We present a time-resolved acoustic method capable of monitoring the variations of linear and nonlinear elastic/dissipative parameters of a medium. After the identification of one of the acoustic resonances of the sample (typically the lowest), alternating probings of this resonance are carried out at different excitation amplitudes, every second typically. The time evolution of the resonance properties (maximum amplitude, resonance frequency, quality factor) are recorded for the different excitation amplitudes. From these data, several linear and nonlinear elastic/dissipative parameters of the probed medium can be extracted, such as the hysteretic nonlinear elastic parameter and the hysteretic nonlinear dissipative parameter.
This method is demonstrated here in the case of the fast relaxation of the linear and nonlinear, elastic and dissipative parameters of a granular packing occurring just after a small mechanical solicitation. This relaxation is observed to last seconds to minutes and is of strong amplitude (several tens of % change in the parameter values). The extracted parameters exhibit different characteristic relaxation times that are studied as a function of several experimental control parameters (humidity, temperature, compacity) with the help of inside a climatic chamber. The obtained results on relaxation are consistent with a thermally activated process of capillary condensation at the nanometric asperities of the inter-bead contacts. Physical interpretations based on this process are discussed. It is concluded that effects of the same nature as those reported here, are observed in cracked materials and could be probed by this method. Applications of the developed method to the non destructive evaluation of materials is finally proposed.