Development of an electromagnetic acoustic transducer using a complex magnetic pole for direct and alternating current

Abstract

Nondestructive inspection of structures in a high temperature environment is requested for safety and economic. An ultrasonic flaw detection system using an electromagnetic acoustic transducer (EMAT) is said to be suitable for high temperature environments. However, the electromagnetic induction coil (EM-coil) in part of a conventional EMAT is required to approach the surface of the specimen in principle. Therefore, it will be easy for it to undergo thermal and mechanical damage. In this research, a new type Lamb wave EMAT wound EM-coil around a ferromagnetic core has been developed to overcome such weak points. As the first step, a trial EMAT with a steel core and permanent magnet to add a biased magnetic field was tested to evaluate the influence of the direction of the bias magnetic field and the optimum magnetic flux density. It was proven that there is an optimum strength of the magnetic field, and the horizontal biased magnetic field was effective. Next, a trial EMAT with a steel core for a static and dynamic magnetic field was fabricated. The received signal by the electromagnet type trial EMAT was much stronger than that of the trial EMAT with permanent magnets. However, the strength was much weaker than that using a conventional Lamb wave EMAT. Therefore, a trial EMAT installed with a magnetic pole made of a PC-perm alloy was fabricated. The best signal was obtained without a biased magnetic field, but the signal was weaker than that of the trial EMAT with the steel core. Finally, the best signal the same as that by the conventional Lamb wave EMAT, was obtained using the trial EMAT installed with a steel core for the biased magnetic field and the PC-perm alloy for the dynamic magnetic field which are magnetically shielded.