Wood-Inspired 3D-Printed Helical Composites with Tunable and Enhanced Mechanical Performance

Helical fibers are versatile building blocks used by Nature to improve mechanical performance and to tune local behavior of load-bearing materials. Helicoidal biocomposites are arranged in multiple layers with different fiber orientations. Such heterogeneity, not matched in synthetic materials, provides biological structures with superior properties. This is the case of the multilayer tube-like structure of the wood cell wall, where each ply features a compliant matrix reinforced by stiff helicoidal microfibrils. Here, 3D polyjet printing and computer simulations are combined to investigate wood-inspired helix-reinforced cylinders. Composites with a main layer containing helicoidal fibers, bordered by inner and outer plies having thinner fibrils are considered. It is shown how the mechanical functionalities of the synthetic structures can be programmed by varying fibers/fibrils orientation and matrix compliance. It is demonstrated that failure resistance can be enhanced by enclosing the main helicoidal layer with a minimum amount of thin fibrils oriented perpendicular to the applied load, as observed in wood. Finite element simulations are used to highlight the critical role of the matrix in load-transfer mechanisms among stiff elements. These structures have the potential to be assembled into larger systems, leading to graded composites with region-specific properties optimized for multiple functionalities.