Kostson, E; (2010) Fatigue crack monitoring in multi-layered aircraft structures using guided ultrasonic waves. Doctoral thesis , UCL (University College London).

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Abstract

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

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