Experimental verification of a Kalman filter approach for estimating the size of fastener hole fatigue cracks

Ultrasonic methods have been implemented for in situ sizing of fatigue cracks near fastener holes. These techniques, however, only provide an estimate at the time of the measurement and cannot predict the remaining life of the structure. In contrast, statistical crack propagation approaches model the expected fatigue life based on worst-case fatigue process assumptions. The authors have recently developed a Kalman filter approach for combining ultrasonic observations with crack growth laws. An ultrasonic angle-beam technique, combined with an energy-based wave propagation model, serves as the measurement model. Paris’s crack growth equation acts as the system model for crack propagation. For simulated data, this approach provided more accurate crack size estimates than either the ultrasonic measurements or crack growth approach alone. Presented here are experimental results to assess the ability of the Kalman filter to provide reasonable crack size estimates.