ABSTRACT

In this paper the results of a study on two main acoustic emission (AE) parameters – hit rate and energy- measured in laboratory on rock samples prior to failure are presented. Uniaxial test were carried out on Lavasan granite specimens, dimensions 45 mm diameter and 100 mm height, using a servo–controlled compression device with a constant rate of displacement. Four transducers were used for monitoring the acoustic emissions during loading. Acoustic emission event-hit data were recorded continuously until the failure of the specimens. Axial and radial deformations have also been measured by electric resistance gauges. Plotting of accumulated hit and the accumulated absolute energy versus time shows the absolute energy is a more appropriate parameter to monitor crack propagation and to predict catastrophic failure of specimens than the conventional AE's hit. By comparing the AE steps with volumetric deformation, we found that AE energy show definitively each steps of deformation that presented by Bieniawski (1967).

1 INTRODUCTION

A number of techniques have been developed to detect crack growth and to study failure evolution in brittle materials. The most common of these involves the use of electric resistance strain gauges to measure slight changes in sample deformation that can be related to the closing and to the opening of cracks (Bieniawski, 1967). To a lesser extent, acoustic emission monitoring has been used to correlate the number of acoustic events to various strain gauge responses (Ohnaka and Mogi 1982; Khair, 1984; Lockner et al, 1991 and Eberhardt et al, 1998). Acoustic emissions (AE) are the stress waves produced by the sudden internal stress redistribution of the materials caused by the changes in the internal structure. Most of the sources of AEs are damage-related; thus, the detection and monitoring of these emissions are commonly used to predict material failure (Huang et al, 1998).

Recording the number of AE events is the most conventional method for evaluation of damage procedure in rock. In addition, to recording the number of events and correlating this number to the measured deformation response in the rock, it is also possible to record certain properties of the AE waveforms. Generally, these waveforms are complex and using them to characterize the source can be difficult. Due to these complexities, AE waveform analysis can range from simple parameter measurements to more complex pattern recognition (Eberhardt, 1998). The characteristics of an acoustic event may also be used to approximate the release of kinetic energy through the AE event. The true energy is directly proportional to the area under the acoustic emission waveform which in turn can be measured by digitizing and integrating the waveform signal. The energy of event can be approximated as the square of the peak amplitude(Lockner et al., 1991) or the square of the peak amplitude multiplied by the event duration (Hardy, 2003). The resulting values are actually more representative of the event power (the units are given in dB), but are commonly referred to as energy calculations in the literature due to their approximately linear relationship with energy.

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