An examination of acoustic emission (AE) signals in terms of their amplitude allows one to have a better appreciation of the mechanical behavior of rock materials. During a recent experimental study on the development of microfracturing processes in rocks, AE signals generated from rock specimen stressed to failure in uniaxial compressive tests were recorded on a SONY (AV-3650) videocorder. Various AE parameters, including: amplitude, pulse width, ringdown counts, rms value, etc ..., were extracted during later videotape playback. The results of the analysis of amplitude distribution are presented in detail in this paper. The discussion includes precursor of material failure and the phenomenon of AE quiescence during the failure process. The various stages of specimen deformation were also considered in terms of AE activities.
Inelastic deformation due to microfracturing has been extensively studied in recent years in order to understand, analyze and model the basic mechanisms underlying the development of micro-fracturing processes in rock under various loading conditions. The development of such a physically based model is a critical step in the extrapolation of laboratory results to the long-time scales associated with geophysical phenomena such as earthquakes and their recurrence intervals, as well as to the engineering analysis of the stability of mine openings and associated support pillars (Costin, 1987; Atkinson, 1987). It has been found through direct observation under the scanning electron microscope that microcracks originate from local stress concentrations that occur due to mismatches in elastic properties either at grain boundaries or at natural flaws and pores (Tapponnier and Brace, 1976). With increases in stress, these microcracks begin to nucleate and when the stress concentration becomes critical they start propagating in a direction parallel to the greatest compressive stress (Wawersik and Brace, 1971; Hallbauer et al., 1973; Kranz, 1983). Although the rate of crack growth is a strong function of the crack-tip stress intensity factor, it has been noted in recent years that cracks may undergo slow and stable growth (subcritical crack growth) at stresses that are far below those required for fracture initiation either in the presence of chemically active environment due to stress-aided corrosion (Anderson and Grew, 1977; Waza et al., 1980; Atkinson, 1982; Costin and Mecholsky, 1983) or under cyclic fatigue (Hardy and Chugh, 1970; Holcomb, 1981; Sondergeld and Estey, 1981' Costin and Holcomb, 1981; Rao, 1987). Such environmental features are normally encountered in seismically active areas and also in the working panels of underground mines. Therefore more detailed laboratory experiments are necessary to examine the development of nficrofraturing processes in rock and to estimate and evaluate nficrocrack damage in rocks under such conditions.
Figure 1 Differential event amplitude distribution shown at increasing stress levels in Berea sandstone specimen ( S- 11 ). [ Values in brackets beside vertical axis are associated range of applied stress in terms of percentage of failure stress ].(available in full paper)
Figure 2 Detailed record of amplitude distributionear the failure point in Berea sandstone specimen ( S- 11 )
Figure 3 Comparison of AE signals sampled at 70% of the failure stress (3a) and just prior to failure (3b)
