Physics (Internal)

Unlocking mysteries of crack propagation: hidden paths and instabilities

This event is part of the Physics Department Colloquia Series.

Abstract: Crack propagation is the dominant mode of mechanical failure of most man-made and naturally occurring materials. Understanding this phenomenon poses a number of challenges for physicists, materials engineers, and Hollywood film makers that are addressed in this talk. Firstly, and mostly of interest to physicists, crack propagation is a truly multiscale phenomenon with no clear separation of scale between atomic and continuum scales, i.e. no simple hydrodynamic limit. Consequently, how to formulate equations of motion for cracks is still an open question. Secondly, in many situations, crack fronts become dynamically unstable, thereby following convoluted paths inside a material and forming complex patterns that are hard to deduce. Those paths need to be guessed from post-mortem analysis of crack surfaces due to the inherent difficulty to visualize the actual front evolution during the failure process. Thirdly, materials with desirable mechanical properties such as high strength and fracture toughness are generally highly heterogeneous, as exemplified by the hierarchical composite structure of bone, and those heterogeneities greatly complexity failure mechanisms. This talk provides an introduction to the basic physics of cracking and discusses a class of continuum models of brittle fracture that self-consistently bridges the descriptions of short scale failure and continuum elasticity. The power of this approach to reveal hidden crack paths and understand crack-front instability mechanisms is then demonstrated for selected experiments. The talks concludes with prospects for improving materials design and film making.