Development of a Flexible Unidirectional Guided-Wave EMAT for Pipe Inspection

Ultrasonic guided wave testing has unique advantages of long-distance propagation and high detection efficiency, and has become a mainstream pipeline and pressure vessel testing and evaluation method. Currently, conventional guided wave transducers typically excite ultrasonic waves that propagate in both directions along the workpiece, leading to signal aliasing and interference in signal interpretation. To solve this problem, this paper proposes a flexible unidirectional guided wave electromagnetic acoustic transducer (EMAT). It is designed to achieve unidirectional pipeline and pressure vessel inspection and avoid interference from the other side. The transducer consists of dual-layer flexible coils, enabling adaptability to pipelines and vessels with varying diameters. To achieve unidirectional excitation and backward wave cancellation, a unidirectional focusing method based on time-delayed dual-element excitation and spatial interlacing arrangement is proposed. Additionally, a differential signal processing method based on dual-element reception is introduced to enhance the amplitude of the received signal. Through numerical simulations and experiments, the performance of the developed transducer in both achieving unidirectional guided-wave excitation/reception and amplitude enhancement are validated. The unidirectional ratio (UR) of the excited guided waves can reach 93.7%, and the amplitude of the unidirectional guided waves is amplified by 3.43 times. The proposed transducer and excitation method can be effectively applied to multi-scale pipeline and pressure vessel inspection with unidirectional guided waves.