Enhancement of Signal Amplitude of Magnetostrictive-Based EMAT with Terfenol-D Coating for Stainless-Steel Specimens

Stainless steel is widely used in petrochemical, marine, and electrical engineering applications owing to its excellent corrosion resistance and superior mechanical properties. Electromagnetic acoustic transducers (EMATs), which require no acoustic couplant, are an attractive non-contact ultrasonic technique for the inspection of such materials. However, the amplitude of ultrasonic signals generated and received by EMATs is generally low, which is particularly true for stainless steel test pieces. To overcome this challenge, a novel magnetostrictive-based EMAT incorporating a Terfenol-D coating is proposed in this paper to significantly enhance the ultrasonic signal amplitude. A polymer prepared by mixing Terfenol-D powder with epoxy resin adhesive is applied to the surface of the specimen, serving as an integral part of the EMAT transduction mechanism. Finite element simulation models are established for specimens with and without the Terfenol-D coating using COMSOL Multiphysics 6.0. The distributions of the electromagnetic field and ultrasonic velocity field generated by the proposed EMAT are simulated and analyzed. Experiments are subsequently carried out on the Terfenol-D-coated specimens and the uncoated reference specimens. The effects of the polishing of the coating and the Terfenol-D-to-epoxy weight ratio on the signal enhancement are also experimentally investigated. The results indicate that the proposed EMAT with the Terfenol-D coating can efficiently generate and receive ultrasonic signals. Compared with the conventional EMAT on an uncoated stainless-steel specimen, the received signal amplitude is increased by approximately 6.7 × 103 times. These results suggest that the proposed EMAT is a promising candidate for non-destructive testing and evaluation (NDT&E) applications of stainless-steel specimens.