ABSTRACT
Acoustic emissions (AE) are transient elastic waves resulting from the sudden release of energy during the failure process. They can be detected at the surface of a specimen by a transducer that converts an acoustic-pressure pulse into an electrical signal of very low amplitude. The major element of an AE transducer is a piezoelectric ceramic mounted with little backing, so that the response is under damped. At the optimum sensitivity, the voltage generated by a l^bar acoustic pressure has a typical value of 0.3 mV. For laboratory studies on materials such as rock and concrete, transducers that are sensitive between
A problem, however, arises for rock-like materials. Often times a structure composed of rock has been observed to fail at a stress 50% lower than a strength value determined in the laboratory. Finding a lower limit of the apparent strength would then be a valuable exercise.
