Performance Stability of Electromagnetic Acoustic Transducers

The signal amplitude stability of non-destructive testing systems based on electromagnetic acoustic transducers (EMATs) and guided ultrasonic waves is investigated. The study is focused on three factors: sensor lift-off, specimen thickness, and dynamic permeability. The objectives are to determine a minimum lift-off to achieve adequate stability and signal-to-noise ratio, and evaluate the effect of variable specimen thickness on the propagation of the ultrasonic dispersive SH1 mode. The analysis is performed through finite element simulations, which are validated experimentally. It was found that a magnetostriction-based transmitter is less sensitive to lift-off than a Lorentz force transmitter. Specimen thickness variations in a direction perpendicular to ultrasonic wave propagation can focus or divert SH1 wave energy such that the resulting signal fluctuations are large enough to interfere with defect detection. Both magnetostriction-based and Lorentz-based EMAT inspection systems were found to be unaffected by random variations of dynamic permeability.M.A.S