Electromagnetic Acoustic Resonance to Assess Creep Damage in 2.25 Cr-1 Mo Steels

摘要

This paper describes an ultrasonic technique for evaluating creep damage in 2. 25 Cr--l N1o steel exposed to the temperature of 923 K at various stresses. The technique is based on the electromag- netic resonance (EN1AR), 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 ENIAT 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 technique has been proven to possess a large potential for the practical nondestructiveoncontact damage monitoring, the EMAR is capable of sensing the attenuation evolution and indicating the damage advance to predict the creep life of the metals.