives an incomplete picture, and studies at the atomistic level are needed to understand the exact nature of the crack-tip response. Experimental studies usually cannot reach the atomistic scale, and simulation studies are essential in understanding the precise phenomena occurring at the crack tip as fracture advances. In the collection of articles that follow, the authors focus on the role of atomisticlevel phenomena in determining fracture behavior. Two articles address atomistic
MRS BULLETIN/MAY 2000
effects in purely brittle fracture, arguing that continuum fracture mechanics by itself is insufficient to explain common experimental observations. Gumbsch and Cannon explain the phenomenon of “lattice trapping,” first identified by Thomson,1 and show that it gives rise to directional anisotropy and cleavage along surfaces that are not low in energy. The authors analyze in detail the cases where lattice trapping can contribute significantly to the difference between the observed energy-release rate for crack propagation and the work necessary to create two surfaces. Beltz and Lipkin discuss how the structure of a low-angle symmetrical tilt boundary can give rise to directional anisotropy in fracture resistance, a feature not predicted at all by Griffith theory. In both of these articles, the authors take a decidedly multiscale approach and make it clear that microscopic processes at the crack tip produce macroscopic effects that cannot be explained by continuum theory alone. Continuing improvements in highperformance computing over the last decade have made it possible to carry out atomic-scale simulations of fracture, using either large system size or moving boundary co
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