Conference Agenda

Defects in plates significantly affect the local propagation characteristics of ultrasonic guided waves. Specifically, crack-like defects can introduce various non-classical nonlinear wave responses due to clapping, friction and hysteretic effects. These phenomena not only introduce nonlinear elastic wave components into the plate, but also lead to the emission of nonlinear acoustic waves into the surrounding air. Hence, airborne nonlinear emission analysis offers potential to detect and localize defects in the plate, without actually measuring on the plate itself.

In this study, SLDV measurements have been performed using the acousto-optic principle, i.e. the SLDV probes the air column above the plate, rather than the plate surface itself. In our approach, the SLDV scans a single line at some distance above the plate, capturing a line projection (1D) of the nonlinear emission field. The plate is mounted on a rotation stage, and multiple line projections were recorded for various rotation angles. Following the concept of computed tomography, this set of line projections is used to reconstruct a planar cross-section (2D) of the nonlinear emission field. By coupling this planar cross-section with a holographic reconstruction technique, a full volumetric representation (3D) of the nonlinear emission field is obtained. Depending on the excitation strategy, different nonlinear emission components, e.g. higher harmonic and modulation sideband, can be isolated and imaged. This allows to get deeper insight in how guided waves couple with defects in plates, which nonlinear wave components are generated, and finally how these generated nonlinear wave components radiate their energy into the surrounding air.

Our results show that the developed tomographic-holographic methodology is able to convert 1D measurement data to a full 3D volumetric representation, even for nonlinear emission signals with a very low signal-to-noise ratio. With this 3D volumetric representation, the presence and location of defects within the composite plate can be easily visualized. Results are presented from both numerical and experimental datasets obtained on composites with a range of delamination-like defects.

Composite materials are taking increasingly vital roles across industries like aerospace, automotive, and civil engineering, owing to their exceptional strength-to-weight ratios, corrosion resistance, and remarkable design versatility. Nevertheless, the inherent anisotropic nature and layered construction of composite structures make them susceptible to intricate and subtle damage mechanisms, such as delamination, matrix cracking, and fiber breakage. These forms of internal degradation, even if not externally apparent, present significant threats to structural integrity. Consequently, the precise detection of damage within composite structures is paramount, not only for ensuring structural safety but also for mitigating the environmental consequences of structural failure. The development and deployment of advanced structural health monitoring (SHM) systems are thus indispensable for enhancing the reliability and lifespan of composite materials in engineering applications.

This paper presents a numerical investigation on the directionality of nonlinear guided wave path interactions for damage imaging in composite structures. This study initiates with the discussion on mixed frequency response of nonlinear ultrasonics. An in-depth theoretical analysis via Finite Element Modeling (FEM) was further carried out. A numerical model of circular composite laminate is established with non-reflective boundaries surrounding the plate. The effective generation of guided wave modes is investigated, where the pure wave mode excitation is accomplished by applying pin-force pairs on the numerical model, representing the low frequency pumping wave and high frequency probing wave. Then various kinds of delamination damage are implemented at the center of the composite laminate, located across certain layers. Consequently, by setting a series of sensing points surrounding the damage, the wave scatting directionality of the mixed frequency component is determined. Meanwhile, the mechanism of nonlinear frequency response is investigated and the feasibility of damage detection utilizing nonlinear wave mixing phenomenon is verified through numerical analysis. Simulation results show that the pumping and probing waves travel along multiple wave paths simultaneously in the composite plate, crossing each other for involving nonlinear interactions at the damage sites. Nonlinear mixed wave components are generated, which could provide indicative information for localizing particular damage sites on the plate. The findings of this research exhibit promising application potentially in damage detection for composite structures, contributing to the enhancement of manufacturing quality and the prevention of unexpected failures across industries. This paper finishes with summary, concluding remarks, and suggestions for future work.

The detection of bolt loosening based on nonlinear vibro-acoustic modulation (VAM) often shows unignorable measurement uncertainties as a result of the non-monotonic variation of nonlinear intensity, thereby limiting its detectability for early-stage loosening. This challenge originates from the elastic-plastic change of the micro-scale rough contact under a wide range of bolt preload loosening, which cannot be analytically interpreted using conventional macro-scale interfacial stiffness models. To address this challenge, we develop a new theoretical interfacial stiffness model to analytically predict the monotonicity of nonlinear intensity in VAM techniques. In the model, the roughness-related parameters are innovatively incorporated in its power law governing the plastic asperity softening at a rough interface under contact concentration, allowing for a prediction of the non-monotonic VAM response through bolt loosening. This theoretical model is validated both numerically and experimentally, and results have demonstrated that the non-monotonic nonlinear intensity observed during the bolt loosening is attributable to the elastic-plastic evolution of the surface roughness at the bolted interface and can be quantitatively described using the developed model. With this model, , a new bolt loosening detection method based on the vibro-acoustically modulated waves is developed, by leveraging the thermo-elastic-plastic susceptibility of the modulated elastic waves under varying temperatures, referred to as thermally interfered vibro-acoustic modulation (TI-VAM). In the method, a quasi-dynamic simulation of interfacial stiffness during thermal interference on the bolted joint is integrated to predict the optimal temperature conditions for enhancing TI-VAM responses during the early loosening stage of bolted joints. Experimental validation between 10°C and 50°C confirms the effectiveness of the TI-VAM method. As the temperature rises to the optimal temperature as predicted by in the qausi-dynamic simulation, the magnitude of the nonlinear counterpart at 74% yield strength is observed to increase by 15 dB. Additionally, the modulation index shows a monotonic decrease of 25% during the early loosening stage (from 74% to 55% yield strength), thereby improving the reliability of the diagnostic process. Furthermore, the TI-VAM method demonstrates remarkable robustness over the variation of ambient temperature, indicating its adaptability to different experimental and operational conditions and potential for widespread use in practical engineering applications involving bolted connections.