Dynamic cleavage fracture experiments of brittle single crystal silicon revealed several length scales of path and surface instabilities: macroscale path selection, mesoscale crack deflection, and nanoscale surface ridges. These phenomena cannot be predicted or explained by any of the continuum mechanics based equations of motion, as critical energy dissipation mechanisms associated with atomistic scale vibrations are not accounted for in the theories. Experimentally measured maximum crack speed, always lower than the theoretical limit, is another phenomenon that is as yet not well understood. The thermally activated phonon emission energy release rate (or heat) dissipated during dynamic crack propagation on several cleavage systems of brittle single crystal silicon was calculated by means of molecular dynamics atomistic computer simulations. The calculations yielded the anisotropic and velocity dependent thermal phonon emission energy release rate. This energy dissipation mechanism is considered a material property, which explained and rationalized the multi-scale surface and path instabilities phenomena obtained in the cleavage experiments. It also explained the inability of a crack to attain the theoretical limiting speed. It is therefore concluded that the phonon emission energy release rate constitutes an essential energy dissipation mechanism involving dynamic crack propagation and therefore, additional energy term was incorporated in Freund equation of motion. This term includes size effect associated with this dissipative mechanism.
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