Acoustic Emissions Produced by Discrete Fracture in Rock

An experimental programme was undertaken to develop a servo-controlled loading and high-fidelity data acquisition system to ~westigate acoustic emission (AE) waveforms produced during discrete fracture propagation in rock. Large (0.3 × 0.3 × I m) chevron-notched specimens were loaded in controlled conditions, and characteristic acoustic emissions associated with Mode I and Mode l l fracture were isolated. Two different rocks, granite and dolostone, were tested. Only the initial segment of each AE waveform was used for analysis, ensuring an analysis free from effects of sample size and shape, with propagation effectively through an infinite half-space. High-fMelity, NISTtype, piezoelectric displacement transducers were built and used. The entire experimental system was calibrated, and the system inverse filter was applied to recorded signals. The system had little effect on the recorded signals and decotwolution was not necessary for these tests. The effects of AE source position and orientation relative to the transducer were studied. A reproducible step impulse source was used. Changes in source orientation were found to result in two types of waveform from a common source--a step-like waveform which changes to a ramp-like waveform when the source was directed with a large angle to the surface being monitored. The effects of source position were small.