The Effect of Transducer Placement on the Monitoring of Fatigue Cracks Emanating from Fastener Holes

In situ ultrasonic methods for structural health monitoring have the potential to provide early detection of fatigue cracks in aircraft components, specifically cracks initiating from fastener holes. A dual angle beam technique based upon an incident shear wave and a spiral creeping wave propagating around the hole has been shown to be a viable configuration for monitoring of fastener hole cracks. This technique takes advantage of the interference of the creeping wave with cracks opening and closing under load, and the ratio of received energy under a reference load to that under no load is monitored for crack detection and sizing. While prior work with this method has been very promising, transducer placement can significantly influence its performance. Here we consider the effect of transducer placement on the sensitivity of the energy ratio algorithm, using the observed location of cracking to guide placement of transducers. Ray path analysis is performed for determining the optimum transducer locations. Several experiments are performed using these new locations, and results are compared to those obtained using the previous mounting locations. The results with the new transducer placement show increased sensitivity and earlier detection.