Cyclic loading of marble: Correlating the attenuation of the electric and acoustic activities and highlighting criticality indices in terms of natural time

The attenuation of the acoustic activity in marble specimens under uniaxial compressive loading-unloading loops is quantified in juxtaposition to that of the electric activity. In parallel, the existence of “pre-failure indices” warning about entrance into a critical stage, that of impending fracture, is explored. The acoustic activity is quantified in terms of the normalized number of acoustic hits, their average rate of production and their cumulative energy, and, the cumulative counts and their average rate of change. The electric activity is studied in terms of the pressure stimulated currents and the electric charge released. The analysis revealed that the acoustic and electric activities are linearly correlated to each other, suggesting that they are different manifestations of the same damage mechanisms. In addition, Kaiser’s effect, governing the acoustic activity, is found to govern, also, the electric activity. Moreover, it is concluded that entrance into the critical stage is safely predicted by means of a simple criterion, based on the evolution of the average rate of change of the normalized cumulative counts in the natural time domain. These predictions are almost identical with those of the criterion based on the “variance” and the “entropies” of the time series of acoustic events in this domain.