Biomechanical role of thin reinforcements layers in helicoidal composites inspired by wood-cells: from field study to transfer to 3D printed composites

The arrangement of stiff helicoidal fibers into a soft matrix is a strategy used by nature to finely tune local mechanical behavior. As an example, the main structural role in wood-cell is played by a thick middle lamella consisting of a compliant matrix reinforced by a twisted motif of cellulose microfibrils. Their angle formed with the vertical axis (microfibril angle) is pivotal in modulating both stiffness and failure strains up to almost 2 orders of magnitude with the stiffer configuration being the less deformable [1]. The remaining 15% of wood-cell wall thickness is usually occupied by two other layers positioned one externally and one internally with respect to the main layer and with a less defined biomechanical role. The aim of this work is to study the role of thin reinforcement layers in helicoidal composite. To reach this goal, we designed cylindrical shells made of a soft matrix reinforced by a middle layer of thick spiral fibers (60% of wall thickness) with varying microfibril angles (at constant fiber number and volume fraction). The role of secondary reinforcement layers was investigated inserting the layers internally and/or externally with respect to the main one and considering vertical and horizontal fibers, alternatively. The mechanical behavior was characterized through a simulated linear buckling analysis and revealed that the most convenient position of the reinforcing layer for buckling strength enhancement changes with the main layer angle: considering low microfibril angles, the most convenient position of the vertical reinforcement is inside the main layer and the contrary happens in case of horizontal reinforcement. On the other hand, when the main layer has a higher microfibril angle the situation becomes the opposite. Moreover, if the reinforcement is horizontal there is an increase in buckling resistance independent to the stiffness increase, as these reinforcements do not change the overall stiffness significantly: in this way stiffness and strength are decoupled, keeping structure overall dimension constant. The stiffness ratio between the fibers and the matrix was also studied, considering values ranging from 60 to 6 and showing that the more similar the mechanical properties are, the less important the angle of the main layer is. After feasibility tests and subsequent scale-up of the models, samples with spiral angles of the main layer from 0 to 30 degree and double reinforcement layers (both internal and external), were produced using multi-material 3-dimensional polyjet printing. The cylindrical shells having an elastomeric matrix reinforced by spiral fibers of a rigid glassy polymer were tested under uniaxial quasi-static compression and showed not only results in good agreement with the corresponding simulated configurations, but also the fundamental role of horizontal reinforcements in avoiding a catastrophic failure after the maximum stress was reached. The introduction of thin reinforcement layers in helicoidal composites seems, therefore, a promising route to enhance buckling resistance, with or without changing the structure stiffness and to have a less dramatic failure.