Identification of damage in composite structures based on feature guided ultrasonic waves

• Main Author: Yu, Xudong
• Other Authors: Fan Zheng, David
• Format: Theses and Dissertations
• Language: English
• Published: 2018
• Subjects: DRNTU::Engineering::Aeronautical engineering
• Online Access: http://hdl.handle.net/10356/73449
• Institution: Nanyang Technological University

Complex-shaped composite laminates are increasingly utilized as Principal Structural Elements (PSE) such as spars, ribs, and the skin-stiffener combination in the aerospace industry. However, due to the stress concentration and out-of-plane impacts during the service life, various types of defects can be initiated in these structures, which have to be identified timely in case they propagate at subsurface laminae and ultimately lead to catastrophic failure. Therefore, this thesis explores the potential of exploiting a special type of ultrasonic guided wave, i.e. feature guided wave (FGW), for rapid screening of long-range composite structures. Such FGWs are capable of focusing their wave energy along the structural feature, with limited leakage into the adjacent plate. Two typical composite features are investigated in the thesis, which are laminated bends and adhesive composite joints, and the results are encouraging. Modal analysis of the laminated bend is carried out by using the Semi-Analytical Finite Element (SAFE) approach, in which the spatially-varying anisotropy is implemented to reveal the existing FGW modes. A shear horizontal type bend-guided (SHB) mode has been identified. The mode is almost non-dispersive and non-leaky, with strong energy confinement in the bend region, which is attractive to be applied as a screening tool for composite bends. Both 3D Finite Element (FE) simulations and experiments are performed to cross-validate the propagation characteristics of the SHB mode and to study its interaction with different defects occurred in the bend region, such as the interlaminar delamination and cracks, showing good agreement. The wave-defect resonance phenomenon and the reflection behaviour are investigated for localizing these two types of defects, and the capabilities of the SHB mode for efficient damage detection in long-range composite bends have been well demonstrated. Also, adhesive bonding is widely used in aerospace composites, and its integrity is crucial to the performance of the entire structure. This motivates the investigation of FGWs in another typical composite assembly, namely the stiffener-adhesive bond-composite skin combination. The SAFE method is employed to understand the modal properties of the FGWs that exist in such assembly, and criteria are suggested to choose proper mode-frequency combination that is sensitive to adhesive defects. An SH type FGW mode has been identified to be well suited, as it produces sufficient power flow in the bond layer and is little-dispersive and low-attenuative, as well as easy to generate. Its scattering by adhesive defects is studied both numerically and experimentally, and the modelling approaches have been validated. The reflection coefficient spectra of the selected FGW mode and the variation of its wave energy along the propagation path are calculated, identifying the adhesive defect between the stiffener and the composite panel. Additionally, parametric studies of defect geometry illustrate the influences of the variation in defect size and bond thickness on the reflection behaviour, shedding light on its practical employment. This thesis has also extended the study of FGWs to structural health monitoring (SHM) applications. In contrast to conventional guided wave SHM schemes, the structural features are no longer regarded as interference but special 'local' waveguides for the transport of the guided wave energy. Proper FGWs with strong energy confinement are exploited for rapid condition screening of the feature area, and an array of sensors permanently attached to the adjacent plate captures the scattered waves from the localized damage. This technique is combined with the synthetic focusing method for the damage reconstruction. The defect can be identified by the indications coherently enhanced in the reconstructed image. Information about the location and severity of the defect can thus be obtained. The proposed FGW based SHM strategy potentially offers enhanced interrogation of the feature area, which can also be implemented in conjunction with the existing guided wave SHM system to yield richer information about the health state of the entire structure.