| Summary: | A new bio-inspired microstructure design approach was developed to improve the translaminar toughness and damage tolerance of Carbon Fibre Reinforced Plastic (CFRP) structures. The microstructure designs take inspiration from the microstructures of biological composites by adopting the most important toughening mechanisms, and applying them to CFRP laminates. Carefully placed patterns of laser-engraved micro-cuts are inserted in the microstructure of the laminate during the manufacturing process. These micro-cuts change the crack propagation path during translaminar fracture, hence allowing to engineer the fracture behaviour of the composite. The microstructure design approach led to remarkable improvements in the maximum tensile load (up to 189%) and translaminar work of fracture (up to 460%) during Compact Tension test for CFRP laminates with Cross-Ply and Quasi-Isotropic (QI) layups when compared with the corresponding un-modified laminates. Furthermore, a significant improvement in the damage resistance under indentation test was demonstrated for a QI laminate with engineered microstructure. These results demonstrate that microstructure design holds the potential to improve the damage tolerance of CFRP structures in industrially-relevant applications. A semi-analytical Fibre Bundle Model (FBM) was developed to investigate the role of dynamic stress concentrations, and of fracture mechanics-driven failure, on the longitudinal tensile strength of fibre-reinforced composites. In particular, the investigation was focused on the size effect: a decrease in the bundle strength with an increase in the number of fibres. To the knowledge of the author, it is the first attempt in the literature to investigate these two physical mechanisms in a FBM. It was shown that, although the dynamic stress concentrations significantly decrease the predicted bundle strength, do not allow to predict the right trend of the size effect shown by the experimental results. On the contrary, including fracture mechanics-driven failure in the bundle simulation allowed to predict the right trend of the size effects on the bundle strength. These results suggest that fracture mechanics is a physical mechanism which might be necessary to consider to correctly predict the longitudinal tensile strength in large composite bundles.
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