Contactless Measurement of Ultrasonic Attenuation and Average Grain Size with Electromagnetic Acoustic Resonance

Electromagnetic acoustic resonance (EMAR) is applied for determining the frequency dependence of the ultrasonic attenuation and the average grain size of carbon steels. Use of a noncontacting electromagnetic acoustic transducer (EMAT) makes it possible to isolate the attenuation within the plate specimens. The method relies on the Lorentz force mechanism to couple the EMAT to the specimen surfaces and then eliminates the uncorrectable losses, which otherwise occur with the piezoelectric transducers. The measurement is independent of the EMAT used, the specimen thickness, the surface condition, the liftoff, etc., and is stable because of the noncontacting aspect. A measurement has two steps. First, we measure the resonance frequencies, to the accuracy in the 10 Hz order, by sweeping the operating frequency and obtaining its amplitude spectrum. The ringing signals are excited and received by an EMAT and then processed with a superheterodyne phase sensitive detector. The frequency is swept through a band in the 0.5∼20 MHz range. Second, we determine the attenuation coefficient as a function of the resonance frequency. At each resonance frequency, the output signal from the detector rings down exponentially with time and the coefficient is obtained by fitting to an exponential decay. After being corrected for the diffraction effect, the average grain size is obtained from the fourth power term in the frequency dependence. The final results are favorably compared with the average of the 3D distribution of grain sizes calculated from the photomicrographs of the exposed cross sections.