C. M. Fortunko M. A. Hamstad D. W. Fitting
Natl. Inst. of Standards and Technol., 325 Broadway, Boulder, CO 80303
Acoustic emission (AE) source characterization requires the use of broadband, typically 20 kHz--2 MHz, ``point-contact'' sensors. Generally, AE sensors based on the NIST ``conical'' transducer approach can meet this requirement. However, because naturally occurring AE events are considerably less energetic than those produced by artificial AE sources, optimization of the sensor signal-to-noise performance is of critical importance. Using a computer model, a variety of sensor-design approaches have been studied, with particular emphasis on transducer/preamplifier/specimen compatibility issues. In this paper, computer-model predictions are compared with the results of experiments. The use of different piezoelectric transducer materials---PZT-5A, lead metaniobate, LiNbO[sub 3], X-cut quartz, and PVDF---is considered. It is shown that ceramic materials exhibiting the highest dielectric constants, (epsilon)[sub 33][sup S], are best suited for most high-performance AE sensor applications. In addition, the impact of preamplifier input capacitance, including the Miller effect, and specimen mechanical impedance on the performance of a ``point-contact'' sensor is discussed. In particular, using the computer model, the limitations of sensors that use piezoelectric ceramics and single-crystal materials are examined. It is shown that in certain situations, such as very thin metal plates and materials that exhibit very low characteristic impedances, sensors that use piezoelectric ceramic and single-crystal transducers may not be the best choice. Therefore, to address such cases, alternative sensor approaches are needed.