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MRS BULLETIN • VOLUME 31 • MARCH 2006 167 From Mirror to Mist: Cracking the Secret of Fracture Instabilities When cracks in a material propagate, bonds between atoms are broken, generating two new surfaces. Experiments have shown that cracks moving at low speeds create atomically flat, mirror-like surfaces, whereas cracks moving at higher speeds create increasingly rough fracture surfaces. Dynamical instability leads to increasing roughening transitioning from a mirror-like surface to a less reflective (mist) surface to a very rough irregularly faceted (hackle) surface. Using massively parallel large-scale atomistic simulations, Markus J. Buehler from the Massachusetts Institute of Technology (MIT) in Cambridge, Mass., and Huajian Gao from the Max Planck Institute for Metals Research in Stuttgart, Germany, (now at Brown University) have developed a new theoretical model to understand atomistic details of how cracks propagate in brittle materials, revealing the physics of dynamical fracture instabilities. As reported in the January 19 issue of Nature (p. 307; DOI: 10.1038/nature04408), their models indicate, in contrast to current theories, that it is critical to consider the properties of materials at large deformations, close to the crack tip, in order to understand how materials fracture. These findings have major implications for the understanding of fracture at different scales, ranging from the nanoscale to the larger scales of airplanes, buildings, or even earthquake dynamics, and predict that cracks can move at speeds faster than the velocity of sound, which has so far been considered an impenetrable barrier for crackpropagation speed. The deformation and fracture of materials has fascinated scientists for decades, due to both their scientific relevance as well as their significance in engineering and to society. In the past, the classical physics of the continuum has been the basis for most theoretical and computational tools in engineering analyses of these phenomena, and theories relying on numerous phenomenological assumptions have been used. Scientists are now beginning to use atomistic simulation as a tool to study the behavior of materials under extreme conditions in order to gain insights about the fundamental mechanisms of deformation and failure at length and time scales unattainable by experimental measurements and which cannot be predicted using continuum theories. This phenomenon of greater roughness for faster crack propagation is found in many different classes of brittle materials, including metals, polymers, and semiconductors, at a variety of scales. Until now, no sound understanding of the underlying physics or of the particular crack speed at which the instability occurs has been achieved. None of the existing theories explain