Pulsed Electromagnets for EMATs

In many industrial NDE applications pulsed electromagnets may be more desirable than large static electromagnets or permanent magnets for magnetic biasing of electromagnetic acoustic transducers (EMAT\u27s). Since electromagnetic acoustic transduction is confined to one skin depth at the operating frequency of the EMAT, the transduction efficiency can be enhanced by the dynamic concentration of the magnetic flux near the surface. This paper describes a number of physical phenomena associated with EMAT generation under pulsed-magnetic-field bias. In particular, it is observed that for maximum transduction efficiencies the ultrasonic pulse must be retarded relative to the initiation of the current pulse to the electromagnet windings. A second maximum in the transduction efficiency is observed when the pulsed magnet-EMAT system is operated on ferromagnetic steel (1020). The second maximum is associated with magnetostrictive effects. Operation of pulsed magnet-EMAT systems at elevated temperatures (400°C) is demonstrated on aluminum