Ultrasonic Plate Waves for Fatigue Crack Detection in Multi-layered Metallic Structures (preprint)

A representative area of concern for fatigue crack growth in aircraft occurs in multi-layered metallic structures. Ultrasonic plate waves are currently being investigated by multiple initiatives to detect these types of flaws with a minimal number of sensors to enable Structural Health Monitoring (SHM). Previous work has focused on structures with one or two layers, coupled with modeling of the wave propagation within these representative samples. However, it is common for multi-layered structures to have more than two layers in many areas of interest. Therefore, this study investigates ultrasonic wave propagation and flaw detection in a multi-layered sample consisting of 2 to 4 total layers with fatigue cracks located in only one layer. The samples contain fastener holes configured as would be expected to find on typical aircraft structure. The flaws in this study are represented by electric discharge machined (EDM) notches. Preliminary measurements show that EDM notches can be detected by the guided ultrasonic waves, but that the sensitivity to EDM notch location is dependent on the boundary conditions of each layer. The boundary conditions are changed by applying various loads on the surface of each layer by tightening and loosening the fasteners that hold the sample together. This variation depicts representative conditions found of aircraft. The experimental results are supplemented by modeling of the guided wave propagation within the structure using the Finite Element Method. The primary parameter studied in the modeling effort is the effect of the changes in the boundary condition on the mode and amplitude of the guided wave. The results of this investigation establish some guidelines for the use of guided waves in multi-layered structures, plus challenges that exist for their use in SHM applications and strategies to address these challenges.