19 April 2011 Detection of surface breaking cracks using thermographic and non-contact ultrasonic methods
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A combined ultrasound and thermography defect detection system using a raster scanned Q-switched laser as a source of heat and ultrasound has been developed for identifying surface breaking defects. Heat is generated on a sample surface by a laser source and the resultant thermal image is examined by a thermal imaging camera. This can be done using a cw or a pulsed laser, but for ultrasonic generation a pulsed laser beam is required. When a defect is present, the flow of heat in the sample is disturbed and a change in shape of the thermal spot on the sample's surface can be detected. The pulsed laser beam generates simultaneously an ultrasonic wave that can be detected by a suitable transducer, which in this case is an electromagnetic acoustic transducer (EMAT). The presence of a defect changes both the amplitude and frequency content of the received wave. Three dimensional finite element modelling of the interaction between Lamb waves and defects have been studied and compared with experimental data, in order to optimise source and detector positions around a defect. The approach can detect surface crack defects via the ultrasonic and thermography method in one measurement.
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S. B. Palmer, S. B. Palmer, S. E. Burrows, S. E. Burrows, S. Dixon, S. Dixon, "Detection of surface breaking cracks using thermographic and non-contact ultrasonic methods", Proc. SPIE 7983, Nondestructive Characterization for Composite Materials, Aerospace Engineering, Civil Infrastructure, and Homeland Security 2011, 79831V (19 April 2011); doi: 10.1117/12.881198; https://doi.org/10.1117/12.881198

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