Bioinspired pseudo-ductile composite laminates with hierarchical energy absorption mechanism

Abstract

The large diffusion of Carbon Fibre Reinforced Polymers (CFRP) over the last three decades in multiple industrial sectors is due to their excellent in-plane mechanical properties and their exceptional strength-to-weight-ratio. As the use of CFRP moved from non-structural parts to primary structures however, the intrinsic layered nature of these materials and their consequent weak resistance to out-of-plane solicitations has changed the safety approach used for traditional ductile materials, shifting the design paradigm towards more severe safety margins. This zero-damage manufacturing strategy, necessary to prevent catastrophic failures, led to overdesigned composite parts, preventing the full exploitation of their unique characteristics and limiting their use in harsh environments. Based on this premise, the possibility to manufacture composite laminates able to respond with a pseudo-ductile behaviour when subjected to an out-of-plane load is of crucial importance as it would eliminate the need of overdesigned parts and extend the range of applications available to composite structures. This project is aimed to the design, manufacturing and characterisation of a bioinspired CFRP laminate in which the pseudo-ductility arises from an ordered pattern of discontinuities which are created over the surface of the different layers before the curing reaction. The presence of this carved pattern creates a hierarchical interplay of high-strength carbon fibre segments and elastic soft matrix-rich areas which resembles the interaction between the β-sheets crystalline domains and amorphous helical and β-spiral structures typical of spider silk and other biological structures (e.g. cellulose, hair) which enables a combination of high mechanical strength and elasticity. The effect of different geometrical parameters of the carved pattern such as critical length, shape and dimensions, on the mechanical properties of the laminate have been analysed via Finite Element Analyses in order to identify the optimal configuration of the discontinuities, finding the best trade-off between in-plane and out-of-plane mechanical properties. Samples with different carved patterns were then manufactured and their properties were assessed by subjecting them to three-point-bending test. The internal distribution of damaged areas was assessed via different Non Destructive Techniques and was compared with the behaviour of traditional CFRP. Results showed that the presence of the artificial discontinuities is able to induce pseudoductile behaviour into the CFRP, improving the energy absorption mechanism during out-of-plane solicitations without severely affecting the in-plane properties.