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

Abstract

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.

1. Introduction

The acoustic emissions (AE) technique is the most mature and widely used structural health monitoring (SHM) technique. It is a passive monitoring approach, based on the detection of elastic waves generated by damages (such as the initiation and propagation of cracks, failure of steel wires or fibers, failure of bonds etc.) and the corresponding stress-redistribution [1]. The history of the development of the technique and its fundamentals are excellently described in a comprehensive overview by Ohtsu [2]. The major advantage of the technique is that it detects damages in areas which are not easily accessible to visual inspections and direct measurements since the elastic waves propagate throughout the structure. The data provided by the AEs are valuable not only in the direction of detecting damages but, also, in the direction of revealing the sequence of the damage mechanisms activated and quantifying the level of damage [3], permitting, thus, estimations about the remaining load-carrying capacity of the structure (or, equivalently its proximity to critical damage levels) [4].

Besides acoustic emissions, additional sensing techniques have been developed in the direction of taking advantage of electric emissions generated while rock samples are loaded at levels approaching the ones causing fracture. These techniques are analogues to similar ones used for in-field geophysical applications [5]. The electric signals detected are proven to be sensitive to latent damage mechanisms, as it is, for example, the development of networks of microcracks prior to macroscopic failure [6]. In fact, it is experimentally verified that during fracture (of either piezoelectric or non-piezoelectric materials) weak electric currents are emitted, denoted as pressure stimulated currents (PSC) [7–10]. Analogous recordings of electrical signals are detected by means of electric potential sensors denoted as pressure stimulated voltages (PSV) or electric potentials (EP) [11–14].

Concerning the underlying mechanisms responsible for the generation of these electric signals, a generally accepted theory does not exist as yet. For example, for the electric signals detected during uniaxial deformation of dry rock samples (nonpiezoelectric), it is supposed that they are related to the processes of nucleation, propagation and coalescence of micro-cracks, which trigger electrification mechanisms, a little before macroscopic fracture [6]. Other approaches attribute these electric signals to the electro-kinetic effects.