Acoustically induced slow dynamics in nonlinear mesoscopic elastic materials

We have known about slow dynamics in rock due to continuous wave excitation drive for several years (http://www.ees4.lanl.gov/nonlinear). TenCate, Smith, and Guyer (see abstract, this meeting) have recently discovered that both the elastic modulus and the wave dissipation display log time recovery in granular solids, and that it may be thermally or mechanically induced. Much to our surprise, we have discovered that a CW or broad-frequency band acoustic source can also induce slow dynamical response. This response was observed as a variation of the ultrasonic probe wave amplitude, the resonance frequency, and Q factor after the action of a pump wave. The slow time recovery took place in materials such as powdered metals, damaged materials, concrete, and rocks as well. The variations of material properties due to the action of pump waves lead to transient amplification and an obscuration of CW probe waves. The observed behavior may be more universal than was first thought. The results have potential implications to many topics, including laboratory wave studies, earthquake strong ground motion, elastic waves emanating from a point source, damage detection, and manufacturing processes. [Work supported by Stevens and by the Department of Energy: Office of Basic Energy Sciences.]