Continuous monitoring of fatigue process with surface wave resonance: In the case of rotating bending fatigue Free

Prediction of fatigue failure of metals has not become reality like that of earthquake despite the intensive studies over many decades. But, there is an exception. The accumulating damage and the remaining life of rotating bending fatigue can be known from the ultrasonic observations through the lifetime. Electromagnetic acoustic resonance (EMAR) makes it possible to monitor the evolution of phase velocity, attenuation, and nonlinearity of surface shear wave exited and detected exclusively by electromagnetic acoustic transducer (EMAT), which operates without any contact with the rotating shafts. The probing shear wave travels in the circumferential direction with the axial polarization and penetrates into the metal to the submillimeter depth. Clear indications occur in the attenuation and nonlinearity measurements, and tell the pertinent metallurgical events within the penetration depth. They are caused by the dislocation mobility and restructuring due to cyclic loading, and occur at the specific fractions to the fatigue life, being independent of the bending stress amplitude. Continuous observation with EMAR can only detect such precursors and tell the remaining lifetime of metal components being fatigued. Discussions are given for the mechanism being based on the replication of small cracks and the TEM observation of dislocations at the crack-tip zone.