Bio-inspired designs for enhanced damage resistance of composite structures: Bouligand, Herringbone and multi-tailored bone-inspired strategies

Abstract

This thesis presents three novel bio-inspired microstructural concepts, for point-by-point tailorable composite solutions, that exploit the ability to diffuse damage in a sub-critical state; these aim at enhancing the damage resistance and the energy dissipation capability of conventional composite structures. To this end, we investigated three bio-inspired concepts: Bouligand, Herringbone-Bouligand and multi-tailored bone-inspired. The Bouligand concept, inspired by the impact-resistant periodic region of the mantis shrimp’s dactyl club and consisting of a helicoidal lay-up with very small inter-ply (pitch) angles, was used to address the inherent low performance to through-the-thickness loading (static and dynamic) of CFRPs. We designed, manufactured, tested and analysed several ultra-thin-ply CFRP Bouligand microstructures, with the smallest pitch angle (2:5°) ever reached in the literature. Experimental studies, as well as detailed analytical and numerical modelling showed that tailored Bouligand microstructures achieved enhanced damage resistance via: smooth double-helicoidal evolution of damage, reduced delamination areas and sub-critical Bouligand cracks. Quasi-static indentation test showed that, compared to conventional CFRPs, tailored Bouligand microstructures achieved a simultaneous 92% increase in load-bearing capability, 74% delay in catastrophic failure and 97% increase in total dissipated energy. The Herringbone-Bouligand concept, inspired by the high-damage-resistant features of the mantis shrimp’s dactyl club impact region and impact surface, was used to improve further the damage resistance of Bouligand-inspired CFRPs. We designed, prototyped, tested and analysed the first high-performance Herringbone-Bouligand microstructure in the literature. We also devised the first prototyping procedure to manufacture point-by-point tailorable Herringbone-Bouligand CFRPs. Quasi-static indentation tests showed that, compared to standard Bouligand solutions, the Herringbone-Bouligand microstructure delays onset of delaminations, reduces the in-plane spreading of damage (71%), increases the energy dissipation capability (13%) and contains damage within the tailored Herringbone-Bouligand region. The multi-tailored bone-inspired concept, based on the introduction of patterns of discontinuities in the form of laser-cuts across the load-carrying fibres of FRPs, was adopted to devise a versatile design strategy that can be used to locally-tailor different damage resistance requirements at different locations within the same structure. Specifically, for a SRPP/CFPP hybrid composite, we demonstrate that patterns of discontinuities can be successfully tailored to create high-energy dissipation paths through which damage is stabilised and diffused at sub-critical levels. This results in a great increase (90%) in energy dissipation with respect to a non-engineered structure, i.e. a similar SRPP/CFPP hybrid structure without discontinuities. We demonstrate that discontinuities can be successfully tailored to enhance impact damage resistance, achieved with increased energy dissipation at sub-critical levels and delayed catastrophic failure.