Damage Detection of Submerged Structures Using Linear and Nonlinear Guided Waves

Abstract

Metallic plates are one of the major components of liquid containment structures and are widely used in petrochemical and civil engineering. In many cases, the metallic plates have one side exposed to liquid and are subjected to different types of loads with varying amplitudes. Corrosion damage and material degradations are the two major concerns. Damage detection of the submerged plate structures plays an important role in maintaining the structural integrity and safety of high-valued infrastructures (e.g. liquid storage tanks and pipes). Guided wave testing is one of the most promising damage detection approaches. Although guided wave based techniques have been extensively studied on different structures in gaseous environments, the design and implementation for the structures immersed in liquid have not been well investigated. This research aims at enhancing the understanding of guided wave propagation and interaction with damage in submerged structures. The focus of this research is on metallic plates that have one side in contact with liquid and the other side exposed to air. The specific objectives of this thesis include the investigation on the propagation characteristics of guided waves in metallic plates with one side exposed to liquid, the development of numerical models to investigate the scattering characteristics of guided waves at corrosion pit damage, the analyses of the influence of the surrounding liquid medium on the linear and nonlinear guided waves features, and the evaluation of the sensitivity of linear and nonlinear guided waves features to different types of damage in the one-side immersed metallic plate. The main body of the thesis consists of four journal articles (Chapters 2-5). Chapter 2 discusses the propagation characteristics and sensitivity to damage of linear guided waves in a metallic plate loaded with water on one side. The targeted damage is local thickness thinning (e.g. corrosion pits) with a size of around a few millimeters. Chapter 3 further investigates and compares the guided wavefields between a plate surrounded by air and the same plate with one side partly exposed to water. The influence of the surrounding liquid medium on the guided wave propagation is demonstrated experimentally and numerically. Chapters 4 and 5 study two different nonlinear guided wave features, which are second harmonic generation and combination harmonic generation, respectively. The nonlinear guided wave features have better sensitivity to microstructural defects that precede the damage in the macroscale. The targeted damage in Chapters 4 and 5 is fatigue degradation in the early stage, where fatigue appears as multiple micro cracks and is distributed in the structural materials. The microstructural defects are too small to be detected by the linear guided wave feature. However, these small defects can distort the guided waves passing through the material, producing new wave components at frequencies other than the excitation frequency of the incident waves. This provides a way for the nonlinear guided wave technique to evaluate the earlystage damage in submerged structures.