Nonlinear guided-wave-based structural health monitoring : mechanism and system design for material degradation monitoring

Abstract

Structural Health Monitoring (SHM) techniques with nonlinear guided waves offer the ability for the incipient damage detection due to their high sensitivity to microstructural changes in materials. However, amplitudes of damage-related nonlinear guided waves are usually small and their measurements are often confounded by other nonlinear sources in a practical system. Despite the significant progress made, these unavoidable nonlinear sources, as well as their impact on damage detection, have not been fully assessed and understood. The aim of this thesis is to provide a quantitative insight into some typical nonlinear sources and to assess their influences on the nonlinear-guided-wave-based SHM systems, thus facilitating the design of effective SHM systems to monitor material degradation. Two SHM methods, respectively based on the second harmonic Lamb waves and the third harmonic Shear Horizontal (SH) waves, are investigated. In the first part of the thesis, the second-harmonic-Lamb-wave-based SHM systems are investigated with PZT actuation. Two nonlinear sources are considered in the system: the localized material nonlinearity of the adhesive bonding layer which is referred to as the adhesive nonlinearity (AN) and the distributed material nonlinearity of the plate (MNP). Four tasks are accomplished. First, a nonlinear shear-lag model is developed to investigate the mechanism and characteristics of AN-induced Lamb waves in plates, covering the process of wave generation, propagation, and sensing. Second, through combining the nonlinear shearlag model and the normal mode expansion method, a more comprehensive theoretical model is established to study both the AN and the :tv1NP under the same roof. The model is then applied to guide the design of effective second-harmonic-Lamb-wave-based SHM systems with the mitigated AN effect. Third, the nonplanar propagating second harmonic Lamb waves generated by both localized AN and distributed MNP are characterized with a newly proposed refined nonlinear parameter, allowing precise demarcation of the localized and distributed nonlinear sources. Fourth, the designed optimized second-harmonic-Lamb-wave-based SHM system is validated by experiments and applied to monitor the material degradation in plates treated by two different thermal aging schemes. The second part of the thesis focuses on third-harmonic-SR-wave-based SHM systems. Considering both the AN and the MNP, this part includes three tasks. First, the mechanism of the third harmonic SR wave generation is investigated, highlighting a mixed mechanism of the quadratic mutual interaction of the primary SR waves and second harmonic Lamb waves. Second, a fully coupled theoretical model is developed to investigate the linear SRO wave generation with PZT actuators. The peak region and the highly dynamic region on the frequency tuning curves are scrutinized and discussed as well as their impact on the SRO wave generation. Third, some relevant issues which have been discussed for the second harmonic Lamb wave cases are revisited in the context of the third harmonic SR waves, e.g., the excitability of the MNP-induced third harmonic SR waves by the PZT actuators, the influence of the AN on the third-harmonic-SR-wave-based SHM systems and the refined nonlinear parameter to characterize nonplanar third harmonic SR waves generated by both localized and distributed nonlinear sources.