Finite element analysis of torsional behaviour of biomimetic structures

Recently, there has been a growing interest in materials that are able to defeat the classical materials design problems concerning their law versatility. Engineers and scientists have been searching for smart systems based on nature to exploit new possibilities and perspectives. As a consequence of this endless interest, the idea of biomimetics is born, whose fundamental concept is to explore advanced solutions and mimic the natural morphologies taking nature as a source of inspiration. In the literature, the human cortical bone has been extensively studied due to its remarkable mechanical properties comparable to the engineering materials such as ceramic and metal alloys. This has been attributed to the fact that its advanced hierarchical morphology and the presence of its functional unit osteon provide an outstanding fractural resistance. Among other materials, other biological materials have been taken as inspiration such as bamboo. Another intriguing biological material is horsetail. Even though it has not been studied in detail in the literature, it has been reported to have a good absorption capacity due to its multicell tube structure. Regarding this, the morphology of the cortical bone, the bamboo, and the horsetail has been taken as an inspiration. In this project, the purpose lies in designing 3D bio-inspired composite structures that provide a good fracture resistance under torsion. In this context, a macroscopic cylindrical specimen has been characterized by a matrix made of a polymeric stiff material (VeroCyan E ≈ 735 MPa) in which a soft rubber is embedded (Agilus30, E ≈ 0.8 MPa). The stiff matrix is to mimic the lamellae which surround the functional units -osteons in a bone-, whilst the soft inclusions are to mimic the functional units themselves. Some of the parameters are kept constant for each structure such as the volume fraction (50 %) and the cortex thickness (∼ 0.7 mm). Designed structures are divided into two sample groups as bone-inspired and bio-inspired. In the first group, the structures are designed considering the typical features of cortical bone varying the osteon shape as cylindrical and elliptical and adding a hollow bar inside the structures. In the second group of materials, the inner diameter of the structures is increased stepwise in order to see the effect of the hollow bar inside the structures. As a general comparison, the effect of different cross-sectional configurations is examined under torsion. At the end of the analyses, it has been seen that adding an inner hollow bar improved the fracture resistance of the structures, however, it might not be deduced a strictly linear trend between the mechanical properties and the diameter of the inner hollow bar. Numerical results show that a further advantage of these heterogeneous structures is that depending on the configuration of the cross-sections, the mechanical properties of the materials can be tailored based on the application area. In a few words, the numerical results that have been obtained in this project might be confirmed and developed by experimental tests employing additive manufacturing techniques. Later, in order to analyse the other parameters, the level of hierarchy can be increased by adding a third material into the component to be able to optimize the torsional properties of the structures.