電磁超音波共鳴法による2.25Cr-1 Mo鋼のクリープ損傷評価

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This paper describes an ultrasonic technique for evaluating creep damage in 2.25 Cr-1 Mo steel exposed to the temperature of 923 K at various stresses. The technique is based on the electromagnetic resonance (EMAR), which is a combination of the resonant technique and a noncontacting electromagnetic acoustic transducer (EMAT). We obtain the ultrasonic velocity from the resonant frequency and the attenuation coefficient from the ringdown curve at a resonance. Two types of EMATs are used. One is the bulk wave EMAT for measuring the frequency dependence of shear wave attenuation in plate samples. The other is the axial-shear-wave EMAT for detecting the attenuation change of shear waves traveling in the circumferential direction of cylindrical samples. The attenuation showed much larger sensitivity to the damage accumulation than the resonant frequency. Approaching the rapture, it becomes ten time as large as the initial value. The frequency dependence of the attenuation also showed a remarkable change with the damage. The EMAR technicue has been proven to possess a large potential for the practical nondestructive/noncontact damage monitoring ; the EMAR is capable of sensing the attenuation evolution and indicating the damage advance to predict the creep life of the metals.
1999-08-25