Guided wave mixing for damage detection in structural elements

Abstract

Thin-walled components are fundamental to numerous civil structures such as bridges, buildings, storage vessels, pipes, and becoming progressively diverse with their use in wind turbines, aircrafts and shipbuilding. Identification and evaluation of damage in such structures plays a significant role in the early stage of the project conception, given that safety, performance and maintenance costs are three fundamental concepts in any engineering design. Structural Health Monitoring (SHM) was originated with collaboration across many disciplines to address a variety of structural issues and prevent dramatic losses. Nonlinear guided waves combines the benefits of nonlinear ultrasound and guided waves. By means of linear parameters such as wave reflection, attenuation and transition, wave velocity, or wave modes, linear guided waves cannot detect microscale damage such as early stage fatigue, corrosion, micro-crack, or microdelamination. In contrast, nonlinear guided wave have resulted promising due to incipient damage detection capabilities and reference-free potential, and leveraged its advantages over linear guided waves. This thesis investigates the use of nonlinear guided waves via a wave-mixing approach, where two ultrasonic frequencies are used, and the spectral content of the response is expected to carry information of the damage. This thesis provides a physical insight into the wave-mixing technique for damage detection in structures. The phenomenon is investigated theoretically, numerically and through laboratory experiments. A number of published and prepared journal papers under the same topic is included in this thesis. In Chapter 1, an overview of the general concepts of Structural Health Monitoring and connected non-destructive testing techniques are introduced along with nonlinear guided wave techniques. A theoretical derivation to correlate the contact effect on a steel bolted joint with the spectral content of a signal response is proposed in Chapter 2. Thorough experiments were carried out and demonstrated the robustness of the technique. Following, in Chapter 3, identification of debonding type of damage in adhesively bonded joints is investigated through three-dimensional finite element simulations and experiments. Numerical and experimental results revealed that guided wave-mixing technique could effectively detect debonding damage. To further extend the advantages of guided wave-mixing for different materials, a composite laminate plate in studied in Chapter 4. In this study, an imaging technique relying of the combined frequency wave is proposed to identify delamination and locate the defect. The proposed approach relies on network of few transducers and does not require reference data from undamaged samples. Lastly, a short study is presented in Chapter 5, where noncollinear pulses of finite time duration and non-planar wave-front are able to generate a resonant wave that is able to measure material nonlinearity, which is subject of study for many early stage fatigue damage detection techniques. Overall, this thesis systematically revealed and capitalized the advantages of nonlinear guided wave-mixing technique for various types of damage in structures across a wide variety of materials.