ABSTRACT
One of the major problems encountered in using Acoustic Emission(AE) techniques to monitor structures in the field is the difficulty in separating AE signals from crack growth , from signals due to extraneous noise sources. These extraneous noise sources can be created by frictional rubbing, impact of particles on the structure being monitored and leaks by pressurized components. Most extraneous noise sources of this type are out-of-plane (OOP sources) and although they can have very high frequency components in an undamped structure, most of the energy in the stress waves created in most structures constructed from plates, can be found at frequencies below 100KHz. This energy is carried by a low frequency flexure wave in the plate. Since most field tests in the past have been conducted with high pass filters at 100KHz and above using resonant transducers, few practitioners were even aware that these waves were present prior to recent work by (Gorman and Prosser 1990). AE signals generated by crack growth are in-plane sources(IP sources) and most of the energy in the stress wave is carried by high frequency extensional and shear waves. This report shows how special transducer and instrumentation techniques can be used to recognize the type of wave predominate in a plate to allow filters to be constructed in the instrumentation to not only eliminate extraneous noise sources from the AE data, but also have the potential of measuring the depth of a growing crack in a plate.
