The last time that you heard something break, you might have wondered whether the sound emitted is important to the fracture process. On reflection, you probably concluded that it is insignificant, since the sound emitted in the fracture of a brittle material is not very loud. Indeed, the energy contained in the sound wave makes up only a small percentage of the energy released during fracture (about 5% in glass).
But new work by Jean-Francois Boudet and Sergio Ciliberto at the Ecole Normale Supérieure de Lyon in France indicates that the sound emitted during fracture can strongly influence the way in which a crack propagates (Phys. Rev. Lett . 1998 80 341). This result might at first glance seem surprising. After all, the effect of the sound might be expected to depend on its intensity, which is small. Moreover, the solid fractures when the load is relatively small, suggesting that the material's macroscopic behaviour should be well described by linear elasticity.
But the response of the material at the crack tip is almost certainly nonlinear, because the crack magnifies the applied load and leads to high stresses in the space around its tip. It is in this context that the sound makes an impact, since even a small perturbation can significantly alter the behaviour of an unstable nonlinear system.
The full article by Steven Gross of Princeton University appears in Physics World magazine (information).