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Frequency domain Lamb wave analysis for damage detection

Purcell, Frederick 2021. Frequency domain Lamb wave analysis for damage detection. PhD Thesis, Cardiff University.
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Abstract

Non-Destructive Testing (NDT) techniques are prevalent in the aerospace, green energy and automotive industries. With the ability to identify defects in service or at the manufacturing stage, NDT is a vital tool in creating safe and efficient structures. Existing NDT methods face many limitations when working with advanced materials such as composites. Further limitations are met by conventional NDT methods in terms of resolution, measurement time and levels of access required to the structure for measurements to be taken. This work presents the development of a band-pass mode filtering technique in the frequency wavenumber domain for the purpose of damage detection. Data were captured in the temporal and spatial domain using a 3D Scanning Laser Doppler Vibrometer (SLDV) with piezoelectric transducers exciting the structure with a variety of steady-state signals ranging in frequency. A thickness map or damage map was created based on the frequency and wavelength of the A0 Lamb wave mode. The technique was first demonstrated on two aluminium specimens with dimensions of 400mm by 400mm with a thickness range of 0.5mm to 8mm with distinct geometric features. Using multi-frequency excitation combined with mode based filters, an estimation of thickness was achieved with a mean percentage thickness error of 15%. Circular thickness reductions with a diameter of 10mm were clearly identified at the maximum plate thickness of 8mm. The proposed mode filtering technique was furthered to work on highly non-isotropic composites using no prior knowledge of the material. Dispersion characteristics were taken from the measurement data and determined the shape of the mode filters. This method was demonstrated on three different composite specimens and was able to identify single ply changes in a fibre-glass specimen as well as a delamination defect in a carbon fibre plate. Multi-frequency steady-state excitation was also shown using multiple driving transducers on a single structure. Further work was completed to enable these techniques to function with wavefield data gained from non-developable surfaces. Through geometrical transforms of the wavefield it was shown that wave mode filtering could be completed on complex geometries. The application of wavenumber-based NDT was demonstrated to give highly accurate results with good spatial and depth resolution on parts with complex geometries, as well as composite and metallic parts. This work presented a new embodiment of wavenumber-based NDT that showed a significant step towards real world implementation and offers a number of advancements over existing techniques

Item Type: Thesis (PhD)
Date Type: Completion
Status: Unpublished
Schools: Engineering
Uncontrolled Keywords: Non-destructive testing, Wavenumber, Frequency Domain, Composite, Scanning laser vibrometer, Guided Lamb wave
Date of First Compliant Deposit: 28 October 2021
Last Modified: 28 Oct 2021 08:13
URI: https://orca.cardiff.ac.uk/id/eprint/145115

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