Alternating winding magnetostrictive electromagnetic acoustic transducer for pipe torsional guided wave generation

Ultrasonic guided waves propagating in bounded elastic media called waveguide are used extensively nowadays for rapid screening and long distance inspection of defect-prone structures. Various types of transducers are available for guided wave generation, including piezoelectric transducers and electromagnetic acoustic transducers (EMATs) etc.. EMAT is based on Lorentz force and/or magnetostrictive effect, and advantageous sometimes because of its non-contact nature. An alternating winding magnetostrictive type EMAT for torsional mode ultrasonic guided wave generation in a steel pipe is introduced in this paper. This new transducer design is composed of a pre-magnetized Nickel strip with circumferential static magnetic field and a coaxial coil in an alternating winding pattern. Theoretically this transducer design could be viewed as an extension of existing magnetostrictive type EMAT for SH guided wave generation in a steel plate. The alternating winding pattern enables accurate and easy control of guided wave mode. Results of experiments done on a steel pipe were shown to verify this transducer design.