Structural materials used in a nuclear power plant may be degraded mainly by fatigue, neutron irradiation embrittlement or thermal embrittlement. In order to improve the availability and reliability of a nuclear power plant, it is especially advantageous to detect nondestructively the degradation as early as possible and to prevent possible failures. Measurement of change of ultrasonic decay constant has been proposed as promising degradation detection method. In this study, measurement of frequency dependance of decay constant of ultrasonic wave during propagation in materials is introduced. By exciting an EMAT with sine burst wave and processing the ringing signals with a superheterodyne phase sensitive detector, decay constant is measured at the resonance frequency for platetype test specimen. Based on this results, it was confirmed that the relation between frequency and decay constant varied with the degree of material degradation and also with the amplitude of residual stress.
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