Ab initio dynamics of rapid fracture∗

As our title implies, we consider materials failure at the fundamental level of atomic bond breaking and motion. Using computational molecular dynamics, scalable parallel computers and visualization, we are studying the failure of notched solids under tension using in excess of 108 atoms. In rapid brittle fracture, two of the most intriguing features are the roughening of a crack’s surface with increasing speed and the terminal crack speed which is much less than the theoretical prediction. Our two-dimensional simulations show conclusively that a dynamic instability of the crack motion occurs as it approaches one-third of the surface sound speed. This discovery provides an explanation for the crack’s surface roughening and limiting speed. For three-dimensional slabs, we find that an intrinsically ductile FCC crystal can experience brittle failure for certain crack orientations. A dynamic instability also occurs, but brittle failure is not maintained. The instability is immediately followed by a brittle-to-ductile transition and plasticity. Hyperelasticity, or the elasticity near failure, governs many of the failure processes observed in our simulations and its many roles are elucidated. ‘Refrain from immediately resorting to a jackhammer to break open a hazelnut without first checking for an incipient crack on the surface of the shell.’ P-G de Gennes¶ ‘You may think I have used a hammer to crack eggs, but I have cracked eggs.’ S Chandrasekhar+