Numerical investigation of application of unidirectional generation to improve signal interpretation of circumferential guided waves in pipes for defect detection

Monitoring and inspection of pipes are paramount in several industry sectors. A convenient approach employs shear horizontal ultrasonic guided waves that propagate circumferentially around the pipe, known as CSH waves. Typically, a pair of transducers, a transmitter and a receiver, can be used to interrogate the whole circumference. Conventional shear horizontal transducers generate waves that propagate in both directions, namely, clockwise and counterclockwise. Bidirectional generation complicates the interpretation of the received signal since the scattered waves from a potential defect can mix themselves or with one of the two direct waves generated by the transmitter. In this paper, we address how unidirectional generation can ease the signal interpretation task for circumferential guided wave testing utilizing finite element simulations. Unidirectional generation is based on dual periodic elements transducer driven by phased pulses, following a design previously presented. Generation and reception were performed with separate transducers on the outer surface of a 324 mm outer diameter, 6.25 mm wall thickness steel pipe. Corrosion-like defects were modelled on the inner surface. We show that, when the angular position of the defect approaches 180° from the transmitter, the defect echo can be almost completely masked by the wave generated in the opposite direction. Unidirectional generation of CSH waves proves itself to be an important feature in pipeline integrity monitoring, providing more reliable signal interpretation.