Low-speed fracture instabilities in a brittle crystal

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When a brittle material is loaded to the limit of its strength, it fails by the nucleation and propagation of a crack1. The conditions for crack propagation are created by stress concentration in the region of the crack tip and depend on macroscopic parameters such as the geometry and dimensions of the specimen2. The way the crack propagates, however, is entirely determined by atomic-scale phenomena, because brittle crack tips are atomically sharp and propagate by breaking the variously oriented interatomic bonds, one at a time, at each point of the moving crack front1,3. The physical interplay of multiple length scales makes brittle fracture a complex ‘multi-scale’ phenomenon. Several intermediate scales may arise in more complex situations, for example in the presence of microdefects or grain boundaries. The occurrence of various instabilities in crack propagation at very high speeds is well known1, and significant advances have been made recently in understanding their origin4,5. Here we investigate low-speed propagation instabilities in silicon using quantum-mechanical hybrid, multi-scale modelling and single-crystal fracture experiments. Our simulations predict a crack-tip reconstruction that makes low-speed crack propagation unstable on the (111) cleavage plane, which is conventionally thought of as the most stable cleavage plane. We perform experiments in which this instability is observed at a range of low speeds, using an experimental technique designed for the investigation of fracture under low tensile loads. Further simulations explain why, conversely, at moderately high speeds crack propagation on the (110) cleavage plane becomes unstable and deflects onto (111) planes, as previously observed experimentally6,7.

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Figure 1: The simulated (111) crack system.
Figure 2: Ridges formed by low-speed instabilities on the (111) crack plane.
Figure 3: The (110) crack system.


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P.G. acknowledges support from the Deutsche Forschungsgemeinschaft (Gu 367/30). N.B. acknowledges support from NRL and ONR. D.S. acknowledges support from the ISF (grant no. 1110/04). J.R.K., G.C. and M.C.P. acknowledge support from the EPSRC portfolio grant GR/S61263/01. T.A. acknowledges support from ANR–France (grant ANR–05–CIGC:LN3M) and IDRIS (Orsay, France, project 051841). A.D.V. acknowledges support from the EPSRC grant EP/5C23938/1. G.C. acknowledges support from the EPSRC grant EP/C52392X/1. The authors thank A. Sutton for a critical reading of the manuscript. Computer time was in part provided by the US Department of Defense HPCMP and the HPCS at the University of Cambridge.

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Correspondence to G. Csányi.

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