Harmonic generation at a nonlinear imperfect joint of plates by the S0 Lamb wave incidence

Nonlinear interaction of Lamb waves with an imperfect joint of plates for the incidence of the lowest-order symmetric (S0) Lamb wave is investigated by perturbation analysis and time-domain numerical simulation. The imperfect joint is modeled as a nonlinear spring-type interface, which expresses interfacial stresses as functions of the displacement discontinuities. In the perturbation analysis, under the assumption of weak nonlinearity, the second-harmonic generation at the joint is examined in the frequency domain by the thin-plate approximation using extensional waves. As a result, the amplitude of the second-harmonic extensional wave is shown to be in good agreement with the result of the S0 mode in a low-frequency range. However, it is found that the thin-plate approximation does not reproduce the amplification of the second-harmonic S0 mode, which occurs due to the resonance of the joint. Furthermore, the time-domain analysis is performed by the elastodynamic finite integration technique (EFIT). When the amplitude of the incident wave is relatively large, the fundamental wave and the second harmonic exhibit different behavior from the results by the perturbation analysis. Specifically, if the incident amplitude is increased, the peak frequency of the second-harmonic amplitude becomes low. The transient behavior of the nonlinear interaction is also examined and discussed based on the results for the weak nonlinearity

Second-harmonic generation of the lowest-order antisymmetric Lamb wave at a closed parallel crack

The second-harmonic generation of the fundamental antisymmetric Lamb wave at a closed parallel crack in an elastic plate is studied by numerical analysis. The closed crack is modeled as a spring-type interface with quadratic nonlinearity. Based on a perturbation method, the problem of nonlinear Lamb wave scattering is decomposed into two linearized problems, i.e., for the linear reflection/transmission of the incident Lamb wave at the crack and for the generation/radiation of the second-harmonic Lamb waves due to the contact nonlinearity. The reduced problems are solved by the finite element method in the frequency domain. Numerical results demonstrate significant effects of the crack resonance on the linear and nonlinear Lamb wave scattering responses, which appear as sharp peaks/dips in the reflection/transmission spectra and enhanced second-harmonic amplitudes at some frequencies. Two simple frequency selection rules are established which explain the enhanced generation of the second-harmonic Lamb waves. The time-domain analysis is also carried out to supplement the frequency-domain analysis, which confirms that the incident Lamb wave interacts with the crack at some specific frequencies in its bandwidth in a selective manner and enhances the generation of the second-harmonic components

The application of ultrasonic NDT techniques in tribology

The use of ultrasonic reflection is emerging as a technique for studying tribological contacts. Ultrasonic waves can be transmitted non-destructively through machine components and their behaviour at an interface describes the characteristics of that contact. This paper is a review of the current state of understanding of the mechanisms of ultrasonic reflection at interfaces, and how this has been used to investigate the processes of dry rough surface contact and lubricated contact. The review extends to cover how ultrasound has been used to study the tribological function of certain engineering machine elements

Guided wave mixing for damage detection in structural elements

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.Thesis (Ph.D.) -- University of Adelaide, School of Civil, Environmental & Mining Engineering, 202

Nonlinear guided wave mixing in pipes for detection of material nonlinearity

Pipes have multiple applications in daily life and they are subjected to different types of defects. Nonlinear guided wave has attracted significant attention in detecting microstructural change at early stage of material deterioration. Extensive research using wave mixing with different wave modes has focused on plate-like structures. However, limited experimental studies have been conducted on the detection of material nonlinearity in pipes using two interacting guided waves. This study investigates nonlinear features generated due to mixing of torsional guided waves and material nonlinearity in pipes at low frequency range. The nonlinear theory of elasticity is implemented in a three-dimensional (3D) finite element (FE) method to simulate the effect of material nonlinearity on torsional guided wave mixing. The phenomenon of nonlinear features generated due to torsional guided wave mixing is investigated by 3D FE models. There is good agreement between the data obtained in the laboratory and numerical simulation results. This study demonstrates the existence of the combinational harmonic generation experimentally and provides physical insight into the phenomenon of nonlinear wave mixing. The findings of this study can further advance the damage detection techniques based on material nonlinearity in wave mixing.Carman Yeung, Ching Tai N