Letter

Nature 455, 1224-1227 (30 October 2008) | doi:10.1038/nature07297; Received 24 January 2008; Accepted 17 July 2008

Low-speed fracture instabilities in a brittle crystal

J. R. Kermode1, T. Albaret2, D. Sherman3, N. Bernstein4, P. Gumbsch5,6, M. C. Payne1, G. Csányi7 & A. De Vita8,9

  1. Theory of Condensed Matter Group, Cavendish Laboratory, University of Cambridge, CB3 0HE, UK
  2. Université de Lyon 1, LPMCN, CNRS, UMR 5586, F69622 Villeurbanne Cedex, France
  3. Department of Materials Engineering, Technion - Israel Institute of Technology, Haifa 32000, Israel
  4. Center for Computational Materials Science, Naval Research Laboratory, Washington, DC 20375–5343, USA
  5. Institut für Zuverlässigkeit von Bauteilen und Systemen, Universität Karlsruhe (TH), Kaiserstrasse 12, 76131 Karlsruhe, Germany
  6. Fraunhofer Institut für Werkstoffmechanik, Wöhlerstrasse 11, 79108 Freiburg, Germany
  7. Engineering Laboratory, University of Cambridge, CB2 1PZ, UK
  8. King's College London, Department of Physics, Strand, London WC2R 2LS, UK
  9. INFM–DEMOCRITOS National Simulation Center and Center of Excellence for Nanostructured Materials, University of Trieste, Trieste I-34127, Italy

Correspondence to: G. Csányi7 Correspondence and requests for materials should be addressed to G.C. (Email: gc121@cam.ac.uk).

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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