Numerical investigation of unidirectional generation and reception of circumferential shear horizontal guided waves for defect detection in pipe

Circumferential shear horizontal (CSH) guided waves generated by electromagnetic acoustic transducers (EMATs) are convenient for pipe inspection. Periodic permanent magnet (PPM) EMATs generate and receive waves that propagate circumferentially in both clockwise and counter-clockwise directions, which can complicate signal interpretation. We recently presented designs and optimization of dual-EMATs that generate SH waves unidirectionally. In a circumferential inspection with bidirectional generation, a defect echo can be masked by the wave generated in the opposite direction, which is suppressed with unidirectional generation. In the pulse-echo mode, however, unidirectional generation can still produce masked defect echoes by the wave that propagates through a defect. In this paper, we apply combined unidirectional generation and reception in order to overcome this limitation, utilizing finite element simulation. A 324 mm diameter, 6 mm thick steel pipe was modelled with a wall thinning defect. Results show that using only unidirectional generation, the through-transmitted wave can mask the defect echo and, consequently, present no observable difference from a defect-free sample. With combined unidirectional generation and reception, the defect echo is clearly separated from the direct wave, showcasing that it is an important feature for circumferential guided wave testing, enabling more reliable signal interpretation with a single transducer in the pulse-echo configuration.