The detection of fatigue cracks at fasteners in the sub layers of multi-layered aircraft structures can be problematic using conventional nondestructive testingudmethods. In this thesis the sensitivity of low frequency guided ultrasonic waves to detect these defects is studied. Guided ultrasonic waves typically have energyuddistributed through the thickness of such structures and allow for defect detectionudin all sub-layers, but have wavelengths larger than commonly used in bulk waveudultrasonic testing.udThe model aerospace multi-layered structure investigated consists of two aluminiumudplate strips adhesively bonded using a paste adhesive with a fastener hole.udGuided waves were excited by placing piezoelectric (PZT) transducers on the surfaceudof the structure. Experimentally the wave propagation and scattering was measuredudusing a laser interferometer. The wave propagation was studied numerically usingudSemi-Analytical Finite-Element (SAFE) calculations and 3D Finite Element (FE)udsimulations.udThickness and width mode shapes of the guided waves were identified from theudSAFE simulations. By placing PZT discs across the width of the structure the excited udexural wave modes could be controlled to an extent. The thickness modeudshapes of these waves are similar to those in a large multi-layered plate structure. 3DudFE simulations predict a similar amplitude change due to a defect in these structures.udFatigue crack growth monitoring on tensile specimens was realized, measuring theudamplitude at a single point. The measured changes in the amplitude of the ultrasonicudsignal due to a defect agree well with 3D FE simulations.udThese investigations found that using low frequency guided ultrasonic wavesuddefects through the thickness of a hidden sub layer can be detected from measurementsudon the undamaged, accessible layer.
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