Bio-inspired micro-structural design for CFRP: exploring damage mechanisms of nacre

Abstract

Carbon-Fibre Reinforced Polymers (CFRP) are widely regarded as the material of choice for many aerospace and automotive applications, where high specific strength and stiffness are required. However, one of the main limitations to further extending the use of such materials is their inherent brittleness, which results in the difficulty to design damage-tolerant lightweight structures.

Among the innovative solutions recently proposed to improve the damage tolerance of CFRP, biomimetics provides a valuable source of inspiration. In particular, of the many biological composites, nacre is one that provides a remarkably tough behaviour, due to its discontinuous tiled micro-structure leading to damage-diffusion and crack-deflection mechanisms acting at the micro-scale.

In this work, a carbon-fibre/epoxy composite with nacre-inspired tiled micro-structure is firstly designed and synthesised. Analytical and numerical models are developed to identify suitable configurations for the tile geometry with interlocks, leading to tiles of about 600 μm which are then laser-engraved in the laminate plies. In-situ bending tests show how the nacre-like interlocking micro-structure succeeds in diverting cracks and avoiding localised failure, while toughness and heterogeneity of the interface between tiles are identified as key elements to promote further spreading of damage.

In order to improve the ductility of the interface, a film-casting technique is developed to deposit extremely thin layers (~13 μm) of poly(lactic acid) (PLA) onto the interface of carbon/epoxy prepregs. Different patterns of PLA texture (including fractals) are explored, with DCB and 4ENF tests showing an increase in interlaminar toughness of about 80% for Mode I and 12% for Mode II.

The film-casting method mentioned above is then used to modify the interfaces of the nacre-inspired laminate, by depositing thin PLA patches with fractal shape in between plies. This results in a thin texture of thermoplastic material that toughens the interface without significantly increasing the overall thickness of the laminate. Results show that damage diffusion is considerably enhanced by the tougher and more heterogeneous interface, which succeeds in creating more extensive pull-out of the interlocking tiles.

Finally, the interaction between nacre-inspired discontinuous micro-structures and continuous fibre-reinforced layers is analysed. It is shown how continuous layers can be used to trigger unstable failure in the nacre-like material, to act as a crack-propagation barrier, or to change the morphology of damage by promoting a transition from brittle failure to energy-dissipating tile pull-out.