Damage Detection Using Second Harmonic Generation of Low-Frequency Guided Waves

Abstract

The thesis provides physical insights into the nonlinear phenomena associated with propagation of guided waves (GW) in thin walled structures. The non-linear phenomena are investigated using experimental techniques, the three-dimensional (3D) finite element (FE) simulations as well as theoretical studies. The thesis is divided into two main parts. In the first part, the contact nonlinearities are studied, while the focus of the second part is on the intrinsic material nonlinearities. The thesis represent a compendium of published, submitted and prepared for submission papers united by the same topic and methodology. Each paper contains the background information and an extensive literature review. For this reason, the thesis does not have a separate chapter on literature review or background. In the first part of the thesis, the generation of the second order harmonic of Lamb waves as a result of contact non-linearity associated with presence of fatigue cracks (paper 1 & 2), contact loosening in bolted joints (paper 3) and delamination in a composite material (paper 4) are investigated. A relative nonlinear parameter (NP) is introduced and utilised to quantify the rate of the growth of the second harmonic (SH). In the case of fatigue cracks, experimental studies along with the 3D FE simulations are conducted in order to maximise the amplitude of the SH and identify the directivity patterns of NP, influence of incident wave angles, and the external loading on the generation of the rate of SH. A feasibility study on detecting loosen bolts is carried out, and the results demonstrated that the generation of the SH can indicate the bolt loosening, however, there is a fluctuation of the results between different group of tests. Finally, a FE model of an aluminium plate with composite patch repaired weakened by defects i.e. delamination and fatigue crack, is developed. The outcomes of dynamic simulations reveal that when a proper GW mode is selected, then it is possible to not only detect the presences of defects, but also the type of the defects. In the second part of the thesis, a VUMAT (material properties) subroutine is developed and implemented in a 3D FE model. It utilises the Murnaghans’ strain energy function to study the intrinsic material nonlinearities and the effect of the applied stresses in an aluminium plate. The FE model with a weakly nonlinear material model is first validated with theoretical solutions describing the acousto-elastic effect and changes of the phase velocity with the applied stress (paper 5). The variation of the second order NP with the wave propagation distance is studied in paper 6. The developed 3D FE model provides a more convenient and flexible approach to study the acoustoelastic effects in prestressed waveguides with complex geometry. Meanwhile, it also provides more options to select the material behaviour and investigate nonlinear phenomena generated by the propagation of nonlinear GW features in weakly-nonlinear material (paper 6).