Higher torsional mode suppression in a pipe for enhancing the first torsional mode by using magnetostrictive patch transducers

Small-sized defects in pipes can be better detected if the first nondispersive torsional mode is used in a higher frequency range. However, dispersive higher torsional modes accompany the first mode if the actuation frequency is above the first cutoff frequency, thereby making the detection difficult. This study proposes a new technique that is particularly useful for guided torsional waves in a pipe; it enhances the desired first nondispersive mode and suppresses the undesired second mode. The technique uses two transmitting transducers separated by an optimized distance and actuated with an optimized delay time. Unlike previous methods, such as a method tuning the delay time for desired mode enhancement and tuning the distance for undesired mode suppression, the proposed approach determines both the distance and delay time mainly to suppress the undesired second mode. With the selected values, the desired first mode is substantially enhanced. This phenomenon is unique in torsional waves, not longitudinal waves, for which delay time and distance controlling methods have been developed. After wave simulations were carried out to show why the proposed method is more effective for the case of torsional waves, several experiments using magnetostrictive transducers were performed to demonstrate the effectiveness of the proposed method.