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Abstracts
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Authors
Ogi, Hirotsugu;Hirao, Masahiko; Honda, Takashi
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Title
Ultrasonic attenuation and grain-size evaluation using electromagnetic acoustic resonance
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Journal
J Acoust Soc Am 98 (1) July 1995 pp 458-464
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Abstract: Electromagnetic Acoustic resonance is applied for determining the frequency dependence of the ultrasonic attenuation and the average grain size of low-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 other losses, which may otherwise occur with the contacting piezoelectric transducers. The measurement is independent of the EMAT used, the specimen thickness, the surface condition, the lift-off, etc., and is stable because of the noncontacting nature. First, the resonant frequencies are measured, to the accuracy of 10 Hz, by sweeping the operating frequency and obtaining the amplitude spectrum over a band in the 0.5-20 MHz range. The ringing signals are excited and received by a shear wave EMAT and then processes with a superheterodyne receiver. Second, the attenuation coefficient as a function of the resonant frequency is determined. At each resonant frequency, the output signal rings down exponentially with time and the attenuation coefficient is obtained from the time constant by fitting an exponential decay to them. After correcting 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 three- dimensional grain-size distribution of steels.
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