Method to suppress higher modes of the guided torsional wave for sending the non-dispersive mode through a pipe

The first mode of the torsional wave in a pipe is the most preferred mode because of its non-dispersion, but the detection of smaller cracks requires the use of higher frequencies above the first cutoff frequency. In this case, the generation of undesirable higher-mode waves cannot be avoided and thus the resulting signal processing for crack detection can be difficult. Here, we consider a special case of the ultrasonic guided-wave inspection of new pipes before being installed in the field. In this case, one can install a transducer at one end of the test pipe and thus there is room for transducer design. The objective of this study is to design a magnetostrictive transducer system to send only the first wave mode through a test pipe even at a frequency above the first cutoff frequency of the test pipe. Several experiments are carried out to verify the validity of the developed transducer system and results show the successful transduction of the only first mode of the torsional wave.