Sandwich structures are frequently used in automotive, aerospace and marine industries, as they provide adequate functional properties. The two-dimensional regular hexagonal cell shape, i.e. honeycomb is the most used core structure in sandwich panels. Recently, a new type of cellular structures composed of lattice struts has been proposed, as they combine high stiffness, strength and energy absorption with low weight. The main purpose of this research is to investigate the effect of the lattice topology on the flexural behaviour of sandwich panels. Five lattice geometries inspired in crystalline structures were designed, namely, body-centred parallelepiped, body-centred parallelepiped with struts in z-axis, body- and face-centred parallelepiped with struts in z-axis, face-centred parallelepiped with struts in z-axis and parallelepiped simple. The relative density of all the lattices was kept constant as 0.3. Both numerical and experimental approaches were used to evaluate the flexural properties and failure behaviour of the sandwich structures under three-point bending tests. The numerical analysis was undertaken with the finite element software NX Nastran. Taking advantage of additive manufacturing technologies, material extrusion was used to produce polylactic acid samples with the configurations aforementioned. The sandwich panels are composed by a single layer formed by the lattice core and two thin plates, at the bottom and top. The three parts of the panel were manufactured all together. The simulation results indicate that, among the lattices studied, topologies body-centred parallelepiped with struts in z-axis and body- and face-centred parallelepiped with struts in z-axis exhibit higher strength, while body- and face-centred parallelepiped with struts in z-axis shows higher stiffness and higher energy absorption, attaining values that do not differ much from the ones obtained with a two-dimensional hexagonal cellular structure, with the same relative density. As a consequence, some of the geometries studied may have the potential to be considered as alternatives to conventional structures in the design of sandwich structures.

Functionally graded cellular materials, which combine cellular materials with a gradient of properties, have been recently investigated as cores of sandwich panels. The present work aims to evaluate the flexural properties of functionally graded cellular structures with a gradient of cell thickness on configurations recently proposed. The structures hexagonal honeycomb (Hr), lotus (Lt), and hexagonal honeycomb with Plateau borders (Pt) were designed with three different gradients of cell thickness. The parts were manufactured by material extrusion using FFF (Fused Filament Fabrication). To evaluate the mechanical properties, three-point bending tests were conducted, both experimentally and by numerical modelling, by means of the finite element method. The materials used were polylactic acid (PLA) and aluminum. Although experimental and numerical results exhibited some deviations, they revealed the same trends. The results showed that the stiffness and the absorbed energy of the graded Hr, Lt and Pt structures, made of PLA, are higher than such values for the regular hexagonal honeycombs. For the same gradient of cell thickness, the lotus structure tends to exhibit the highest stiffness and absorbed energy, while the hexagonal honeycomb arrangement achieves the largest value of strength. Graded aluminum specimens also attain higher values of stiffness, strength and absorbed energy in comparison with non-graded hexagonal honeycomb configurations. Thus, the use of gradients of cell thickness can promote structures with higher stiffness and absorbed energy, which may compete with the conventional structures of sandwich panel cores.

Biodegradable metals such as iron have become appealing for usage in temporary bone implants. Although iron possesses excellent properties of biocompatibility, it shows a very slow degradation rate and presents a higher stiffness and strength when compared to human bone. Several strategies can be applied to decrease the strength of the iron, one of them being the use of porous/cellular structures. The present work aims to study the mechanical properties and the degradation behaviour of porous iron with cellular structures composed of lattice struts. Six lattice geometries, which were previously studied, and inspired by crystalline structures, were designed. The mechanical behaviour of the lattice arrangements was studied numerically using the finite element software NX Nastran. The corrosion module of the finite element software COMSOL was used to simulate the degradation behaviour of the cellular structure immersed in simulated body fluid. After the degradation process, the compressive properties of the porous cellular structures were also assessed. Under compression, the iron lattice structures showed an elastic limit stress close to the values determined for the trabecular bone yield stress. Geometries selected to simulate degradation have shown mass loss percentage and corrosion rates close to the degradation rate established for an ideal bone substitute. The present study confirms that the structural arrangement has a strong influence on the mechanical properties of iron cellular structures. The degradation behaviour of iron lattices also appears to be affected by the unit cell topology.

Sandwich panels have a wide field of applications from aerospace to automotive industries. These panels are formed by a core and two layers, having the geometry of the core an important role in its mechanical properties. While the most common cores have hexagonal honeycomb structures, there is a recent trend to replace them with other cellular structures based on truss or lattice arrangements. Previous studies on modified atomic-based lattices reached the conclusion that modified body and face-centred parallelepiped, with vertical struts, denoted by the body- and face-centred parallelepiped with struts in z-axis, provided higher stiffness and absorbed energy with low weight, compared with the conventional design. These promising findings have motivated a further comprehensive investigation of this type of structure. The aim of the present study is to get the body- and face-centred parallelepiped with struts in z-axis structure that provides the highest stiffness and absorbed energy, in comparison with a previous work of the current authors, by changing its dimensions and relative density. To accomplish this purpose body- and face-centred parallelepiped with struts in z-axis lattices with three relative densities, namely, 0.25, 0.30 and 0.35 and with truss radii of 0.8, 0.92 and 1.1 mm were designed, respectively. Samples were printed by additive manufacturing of polylactic acid. The two face sheets (skins) and the core of the panel were manufactured altogether. The mechanical behaviour of the sandwich panels was assessed by three-point bending tests both experimentally and in numerical simulations with finite element analysis. A good correlation between experiments and numerical results was achieved. Simulation results revealed that the highest values of strength, stiffness and absorbed energy were obtained for the combination of the lowest relative density with the highest truss radius. A failure analysis revealed two failure modes, namely, cohesive failure and face sheet failure. Results indicate that the dimensions of the struts and the relative density affect the mechanical performance and the failure mode of the panels.

Cellular structures are formed by struts and edges, which makes them difficult to fabricate by conventional technologies. The emergence of 3D printing has enabled the production of such complex structures. In the present work, the Fused Filament Fabrication method was used to obtain cellular structures formed by the repetition of triply periodic minimal surfaces unit cells. Due to the complexity of the cells, several defects occurred until the procedure was fine-tuned. Failure analysis of samples was performed after bending and compression tests. Failure occurs at the same locations of the samples independently of their relative density, for the three densities tested. These fracture initiation locations correspond to the zones where the von Mises stress is highest.