Relationships between acoustic emission signals and physical phenomena during indentation
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Relationships between acoustic emission signals and physical phenomena during indentation D. F. Bahra) and W. W. Gerberich Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 (Received 3 February 1997; accepted 29 November 1997)
A commercial piezoelectric acoustic emission transducer has been used in conjunction with nanoindentation techniques to study the relationship between acoustic emission signals and discrete physical events to identify the type and strength of an event. Indentations into tungsten and iron single crystals have been used to study dislocation generation and passive film failure. In addition, indentations made into a thin nitride film on sapphire have been used to cause film delaminations. Parameters such as signal rise time and frequency for a piezoelectric sensor are related to sample geometry, and not to the type of event which caused the acoustic emission signal. As a possible calibration for acoustic emission sensors, the most meaningful parameter is the acoustic emission energy, which has been shown to scale with the elastic energy released during the event. The measured values of elastic energy released correspond very closely to those calculated using Hertzian contact mechanics.
I. INTRODUCTION
Low load indentations, commonly referred to as nanoindentation, are used to measure a variety of mechanical properties of materials. These indentations monitor the relationship between the load applied to an indenter tip and the depth of penetration into a sample. Common measurements in both bulk and thin film systems focus on determination of the hardness and elastic modulus of a material.1 However, when testing both single crystals and thin film systems it is possible to generate phenomena which cause discontinuities in the load depth relationships.2,3 This type of behavior has been associated with dislocation emission,2,3 oxide film failure,2 and thin film delaminations.4 In principle, characterization of a physical event using acoustic emission would allow an in situ measurement of both the magnitude and type of event. Previous studies on the acoustic emission behavior of a variety of materials during indentations4 have shown that the speed at which an event occurs can be correlated to the type of event which led to the acoustic emission.5 The purpose of the present work is to further explore the relationships between an acoustic emission signal and the type and magnitude of the event that caused the acoustic emission. Since nanoindentation causes discrete, localized events, the ability to identify each physical event and correlate those events to the acoustic emission behavior will allow a direct comparison between the event and the individual acoustic emission event. a)
Present address: Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164. J. Mater. Res., Vol. 13, No. 4, Apr 1998
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