|Ph.D Student||Atrash Fouad|
|Subject||The Role of Phonon Emission in Dynamic Crack Propagation in|
Brittle Single Crystals
|Department||Department of Materials Science and Engineering||Supervisor||Dr. Dov Sherman|
|Full Thesis text|
Dynamic crack propagation is a ubiquitous problem with still several fundamental questions that have to be solved and that are of broad interest for various engineering and scientific communities. For example continuum mechanics sets the Rayleigh surface waves speed, CR, as the upper limit for crack propagation under tensile stresses but the fastest crack speed measured experimentally is only 0.7-0.9 CR. In this research, based on previous experimental work, we suggest that the key for resolving these inquiries is hidden in the lattice vibrations or thermal phonons, emitted from the crack tip during the crack dynamics. Thus, the goal of the present work is to quantify, computationally, the contribution of the phonon emission energy to the total energy balance and how it determines the fracture dynamics, specifically in brittle single crystals.
Molecular dynamics simulations of brittle fracture in silicon-like brittle single crystal were performed. We suggest a novel computational procedure to evaluate the contribution of phonon emission to the dynamical energy release rate in the modeled system. We used a pre-cracked strip-like specimen, subjected to prescribed displacement on the boundaries, commonly used in atomistic simulations. A typical computational specimen included about 120,000 atoms. Different cleavage planes and directions for a wide range of crack velocities were investigated.
We show that the phonon emission energy is different for dissimilar crack planes and propagation directions and that it is responsible for various phenomena at several length scales. Our MD results predicted the existence of energetically preferred crack systems in silicon at the atomic scale, it was found that crack propagating along the [21-3] direction on the (111) cleavage plane dissipate less energy by phonon emission than other directions on the same plane, this result was experimentally observed by us. Our MD results help in the description of the cleavage plane deflection phenomena observed in silicon fracture experiments. We investigated the variation of the phonon energy with the sample’s size. It was found that the terminal crack speed increases with volume at the micrometer scale; we also show that the phonon emission energy is responsible for the inability of cracks to attain the theoretical limiting speed on the macro scale. We propose to include the contribution of the phonon emission energy in the Freund equation of motion for dynamically propagating cracks.