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Baseline-free Method for Acousto-Ultrasonic On-Line-Monitoring of Cracks in Mechanically Loaded and Rotating Axles



In real operation, components are exposed to varying environmental and operational conditions (EOC), e.g. temperature, speed of rotation, etc. For Acousto- Ultrasonics (AU)-based methods in Structural Health Monitoring (SHM), it might be not economically feasible to collect data and construct baselines for all possible combinations of EOCs. In consideration of such practical factors, this paper presents a novel baseline-free method for detection of transverse cracks in axles. The underlying effect is the opening and closing mechanism of the crack during a revolution of a mechanically loaded and rotating axle which leads to a difference in the wave propagation depending on the angle of rotation. During the short time of data acquisition, the EOCs remain nearly constant and the methodology focuses only on the difference in wave propagation due to the rotating stress distribution and the rotating crack. The differences of the structural responses result from the change of wave velocity and reflection behavior. Mechanically loaded and rotating axles can be found in many industrial applications for example in running gears. The presented method actually provides the possibility to realize Condition Based Maintenance (CBM) for safety critical components, which has the potential to significantly reduce the maintenance cost and to increase the availability level of the component. In this study, Piezoelectric Wafer Active Sensors (PWAS) are applied to excite the elastic waves and to measure the structural response during rotation. In situ ultrasonic measurements and an on-line analysis of the structural response were performed during a rotating bending fatigue test of an axle. The experimental results demonstrate the effectiveness of the presented method. The growing crack shows a progressive change of a characteristic pattern.


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