NONLINEAR INTERACTION OF GUIDED WAVES WITH DELAMINATION DEFECTS IN LAMINATED COMPOSITE PLATES

Abstract

Advances in manufacturing have expanded the use of layered composites in aerospace, civil, and mechanical industries. However, their susceptibility to defects requires frequent inspections to guarantee safety and reliability. Guided wave-based non-destructive evaluation (NDE) is a preferred method for inspecting layered composites, due to its ability to detect defects over long distances and scan the entire thickness. Interlaminar delamination is a common mode of failure in layered composites, making early detection and characterization of defects critical for ensuring safe operation, which is a main goal of NDE. Guided wave-based nonlinear damage detection techniques are more effective and sensitive to incipient delamination defects than conventional linear damage detection methods. In its early stages, damage can create nonlinearity through various mechanisms. These may include the inelastic behavior of the material, thermoelastic coupling, and intermittent contact between damaged surfaces, leading to a nonlinear wave damage interaction. The incident wave interacts nonlinearly with the breathing or clapping of the delamination giving nonlinear acoustic signatures in the response spectrum, such as the generation of nonlinear harmonics (sub and /or super harmonics), mixed frequency response, and shift in the resonance frequency. These signatures can be a potential precursor of the presence of a delamination defect. This work explored the generation of these signatures resulting from contact acoustic nonlinearity (CAN) in glass fiber-reinforced plastic (GFRP) composites with defects. The focus of this thesis is on the development of a numerical framework and experimental in vestigation for examining the interaction between waves and delaminations in GFRP composites. Thestudy begins with a review of previous research on the generation of nonlinear acoustics from delaminations and their use in identifying damage. The numerical framework and experimental method used in this work are then discussed. Several GFRP composite plates are fabricated for the experiment using hand layup and vacuum-assisted resin transfer molding (VARTM). Both unidirectional, as well as angled laminates containing single as well as multiple delaminations, are used in the investigations. The analysis of the effects of nonlinear interactions between the A0-guided mode and delaminations in GFRP composite laminates is being carried out using a f inite element (FE)-based framework. The experimental investigations conducted in-house on GFRPcomposite laminates, with and without delaminations, have confirmed the accuracy of the FE analysis outcomes. Finally, the effects of the delamination feature and stacking sequences on nonlinear acoustics generation using single as well as dual frequency excitation signals are discussed in detail. The process of quantifying the nonlinear wave-damage interactions and their connection to the width, stacking sequence, and interlaminar location of delamination involves introducing parameters such as the higher harmonics index (HHI) and combination tone index (CTI). The underlying contact phenomenon is governed by two fundamental parameters; the phase difference between the wave packets passing through the sub-laminates and the flexural rigidities of the sub-laminates at the delamination site. The phase difference controls the sub laminate relative displacement, while the flexural rigidities determine the likelihood of collisions. Ultimately, the impact of these two parameters on the creation of nonlinear acoustics with various delamination features is revealed. It is explained further that the CTI is highly dependent on the amount of temporal overlap between the two wave envelopes as they travel through the breathing delamination. When the two wave envelopes are in sync (with 100 % temporal overlap), the CTI is at its highest, and it decreases as the temporal overlap decreases. A relation is established that links the temporal separation of the wave envelopes at the actuator, the group speeds, and the distance from the actuator to the delamination by utilizing the dispersive nature of the chosen guided mode. Based on this information, a method for detecting breathing delamination is proposed. This involves changing the temporal overlap in the input waveform and monitoring the CTI to find its maximum. The effectiveness of this localization technique, which is close to 90 %, is shown through both numerical and experimental analysis of an illustration. Specifically, the output from nonlinear interactions between guided waves and delaminations in GFRP composites is analyzed to characterize the delaminations using a probabilistic Bayesian approach with a focus on structural reliability. A Bayesian model-based technique, utilizing subset simulation, is introduced and applied to characterize multiple overlapping delaminations. The results show that the characterization process is determined by both log-likelihood and log evidence. Log-likelihood represents the agreement between actual and simulated data, while log evidence supports the existence of the model. According to the findings, the proposed method was highly successful in characterizing and estimating the number, position, width, and type of multiple overlapped delaminations.