27 May 2011 Lockin-interferometric imaging of thermal waves for nondestructive testing
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Phase shifting shearography monitors the mechanical behaviour of an object under load, which makes it a valuable tool for non-destructive testing. However, it cannot determine the depth of defects, and sometimes, the gradient of the displacement of the whole object is so large that it hides small deviations caused by flaws. Our approach to overcome these limitations is based on shearographic imaging of the gradient of the displacement field of an object that is periodically loaded by a modulated excitation. After unwrapping the stack of fringe images, the local phase and amplitude of the periodical object displacement can be retrieved by a pixelwise discrete Fourier transformation. The displacement of the test object itself is mathematically reduced since only the sine-coded object response is extracted by the Fourier transformation. Depth range can be adjusted since the thermal diffusion length of the thermal waves depends on their frequency. Since all images are used for evaluation (and not only one fringe image like in conventional speckle-interferometry), the signal-to-noise ratio is substantially increased. This paper discusses the performance of this technique on model samples and demonstrates the advantages of this approach on modern automotive and aerospace structures.
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Philipp Menner, Philipp Menner, Gerd Busse, Gerd Busse, } "Lockin-interferometric imaging of thermal waves for nondestructive testing", Proc. SPIE 8082, Optical Measurement Systems for Industrial Inspection VII, 808223 (27 May 2011); doi: 10.1117/12.889136; https://doi.org/10.1117/12.889136

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