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A wavefront analysis indicates that a flexural wave propagates at a helix angle with respect to the pipe axis. The expression for calculation of the helix angle for each flexural mode is given, and the helix angle dispersion curves for flexural modes are calculated. According to the new understanding of flexural guided waves, a magnetostrictive helical array transducer (MHAT) is proposed for selectively exciting a single predominant flexural torsional guided wave in a pipe and inspecting spiral welded pipes using flexural waves. A MHAT contains a pre-magnetized magnetostrictive patch that is helically coupled with the outer surface of a pipe, and an array of novel compound comb coils that are wrapped around the helical magnetostrictive patch. The proposed wideband MHAT possesses the direction control ability. A verification experiment indicates that flexural torsional mode T(3,1) at center frequency f=64kHz is effectively actuated by a MHAT with 13-degree helix angle. A 20-degree MHAT is adopted to inspect a spiral welded pipe, an artificial notch with cross section loss CSL=2.7% is effectively detected by using flexural waves. [ABSTRACT FROM AUTHOR]

Millions of miles of pipes are being used in both civil and industrial fields. Spiral-welded pipes, which are widely applied in fields such as drainage, architecture as well as oil and gas storage and transportation, are difficult to inspect due to their complex geometry. Guided waves have shown a great potential in Non-Destructive Evaluation (NDE) and Structural Health Monitoring (SHM) for such cases. Flexural guided waves that propagate at a helix angle relative to the axial direction of pipe, are the most appropriate modes for inspecting spiral-welded pipes. The classical Normal Mode Expansion method (NME) is adopted to disseminate the forced response and perturbation analysis of a steel pipe with respect to a time delay circular loading. A time delay circular array transducer (TDCAT) is proposed for the purpose of exciting pure flexural mode in pipes. Pure flexural mode can be excited when the time delay parameter is specifically designed. The theoretical prediction is verified by finite element numerical evaluation and spiral-welded pipe inspection experiment. [ABSTRACT FROM AUTHOR]