Damage identification in composite laminates and sandwich structures using ultrasonic guided waves and a 3D laser vibrometer

This thesis addresses the feasibility of using ultrasonic guided waves (UGWs) to detect and characterise the barely visible impact damages (BVID) that can develop in thin composite laminates and composite sandwich structures (CSS) by carrying out a fundamental investigation into wave-damage interaction. The interaction of UGWs with BVID in a structure is analysed using full wavefield data obtained by a Laser Doppler Vibrometer (LDV) and by numerical simulations. Multiple signal and image processing techniques are proposed to enhance the features relating to damage. The findings from this analysis are then incorporated into an in-service structural health monitoring (SHM) methodology using a sparse network of piezoelectric transducers. For the laminated composite panels, isolated subsurface delaminations between plies and complex BVID caused by a low velocity impactor are investigated. Both cases show that the first symmetric mode, S0, causes mode conversions when interacting with the defects whilst the first anti-symmetric mode, A0, mainly causes a change in phase and amplitude across the defects. Both cases also show that as the damaged area got more severe, the effects of the damage became more pronounced. The findings are then integrated and validated by a delay & sum algorithm to show the UGWs potential as an in-service SHM methodology. The focus of research then turns to the theoretical fundamentals of UGW propagation through CSS. The underlying mechanics of UGWs in CSS, including the relation between panel thickness and the UGW wavelength as well as the energy transfer through the core are presented. It is noted that three main types of propagation can exist in CSS which are global Lamb waves, leaky Lamb waves and Rayleigh waves. Dispersion curves are obtained for the CSS and polar plots of group velocities show their anisotropic nature. The final part of the thesis focuses on damage detection and localisation in CSS using full wavefield analysis and a sparse network of transducers. Both fundamental modes can localise the BVID in the structure, even with the anisotropic behaviour of the core. Based on these results, this thesis concludes that UGW based SHM shows great promise as an in-service damage detection technique that can detect, localise and, in some cases, characterise impact induced defects in thin composite laminates and CSS.Open Acces

Effect of material anisotropy on guided wave propagation and scattering in CFRP laminates

Carbon fiber laminates, consisting of highly anisotropic fiber-matrix ply-layers, are widely used in aerospace applications due to their good strength to weight ratio. However, poor interlaminar strength makes them prone to barely visible impact damage (BVID), significantly reducing the load bearing capacity of aircraft components. Guided ultrasonic waves have been widely used for structural health monitoring (SHM) of composite structures. Guided wave propagation and scattering at circular delaminations in a quasi-isotropic laminate was modelled using full three-dimensional (3D) Finite Element (FE) simulations in ABAQUS. Non-contact laser measurements were performed to obtain the scattered wavefield at a film insert delamination. The influence of ply layer anisotropy and incident wave direction were investigated both numerically and experimentally. Scattering directivity patterns were calculated using a baseline subtraction method and 2D scattering matrices were obtained for all incident wave directions. Circular magnets were used as a scattering target and numerical and measured scattering patterns were compared with those of the insert delamination. Strong directional dependency was observed for incident and scattered waves around both delamination and magnets, indicating energy focusing along the outer ply layers of the laminate. For the delamination a strong forward wave was observed, with low amplitude in other directions, whereas the magnet blocked forward transmission of the wave, demonstrating distinct scattering behavior. The anisotropic effects and different scattering patterns should be considered for guided wave sparse array SHM to ensure the robustness of imaging algorithms

Nondestructive Testing in Composite Materials

In this era of technological progress and given the need for welfare and safety, everything that is manufactured and maintained must comply with such needs. We would all like to live in a safe house that will not collapse on us. We would all like to walk on a safe road and never see a chasm open in front of us. We would all like to cross a bridge and reach the other side safely. We all would like to feel safe and secure when taking a plane, ship, train, or using any equipment. All this may be possible with the adoption of adequate manufacturing processes, with non-destructive inspection of final parts and monitoring during the in-service life of components. Above all, maintenance should be imperative. This requires effective non-destructive testing techniques and procedures. This Special Issue is a collection of some of the latest research in these areas, aiming to highlight new ideas and ways to deal with challenging issues worldwide. Different types of materials and structures are considered, different non-destructive testing techniques are employed with new approaches for data treatment proposed as well as numerical simulations. This can serve as food for thought for the community involved in the inspection of materials and structures as well as condition monitoring