Fatigue Damage Evaluation in Thin-Walled Structures Using Guided Waves

Abstract

Thin-walled structural components are common in engineering design across many industries and applications. These components are often subjected to cyclic loading, which can lead to fatigue damage accumulation, nucleation and propagation of defects and, finally, to structural failures. The use of damage detection techniques as a part of safety inspections allows to monitor defects and prevent structural failures. However, damage evaluation in thin-walled structures can be challenging with the traditional non-destructive evaluation (NDE) techniques. Therefore, development of new NDE techniques is important to maintain structural integrity and safe operation of infrastructure. The overall aim of this thesis is to develop new NDE techniques, specifically for thin-walled structures and non-ideal geometries, for the detection and evaluation of early-stage fatigue damage. These new techniques are based on fundamental modes of Lamb and edge waves, which are the most suitable for thin-walled structural components, as these modes disperse rapidly over propagation distance in thick structures. In Chapter 2, the accumulation of low-cycle fatigue damage is investigated using the low-frequency Lamb wave mixing method combined with phase-reversal approach. Chapter 3 proposes a new approach for the frequency selection and a time-shifting technique to improve the efficiency of wave mixing method for the fatigue damage evaluation. Detection and evaluation techniques for fatigue edge cracks in realistic structures are developed in Chapters 4, 5 and 6. Chapter 4 investigates the propagation of edge waves along corners of thin-walled structures for the detection of edge cracks. Chapter 5 explores the propagation of the fundamental quasi-edge wave modes in thin-walled structures with non-ideal (curved) edges and the use of these modes for defect evaluation. In Chapter 6, the edge crack length is evaluated using the fundamental mode of edge waves. Moreover, a new Finite Element (FE) model is proposed to simulate the interactions of elastic waves with fatigue cracks, which also accounts for the plasticity-induced closure phenomena. The main outcomes of the thesis are briefly summarised below, (1) Development of a new technique for the evaluation of early-stage fatigue damage using the fundamental mode of Lamb waves and wave mixing method; (2) Development of a new approach for the frequency selection and a time-shifting method, which can improve the efficiency of wave mixing method for damage evaluation; (3) Investigation of the propagation of edge waves in plates with sharp and rounded corners, which supports the further development of a NDE technique to evaluate damage in inaccessible locations; (4) Investigation of the quasi-edge wave modes and the development of a Semi-Analytical Finite Element (SAFE) model to characterise the wave propagation properties; (5) Development of a new technique for the evaluation of the edge crack length based on the fundamental mode of edge waves, and (6) Development of an advanced FE model to simulate the interaction of elastic waves with cracks considering the plasticity-induced closure phenomena. Overall, the findings of this thesis provide knowledge and deeper understanding of fatigue damage evaluation using guided waves. The outcomes will help apply guided wave based NDE techniques to real structures.