Instability, chaos, and "memory" in acoustic-wave-crack interaction.

A new class of nonlinear acoustic phenomena has been observed for acoustic wave interactions with cracked defects in solids. Parametric modulation of crack stiffness results in fractional acoustic subharmonics, wave instability, and generation of chaotic noiselike acoustic excitations. Acoustic-wave impact on a crack is shown to exhibit amplitude hysteresis and storage for parametric and nonlinear acoustic effects. The measured storage time amounts to several hours and is believed to be due to a long-term relaxation of thermally induced microstrain within a crack area.