Macro-fibre composite actuators for the transduction of Lamb and horizontal shear ultrasonic guided waves

摘要

Long-range ultrasonic testing (LRUT) makes use of ultrasonic guided waves (UGW) to screen large structures for defects whilst only requiring limited access. Generally, arrays of transducers are used to excite and sense particular ultrasonic wave modes preferentially and to control their direction. These transducers can have a directional efficiency that is both frequency and wave mode dependent. This is often because they act on a surface of the structure that is parallel to the plane in which the waves propagate and because they are large with respect to the ultrasonic wavelengths used. In this case, the directional characteristics of a transducer type should be known to allow them to be used optimally. Macro-fibre composite (MFC) actuators are capable of exciting and receiving ultrasonic guided waves. They are desirable for use due to their light weight and flexible, film-like nature. In this paper, their directional transmission of both Lamb and shear horizontal plate waves (in 10 mm-thick steel plate) are evaluated. Their frequency and wave mode dependent characteristics are assessed at low ultrasonic frequencies typical for use in the inspection or monitoring of such plate structures. Also, the characteristics of a more conventional uniform in-plane transducer are evaluated to allow comparison. The bipolar nature of the MFC is demonstrated to give rise to significantly directional and frequency-dependent Lamb wave sensitivity in the direction of polarisation, but with some orthogonal sensitivity due to a Poisson's ratio effect. It is also shown how their bipolar nature excludes SH0 in the directions parallel to the piezo-fibre and orthogonal axes, yet significant SH0 transmission does occur in the diagonal directions.