Investigating the multilayer fiber-reinforced structure of the wood cell using computer simulations and additive manufacturingPart of:
The arrangement of stiff helicoidal fibers into a softer matrix is a widespread strategy adopted by nature to finely tune local mechanical behavior of biological materials. The wall of wood cells, for instance, is traditionally described as three-layer structure; each layer consists of a matrix reinforced by cellulose microfibrils. In the thick middle layer, microfibrils show a twisted motif forming an angle with the cell axis, pivotal in modulating both stiffness and failure strains of the wood material, with the stiffer configurations being also the less deformable. Microfibrils are less ordered in the outer and inner layers, that occupy 15% of wall thickness and whose potential biomechanical role is less understood. The aim of this work is to investigate the mechanical behavior of wood-like synthetic composites as a function of fiber orientation in different concentric layers. As starting configuration, we considered single-layer cylindrical shells made of a soft matrix reinforced by stiff spiral fibers at various winding angles. The role of thin reinforcing layers was investigated considering their position (inner and/or outer with respect to the main middle layer) and fiber direction (vertical or horizontal). We used finite element simulations to compute apparent stiffness and buckling resistance of the virtual composites. The most efficient position of the reinforcing layer for buckling resistance resulted to depend on the orientation of the fibers in the middle layer: increasing the winding angle shifted it from the outside to the inside of the cylinder (with horizontal reinforcing fibers). The opposite was true for reinforcing layers having vertical fibers. Moreover, three-layer configurations with both inner and outer layers having horizontal fibers resulted in the highest increase in buckling strength with no modification of apparent stiffness and sample dimensions, uncoupling strength from stiffness by adding inner architecture. We then used multi-material 3-dimensional polyjet printing to fabricate three-layer cylindrical shells, with each layer featuring an elastomeric matrix reinforced by spiral fibers of a rigid polymer. Quasi-static compression tests revealed the fundamental role of horizontal reinforcements in avoiding catastrophic failure after the buckling critical stress peak. Therefore, construction principles observed in wood can enhance mechanical performance and could extend functionalities of polymer-based composites.