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12 April 2004 Defect depth retrieval from pulsed phase thermographic data on Plexiglas and aluminum samples
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In the last few years, several quantitative inversion methods have been proposed to analyze pulsed phase thermographic data: statistical methods [1], Neural Networks [2] and wavelets [3], with a wide range of reported accuracies. In the present paper a new approach is proposed based on absolute phase contrast computations defined in a similar way as for absolute temperature contrast [4]. Phase contrast data is then used to estimate the blind frequency, i.e. the frequency at which the defect becomes visible for the 'first' time [5]. It was found an excellent agreement between defect depth z, and the corresponding blind frequencies fb. Experimental tests on Plexiglas and aluminum specimens demonstrate the potential of the technique on retrieving the depth of flat-bottomed holes. We also discuss temporal aliasing and its relationship with the phase delay images. As will be stressed, the unavoidable differences between the Continuous and the Discrete Fourier Transform of a time-dependent temperature decay signal can be effectively minimized not only by selecting a sampling frequency rate according to Shannon's Sampling Theorem (as is well-known [6]), but also by choosing an appropriate truncation window size [7].
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Clemente Ibarra-Castanedo and Xavier P. Maldague "Defect depth retrieval from pulsed phase thermographic data on Plexiglas and aluminum samples", Proc. SPIE 5405, Thermosense XXVI, (12 April 2004);


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