Abstract
For the maintenance of machine structures from fatigue damage, it is important to evaluate the fatigue damage before fatigue crack initiation and growth. Therefore, in this study, we propose a new method to evaluate microstructure changes during fatigue process using ultrasonic spectroscopy. The dislocation behavior, i.e. density of movable dislocation and eigen-frequency of dislocation strings, are closely related to fatigue damage of materials. We can estimate the density of movable dislocation under fatigue process by measuring ultrasonic surface wave velocities with three different propagating frequencies via Granato-Lucke's dislocation string theory. This proposal using the method of presuming movable dislocation density is examined whether it is one index of evaluating fatigue damage in engineering or not. Therefore, we experimented in two cases, i.e. the first experiment is a pulsating tension-compression fatigue test, and the second is a rolling contact fatigue test. In the first case, the correlation of the movable dislocation density with fatigue damages concerning both of aluminum alloy and chromium molybdenum alloy steels was examined. The results obtained are as follows. The ultrasonic surface wave velocities with three different propagating frequencies (5, 10 and 15 MHz) decreased due to increasing of number of fatigue cycles in the both cases of aluminum alloy and chromium-molybdenum alloy steel. The density of movable dislocation in the both cases estimated by Granato-Lucke's model increased in the fatigue damage progress. In the second case, we estimated the density of movable dislocation under the rolling contact fatigue process of chromium molybdenum forged steel by measuring ultrasonic surface wave velocities with three different propagating frequencies via Granato-Lucke's dislocation string theory. As a result, the ultrasonic surface wave velocities with three different propagating frequencies (5,10 and 14 MHz) decreased due to increasing of the rolling number of rolling contact fatigue and the density of movable dislocation estimated by Granato-Lucke's model increased in the fatigue damage progress. The comparison of changes of the density of movable dislocation with changes of half value breadth of X-ray diffraction line profile analysis under the rolling contact fatigue process suggests promising availability of the proposed method.
