Acoustic Emission (AE) techniques have long been used as a non-intrusive and non-destructive method to study brittle fracture, as well as other avalanche phenomena.
In our experiments we use piezoelectric transducers(micro-80) to detect ultrasonic (100 kHz-MHz) AE signals produced during the collapsing of different porous materials (Vycor (SiO2), Goethite (FeO(OH) and Alumina (Al2O3) ) subject to different steady uniaxial compression rates. The identification of signal events related to internal microcracks allows us to study the phenomena as a point-process and identify the statistical laws that depicts the dynamics of the underlying avalanche process.
Non-surprisingly, we found profound analogies with seismological laws: from the scale-free distribution of energies, to the production of aftershocks and foreshocks, and the recent Unified Scaling Law (USL) for the distribution of waiting times. The stationarity, rate independence and scale invariance of these laws in the case of Vycor, could be a trademark of an underlying critical phenomena.