ABSTRACT:

Lattice materials are materials constituted by the repetition of small cells composed of structural units as bars, beams or shells, which form a lattice. The cell geometry can be designed to have an outstanding macroscopic response such as very high specific stiffness or energy absorption capacity in relation to its low weight. With the recent improvements in additive manufacturing techniques, it is possible to fabricate lattice cells using standard Selective Laser Sintering with sizes below the millimetric scale. However, additively manufactured components often present internal defects, such as porosity, which affect the mechanical properties.

The objective of this work is to perform an analysis of the non-linear response of lattice metamaterials fabricated with AM by integrating full-field numerical simulations and experimental analysis of lattice materials made of PA12 and TPU. First, the response of the two materials in which cells are fabricated, three dimensional printed TPU and PA12 bulk material samples, are characterized experimentally with uniaxial tensile, compression, cyclic and relaxation tests in order to calibrate material models that predict their behaviour. A parametric study of 11 cell candidates has been done with FFT and FEM numerical simulations using material models that predict TPU and PA12 behaviour to see which ones present the best impact behaviour. Finally, the influence of porosity on the mechanical behaviour of 3D printed lattices is studied combining experimental and numerical techniques. To this aim, the real microstructure including defects of two unit cell geometries with different strut diameters are characterized using X-Ray tomography, and their compression response is obtained experimentally. The tomographic data is used to generate FFT models and virtual tests are performed to quantify the difference ob the response between the actual microstructure and the defect-free designed one.

The results from the studies show that PA12 presents a elastic-viscoplastic response with a moderate strain sensitivity in the yield stress and presents anisotropy under tensile load. On the other hand, non-linear hyperelastic behaviour is observed on TPU which also presents anisotropy under tensile loads. From the 11 cell candidates, it is observed that the octet lattice, and an isotropic auxetic geometry with a bow-tie architecture, present the best behaviour of all geometries. Finally, 3D printed geometries present porosity and high surface roughness that affect not only the mechanical properties, but also the strut diameter.