Calibration-free SH guided wave analysis for screening of wall thickness in steel with varying properties

Guided shear horizontal (SH) ultrasonic wavemodes show promise for fast screening of wall thinning and other defects, with miniaturised electromagnetic acoustic transducers (EMATs) offering the potential for automated robotic inspection. However, the use of guided waves for full defect characterisation is strongly affected by mode conversions, changes in lift-off from the sample, variations in material properties and other effects, and these are not typically considered when reporting laboratory results. We show that these factors can have a significant effect on the data in standard measurements, but that a combination of different analysis methods on the SH0 and SH1 wavemodes can offer a high reliability of detection, and mitigate some of the issues with changes in experimental conditions and mode conversions. For inspection of 10 mm thick steel plates, the proposed processing of the signals is shown to reliably detect 40 mm diameter at bottomed holes with 5% - 50% wall thinning. A rough, approximately 20x20 mm square defect with up to 20% wall thinning is also detected.