Investigation of mechanical properties and damage behaviour in TFP composites: a multiscale characterisation and modelling framework

The application of carbon fibre reinforced composites is gradually increasing in product fabrication in the aerospace and automotive industries due to their superior mechanical performance. However, the inherent anisotropic properties of fibre reinforced composites present challenges when these structures are subjected to multi-axial loads. On the other hand, the manual placement of fibre preforms for manufacturing complex structures leads to material wastage, as preforms need to be trimmed to fit the mould. In recent years, automated fibre placement techniques have been developed to enable multi-directional fibre orientations within the same ply, reducing material wastage while maintaining high mechanical performance. Tailored Fibre Placement (TFP) technique, as one of the advanced fibre placement approaches, provides greater manufacturing flexibility and reduced material usage without compromising the quality of composite structures. Nevertheless, there remains a lack of numerical tools capable of predicting the mechanical behaviour of composite laminates produced using the TFP technique. Therefore, this thesis seeks to develop multiscale models to predict the elastic properties and strength of composites laminates, focusing on characterising their microstructure morphologies at the mesoscale level and the development of the unit cell model.
To create highly precise multiscale models of composites, experimental characterisations were conducted at three scales, namely micro, meso, and macro, with particular emphasis on the development of the mesoscopic model. Fibre distribution behaviour was statistically analysed at the microscopic scale using Scanning Electron Microscopy (SEM) alongside several statistical descriptors. A microscopic model was developed using a Python script. The fibre bundle structure was characterised using X-ray imaging, and an idealised mesoscopic model of the TFP composite was designed, identifying the fibre bundle structures. The homogenised properties, simulated from the mesoscopic model with different morphologies, were explored through the macroscopic model. The elastic and strength properties of TFP composite laminates were predicted and validated. The stress-strain response from the TFP composite laminate model was compared to in-situ experimental results obtained from X-ray Computed Tomography (XCT). Modelling results were compared to three-dimensional strain distribution measurements obtained using a novel Digital Volume Correlation (DVC) algorithm developed in this research. The Hashin damage criterion was employed to predict crack initiation and propagation in the TFP composite. Numerical results were compared and validated with experimental results from XCT images.
This work has highlighted the critical role of experimental characterisation in developing the multiscale models for composite laminates. By considering manufacturing parameters, the multiscale model developed in this work for TFP composites shows reasonably good agreement with experimental results. Moreover, it highlights significant potential for predicting and optimising the mechanical performance of TFP composite structures fabricated using varying manufacturing parameters.