Acoustic emission waveforms produced by discrete crack propagation in rock

An experimental program was undertaken to investigate acoustic emission (AE) waveforms produced during discrete fracture propagation in rock. Large (0.3 m x 0.3 m x 1 m) chevron-notched specimens were loaded in controlled conditions, and characteristic acoustic emissions associated with Mode I and Mode II fracture were isolated. Two different rocks, granite and dolostone, were tested. Load, load point displacement, crack mouth separation, and time of AE event occurrence were logged.; Only the initial segment of each AE waveform was used for analysis, insuring an analysis free from effects of sample size and shape, with propagation effectively through an infinite half-space. High-fidelity, NIST-type, piezoelectric displacement transducers were built and used. The entire experimental system was calibrated, and system inverse filter applied to recorded signals. The system had little effect on the recorded signals, which were low-pass filtered to enhance waveform characterization procedures. Deconvolution was not necessary for these tests.; The effects of AE source position and orientation relative to the transducer was 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.; The AE waveforms belonged to five characteristic classes. By far the most common three classes are related to basic step impulses. The variety of AE event classes was the same in Mode I and Mode II loadings for both rock types. The mechanism for crack propagation in rock is thought to be due to localized tensile failure for both tensile and shear loadings.; Rates of occurrence of AE events varied during loading modes, and for the two materials. Very few events events occurred during load increase to peak or during unloading. Event rates increased as the crack propagated through the notch, and rates for subsequent constant-length crack propagation were constant. Relative roughness of the crack surface corresponded closely with relative distributions of the different characteristic event classes.