Design, analysis, and optimisation of discontionuous tailorable composites
Creators: name: Finley, James Michael
Publisher: name: Imperial College London
Year: 2019

description: Composite materials are an attractive material for use in the aerospace and automotive sectors, due to their high specific strength and specific stiffness. However, most composites fail suddenly in a brittle manner, which may result in catastrophic failure. This inherent lack of tailorability in the mechanical response of composites has made their widespread application limited. Aligned discontinuous composites (ADCs) are a new type of material that offers increased mechanical tailorability, as subtle changes to the ADC microstructure can change its stress-strain response from high-strength linear elastic behaviour, to a non-linear response with high levels of pseudo-ductility. This work aims to understand mechanisms for mechanical tailorability in ADCs, and determine the suitability of ADCs for applications in real-world structures. A Virtual Testing Framework (VTF) is used and developed throughout this thesis, to give an in-depth understanding of the behaviour of ADCs. The VTF consists of a multiscale framework to simulate complete ADC specimens with hybrid or non-hybrid reinforcement; final failure of ADCs is simulated in the VTF by a non-linear fracture mechanics failure criterion. The VTF is successfully validated throughout this thesis against a variety of experimental data with good agreement. The first study explores the use of different hybrid fibre-type arrangements, to explore their influence on the pseudo-ductile behaviour of hybrid ADCs. The addition of a fibre-migration algorithm (used to capture the effects of fibre movement in ADCs during their manufacture), and developments to the fracture-based failure criterion, enable the VTF to successfully capture the effects of various levels of fibre-type grouping. Hybrid ADCs with larger groups of low-elongation fibres demonstrate a more brittle response, whereas intimate intermingling of the fibre-types increases pseudo-ductility. The ultimate strength and pseudo-ductility are then maximised by employing a fibre-type arrangement where the low-elongation fibres are completely isolated from one-another. Having shown variability in the fibre-type arrangement has a significant influence on tailorability, the concept of variability is explored further. Several new sources of variability and defects are added to the VTF, and the non-linear fracture-based failure criterion is further improved to accurately predict the failure of any type of ADC microstructure. The VTF indicates that the most critical source of variability (i.e. the source of variability that most readily promotes premature failure) depends on the microstructural design of the composite: hybrid-fibre ADCs are most sensitive to the fibre-type arrangement, while short-fibre non-hybrid ADCs are most sensitive to variability in the fibre overlap lengths, and long-fibre non-hybrid ADCs are more sensitive to weaker fibres. The VTF is then coupled with an intelligent optimisation routine to maximise the mechanical performance of ADCs that feature inherent variability. The use of a surrogate model helps minimise the number of design iterations before optimal design(s) are converged upon; surrogate modelling is also demonstrated to be invaluable when assessing the optimality of materials which feature high levels of inherent variability (such as ADCs). The true tailorability of ADCs is showcased, with an initial stiffness of 505 GPa, an ultimate strength of 1.92 GPa, or an ultimate strain of 3.92 % all achievable in ADCs via single-objective optimisation. While significant trade-offs between competing mechanical performance characteristics are found, useful combinations of mechanical properties remain, with an optimal ultimate strength &amp; ultimate strain combination of 982 MPa and 3.27 %, or an optimal combination of 720 MPa yield strength &amp; 1.91 % pseudo-ductile strain all possible. ADCs are shown throughout this thesis to fail by translaminar fracture, yet the resistance of ADCs to translaminar cracks has never been measured; the translaminar fracture toughness is therefore a key material property that must be characterised, to enable accurate failure predictions using the VTF, and to enable the use of ADCs as part of a damage tolerant structure. Specially-designed compact tension tests are performed to measure the translaminar fracture toughness of hybrid and non-hybrid ADCs for the first time in literature. Results show that the translaminar fracture toughness of non-hybrid ADCs is significantly higher than hybrid ADCs, and that analytical predictions of the translaminar fracture toughness are sufficiently accurate to ensure realistic failure predictions using the VTF. The new understanding and optimisation of aligned discontinuous composites demonstrates the feasibility of ADCs as an alternative material for the aerospace and automotive sectors. The models developed as part of this study can be used to support the design of new materials, perform parametric studies, and perform intelligent optimisation campaigns.