EMAT Generated Shear Horizontal Wave Interactions with Defects in Welded Steel Plate Structure

Remote inspection capabilities for double shell tank bottoms are desired for managing hazardous waste. The effectiveness of electromagnetic acoustic transducers (EMATs) to be used as a remote inspection tool for tank bottoms was evaluated on specially constructed mockups. The EMATs are designed specifically for the access limitations and to enable ultrasonic guided wave-based methods that provide optimal coverage. Lorentz force EMATs were selected because they are noncontact devices that are suitable for robotic delivery. They are effective at emitting and receiving shear horizontal (SH) waves that can propagate long distances. It is the scattering and reflection of the SH waves from cracks and corrosion-induced defects that makes defect detection possible. Artificial defects (i.e., notches, pits, and wall-thinning) seeded in the welded steel mockup provided a means of assessing effectiveness of the EMATs and the pitch-catch, pulse-echo, and through-transmission inspection methods. Signal processing of pulse-echo and through-transmission results was very straight-forward. Defect detection in pulse-echo mode is based upon receiving an echo from a defect when previously there was no signal. While in through-transmission mode, detection is based upon a reduction in amplitude from a strong signal. Moreover, due to the large beam divergence, the pitch-catch mode was quite versatile for defect detection when using the synthetic aperture focusing technique (SAFT). Results are presented for various defects located in welds, near welds, and away from welds using the SH0 and SH1 modes emitted at a central frequency of 250 kHz. The presence of debris and general corrosion had little effect on EMAT measurements.

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