MECHANICAL DESIGN OF A NOVEL LATTICE STRUCTURE

The main objective of the present PhD thesis is the design of a novel lattice material, named Triply Arranged Octagonal Rings (TAOR). Lattice materials, thanks to their unique combination of properties, attract a great deal of interest in several engineering field, for instance automotive, aerospace and biomedical. The design and the subsequent investigations of the novel lattice material aimed to meet the mechanical and morphological requirements for biomedical applications. Biomimetic theory, the Gibson-Ashby model and a static finite element (FE) analysis of regular polygons were taken into account in the development of the new lattice material. Eight octagons of the same dimensions were combined to each other forming a ring; to obtain an isotropic lattice structure, cubic symmetry was imposed, each unit cell is made of three rings mutually perpendicular, one ring for each principal direction. Extensive experimental and numerical investigations were conducted to evaluate the mechanical performance of the TAOR lattice and assess its suitability for applications in biomedical devices used as bone substitutes. Compressive tests were carried out and a comparison with other lattice materials currently used in the biomedical field was conducted. A scanning electron microscope (SEM) was useful to perform a morphological analysis in which the matching between designed and actual geometric parameters was evaluated. Moreover, SEM was used to evaluate the failure mode of the structures together with visual inspection. The Gibson-Ashby model was applied for the mechanical characterization of the TAOR cell. The model allows a direct comparison between the mechanical properties of given lattice materials and it helps to estimate their compressive behaviour. The mechanical properties, calculated through the compressive tests, were plotted in a graph against the relative density and their power-law relationships were estimated. The Gibson-Ashby model confirmed that the TAOR lattice presents a bending-dominated behaviour, already previously estimated from the application of the Maxwell stability criterion. A linear static FE model of the TAOR lattice was developed to estimate its elastic modulus, the stress distribution within the structure and the cell size effect. Non-linear FE models of the single unit cell and of lattices with 2 or 3 unit cells for each specimen edge were developed. The model of the single unit cell aimed to investigate the compressive behaviour and the failure mode of the cell, without the presence of adjacent cells that affect the deformation process. The model of the specimens aimed to evaluate the mechanical properties of the lattice material and compare the results with those obtained in the experimental tests. The results of the investigations allow to assert that the proposed TAOR cell meets the mechanical and morphological requirements for application in the biomedical field, thus it can represent a relevant design choice to produce bone scaffold. Further investigations are needed to assess the TAOR suitability with the biological requirement for application in the biomedical field. A case study, regarding the application of lattice structures in biomedical implants was conducted. In the present study, the rhombic dodecahedron, which is one of the most used lattice structures for applications in the biomedical field, was selected; in preparation for a future development of the research with the inclusion of the novel TAOR cell. The risk of subsidence was analysed for two different intervertebral body fusion devices. The subsidence represents a serious clinical issue during the healing process, mainly when the interfaces between the implant and the vertebral bodies are not well designed. The devices present the same shape, but one of them includes a filling rhombic dodecahedron structure. The effect of the lattice structure on the subsidence behaviour of the implants was evaluated by means of experimental tests and finite element analyses. Compressive tests were carried out by using blocks made of grade 15 polyurethane, which simulate the vertebral bone. Non-linear, quasi-static finite element analyses were performed to simulate experimental and physiological conditions. The experimental tests and the FE analyses showed that the subsidence risk is higher for the device without the lattice structure, due to the smaller contact surface. Moreover, in this device an overload in the central zone of the contact surface was detected and it could cause the implant failure. On the contrary, the presence of the lattice structure allows a homogenous pressure distribution at the implant - bone interface. The first part of the PhD was mainly focused on the study of strut-based lattice materials. To have a global view of the issue, a period of six months was spent at KU Leuven University in Belgium. During the stay, the focus of the research shifted on TPMS lattice materials. Compressive tests were carried out, in conformity to the ISO 13314 standard, to evaluate the mechanical behaviour of TPMS gyroid scaffolds. Gyroid lattice has been vastly investigated in literature and a wide range of mechanical properties have been evaluated; thus, an in-depth analysis was conducted to compare the results of the compressive tests with published models from literature. The data of the experimental tests and of the selected models from literature were plotted in a Gibson-Ashby diagram. The comparison shows that the mechanical properties of all the selected models match well for lower relative densities, while the results diverge at the increase of the relative density.