Non-contacting in-situ evaluation of torsional fatigue damage of carbon steel using axial-shear-wave resonance

Abstract Electromagnetic acoustic resonance (EMAR) has been applied to fatigue-damage assessment because of its ability to accurately measure ultrasonic velocity and attenuation contactlessly, which are sensitive to microstructural changes. We applied EMAR using axial-shear-wave resonance, which can sense different depth information, to non-contacting in-situ evaluation of torsional fatigue damage of a carbon-steel rod. While previous studies have shown that ultrasonic velocity decreases monotonically as fatigue damage progresses and attenuation peaks before failure, we found an unusual increase in the velocity, which has never been reported. For the mechanism analysis, we observed the microstructures and measured the residual stress near the specimen surface. As a result, we found that it is difficult to explain the velocity change only by the dislocation-structure changes, by which ultrasonic behavior has been explained in previous studies, and that residual stress and solid-soluble precipitation are likely to contribute to the initial increase in the velocity.