E. A. Lindgren, US Air Force Research Laboratory, Wright-Patterson AFB, OH; K. V. Jata, B. Scholes, J. Knopp, AFRL/MLLP, Metals, Ceramics and NDE Divison, Wright-Patterson AFB, OH; J. C. Aldrin, Computational Tools, Gurnee, IL
A representative area of concern for fatigue crack detection in aircraft is 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 fatigue crack 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 be found on typical aircraft structure. The fatigue cracks were grown using cyclic fatigue loading after the fastener holes were placed in the samples.Preliminary measurements show that fatigue cracks can be detected by the guided ultrasonic waves, but that the sensitivity to crack size 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 on aircraft. The experimental results are supplemented by modeling of guided wave propagation within the structure using Finite Element Methods. 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 recommendations for the use of guided waves in multi-layered structures, plus challenges that exist for their use in SHM applications and potential strategies to address these challenges.
Summary: Summary to come March 5th.
