Time of flight diffraction and imaging (TOFDI)
Petcher, P. A.
Time of flight diffraction and imaging (TOFDI) is based on time of flight diffraction (TOFD),\ud adding cross-sectional imaging of the sample bulk by exploiting the scattering of ultrasonic waves\ud from bulk defects in metals. Multiple wave modes are emitted by a pulsed laser ultrasound ablative\ud source, and received by a sparse array of receiving electromagnetic acoustic transducers (EMATs),\ud for non-contact (linear) scanning, with mode-conversions whenever waves are scattered.\ud Standard signal processing techniques, such as band-pass filters, reduce noise. A B-scan is\ud formed from multiple data captures (A-scans), with time and scan position axes, and colour\ud representing amplitude or magnitude. B-scans may contain horizontal lines from surface waves\ud propagating directly from emitter to receiver, or via a back-wall, and angled lines after reflection\ud off a surface edge. A Hough transform (HT), modified to deal with the constraints of a B-scan,\ud can remove such lines. A parabola matched filter has been developed that identifies the features\ud in the B-scan caused by scattering from point-like defects, reducing them to peaks and minimising\ud noise. Multiple B-scans are combined to reduce noise further. The B-scan is also processed to form\ud a cross-sectional image, enabling detection and positioning of multiple defects.\ud The standard phase correlation technique applied to camera images, has been used to track\ud the relative position between transducer and sample. Movement has been determined to sub-pixel\ud precision, with a median accuracy of 0.01mm of linear movement (0.06 of a pixel), despite uneven\ud illumination and the use of a basic low resolution camera.\ud The prototype application is testing rough steel products formed by continuous casting, but\ud the techniques created to facilitate operation of TOFDI are applicable elsewhere.
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