Steel billet inspection using laser-EMAT system

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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“…Each [1], [14] . Th obscured by a , as depicted 0mm × 123mm block is inspe e top and bot or, the standa [15] .…”

Section: Time Of Flight Diffraction (Tofd)mentioning

confidence: 99%

Obtaining more information from time-of-flight-diffraction measurements

2012

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“…Non-contact ultrasonic techniques, including laser ultrasonics and electromagnetic acoustic transducers (EMATs), are becoming more widely accepted [4]. These are inefficient when compared to piezoelectric transducers, but this can be mitigated by their lack of requirement for couplant and the ability to operate in harsh environments out of contact with the sample [3,[7][8][9].…”

Section: Introductionmentioning

confidence: 99%

“…They can work without direct physical contact to the samples, with no coupling gel required, allowing them to be utilised in a variety of situations where standard piezoelectric ultrasonics are not viable, such as on moving, hot, rough, or rusted surfaces [3,[7][8][9]. EMATs as generators consist in their simplest form as a coil of wire through which a current is pulsed [4].…”

Section: Introductionmentioning

confidence: 99%

Focused Rayleigh wave EMAT for characterisation of surface-breaking defects

Thring

Yang

Edwards

2016

NDT & E International

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Developments towards higher resolution and the ability to detect small defects are bringing a step-change in non-destructive testing. This paper presents a new method for increasing resolution, using a focused electromagnetic acoustic transducer (EMAT) optimised to generate Rayleigh waves at 2 MHz. This high frequency allows detection of mm-depth defects, and the focusing allows sizing of much shorter defects than is possible when using standard EMATs. The focusing behaviour and the aperture angle effect are analysed using laser vibrometry and finite element modeling, showing that a reduced aperture shifts the focal point from the designed value and increases the focal depth. The dual-EMAT has excellent signal to noise ratio (up to 30 dB) and has been used in single shot mode to image a variety of surface-breaking defects, including detecting and positioning a pair of real defects in an aluminium billet sample, and a machined defect of 2 mm length, 0.2 mm width, and 1.5 mm depth, giving an upper limit on the defect length of 2.1±0.5 mm. The results can be used to design an EMAT with optimised focal behaviour for defect detection.

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“…Its applications include weld quality inspection [1], surface and internal defect testing [2][3][4][5], thickness estimation [6], testing of bonding and characterization of materials [7]. Its non-contact feature during operation makes it very attractive for on-line inspections [8].…”

Section: Introductionmentioning

confidence: 99%