Strain localization peculiarities and distribution of acoustic emission sources in rock samples tested by uniaxial compression and exposed to electric pulses

Abstract

Results of uniaxial compression tests of rock samples in electromagnetic fields are presented. The experiments were performed in the Laboratory of Basic Physics of Strength, Institute of Continuous Media Mechanics, Ural Branch of RAS (ICMM). Deformation of samples was studied, and acoustic emission (AE) signals were recorded. During the tests, loads varied by stages. Specimens of granite from the Kainda deposit in Kyrgyzstan (similar to samples tested at the Research Station of RAS, hereafter RS RAS) were subject to electric pulses at specified levels of compression load. The electric pulses supply was galvanic; two graphite electrodes were fixed at opposite sides of each specimen. The multichannel Amsy-5 Vallen System was used to record AE signals in the six-channel mode, which provided for determination of spatial locations of AE sources. Strain of the specimens was studied with application of original methods of strain computation based on analyses of optical images of deformed specimen surfaces in LaVISION Strain Master System. Acoustic emission experiment data were interpreted on the basis of analyses of the AE activity in time, i.e. the number of AE events per second, and analyses of signals' energy and AE sources' locations, i.e. defects. The experiment was conducted at ICMM with the use of the set of equipment with advanced diagnostic capabilities (as compared to earlier experiments described in [Zakupin et al., 2006a, 2006b; Bogomolov et al., 2004]). It can provide new information on properties of acoustic emission and deformation responses of loaded rock specimens to external electric pulses. The research task also included verification of reproducibility of the effect (AE activity) when fracturing rates responded to electrical pulses, which was revealed earlier in studies conducted at RS RAS. In terms of the principle of randomization, such verification is methodologically significant as new effects, i.e. physical laws, can be considered fully indubitable if they prove stable when some parameters of the experiment are changed. Parameters may be arbitrarily modified within a small range, and randomization is thus another common statistical significance criterion for sample sets obtained at the same conditions. At ICMM, the experiments were conducted in compliance with the principle of randomization [Bogomolov et al., 2011]. In this respect, the material of specimens, loading conditions and characteristics of the electrical pulses source were similar to those in the experiments at RS RAS. As evidenced by the experiments, during electromagnetic field stimulation, the AE activity is manyfold higher than the background activity before the impact. This supports the research results reviewed in [Bogomolov et al., 2011] concerning the AE activity increment of 20 % due to electric pulses in the field twice less strong than that in our experiments at ICMM. The AE energy distribution analysis shows that cumulative distributions of the number of AE signals vs energy (i.e. the number of AE signals which energy exce...