Influence of the loading direction on the mechanical behavior of ceramic foams and lattices under compression

In this study, the geometry of a polyurethane foam template was reconstructed via X-ray computed tomography, and its average morphological parameters (e.g. cell aspect ratio) were retrieved. Using these parameters, numerical models of a random foam and of lattices based on an array of tetrakaidecahedra with similar aspect ratios, dimensions and number of cells per edge were generated. Two different types of lattice were considered: an isotropic lattice and an elongated (stretched) lattice, the latter simulating the geometrical anisotropy found in real foams. The numerical models were then rotated with a dedicated script in order to investigate the effect of the loading direction. Boundary conditions were applied to each rotated model and compression tests in the linear elastic regime simulated. In order to validate the simulations, ceramic foams produced by the replica technique using a polyurethane foam template or polymeric lattice structures fabricated via rapid prototyping were mechanically tested. Results indicate that an elongated lattice presents a markedly orthotropic behavior, with the elastic modulus varying abruptly when changing loading direction, while a random foam shows a lower degree of elastic anisotropy and also some transverse isotropy. When the distribution of stresses is concerned, a random foam shows a region of much higher stress concentration than regular lattices, owing to the random orientation of the struts in space.