Cracks faster than the speed of sound

How fast can cracks move? This question is not only of significance to safety engineering but also to earthquake dynamics.

There are three fundamental wave speeds in solids. In order of increasing magnitude, they are the Rayleigh wave speed, or the speed of sound on a solid surface, the shear (transverse) wave speed and the longitudinal wave speed. Existing theories of fracture have led many scientists to believe that the Rayleigh wave speed is the limiting speed of crack propagation.Is this really true?

Glass breaks, rubber bursts—there are numerous ways how materials can fail under extreme conditions. However, many of the atomic mechanisms of materials failure still remain a mystery. Some materials harden when they are stretched, others soften under large deformation. This phenomenon is referred to as hyperelasticity. We study the dynamics of cracks using the world's most powerful computers, whereby the motion of every single atom in the material is calculated according to Newton's laws of motion.

Combining theoretical considerations and large scale molecular dynamics simulations, we derived the conditions under which hyperelasticity governs dynamic fracture. We discovered that cracks can propagate supersonically when hyperelasticity, the elasticity at large strains, becomes dominant within a zone of high energy transport near the crack tip. This is important in understanding the dynamics of earthquakes or nucleation and propagation of cracks in aircrafts and space shuttles. The results are in clear contrast to classical theories in which the speed of elastic waves was considered the limiting speed of fracture, analogous to the speed of light in the theory of relativity (M.J. Buehler et al., Nature 426 , pp. 141-146 , 2003).

Copyright (c) 1999-2005 Markus J. Buehler. All rights reserved.