Instability in dynamic fracture

The fracture of brittle amorphous materials is an especially challenging pr oblem, because the way a large object shatters is intimately tied to detail s of cohesion at microscopic scales. This subject has been plagued by conce ptual puzzles, and to make matters worse, experiments seemed to contradict the most firmly established theories. In this review, we will show that the theory and experiments fit within a coherent picture where dynamic instabi lities of a crack tip play a crucial role. To accomplish this task, we firs t summarize the central results of linear elastic dynamic fracture mechanic s, an elegant and powerful description of crack motion from the continuum p erspective. We point out that this theory is unable to make predictions wit hout additional input, information that must come either from experiment, o r from other types of theories. We then proceed to discuss some of the most important experimental observations, and the methods that were used to obt ain the them. Once the flux of energy to a crack tip passes a critical valu e, the crack becomes unstable, and it propagates in increasingly complicate d ways. As a result, the crack cannot travel as quickly as theory had suppo sed, fracture surfaces become rough, it begins to branch and radiate sound, and the energy cost for crack motion increases considerably. All these phe nomena are perfectly consistent with the continuum theory, but are not desc ribed by it. Therefore, we close the review with an account of theoretical and numerical work that attempts to explain the instabilities. Currently, t he experimental understanding of crack tip instabilities in brittle amorpho us materials is fairly detailed. We also have a detailed theoretical unders tanding of crack tip instabilities in crystals, reproducing qualitatively m any features of the experiments, while numerical work is beginning to make the missing connections between experiment and theory. (C) 1999 Elsevier Sc ience B.V. All rights reserved.