4 April 2012 Obtaining more information from time-of-flight-diffraction measurements
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Abstract
Time of flight diffraction and imaging (TOFDI) is based on time of flight diffraction (TOFD); it adds cross-sectional imaging to examine the bulk of a sample. Multiple wave modes are generated by a pulsed laser beam, ablative source and are received by a sparse array of non-contact electromagnetic acoustic transducers (EMATs). A B-scan is formed from multiple data captures (A-scans), with time and scan axes, and colour representing amplitude. B-scans may contain horizontal lines from surface waves propagating directly from emitter to receiver, or via a back-wall reflection, and angled lines after reflection off a surface edge. A Hough transform (HT), modified to deal with the constraints of a Bscan, can remove such lines. A parabola matched filter has been developed to identify features in the B-scan caused by scattering from point-like features, reducing them to peaks. The processed B-scan is processed further to form a crosssectional image, enabling detection and positioning of multiple defects. Phase correlation of camera images is used to track the relative position between transducer and sample to sub-pixel precision.
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Stuart B. Palmer, Steve Dixon, Phil A. Petcher, "Obtaining more information from time-of-flight-diffraction measurements", Proc. SPIE 8347, Nondestructive Characterization for Composite Materials, Aerospace Engineering, Civil Infrastructure, and Homeland Security 2012, 83470Z (4 April 2012); doi: 10.1117/12.913605; https://doi.org/10.1117/12.913605
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