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12 April 2004 Experimental considerations in vibrothermography
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Sonic, or thermosonic nondestructive testing, which is based on the vibrothermography method introduced in the late 1970’s, has attracted a great deal of recent interest as a means for detection of cracks that were previously considered to be undetectable using thermographic inspection methods. Excitation of a solid sample with bursts of high-energy (500 - 3000 Joule), low-frequency (10 - 50 kHz) acoustic energy has been demonstrated to be effective in generating transient localized heating at crack sites, making them detectable by an infrared camera. Despite the apparent simplicity of the scheme, there are a number of experimental considerations that can complicate, or in some cases even prevent, the implementation of vibrothermography-based inspection. Factors including acoustic horn location, horn-crack proximity, horn-sample coupling, and effective detection range all significantly affect the degree of excitation (or whether any excitation occurs at all) that occurs at a crack site for a given energy input. In cases where the experimental objective is precise measurement of crack length, the method used to visualize the data from the IR camera and its optic must also be taken into consideration.
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Steven M. Shepard, Tasdiq Ahmed, and James R. Lhota "Experimental considerations in vibrothermography", Proc. SPIE 5405, Thermosense XXVI, (12 April 2004);

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