Lattice materials, composed of the repetition of cells made of struts, can be designed to achieve high specific stiffness and energy absorption with minimal weight. Recent improvements in additive manufacturing allow the fabrication of lattice cells using standard Selective Laser Sintering at the millimetric scale. However, these components often contain internal defects, such as porosity, affecting mechanical properties. It is generally observed that smaller cell sizes tend to reduce the lattice’s effective elastic modulus, yet the cause of this size effect is not well understood. The objective of this work is to comprehensively analyze the non-linear behavior of PA12 lattice materials by combining experimental analysis and full-field simulations.

The bulk response of PA12 is characterized experimentally with uniaxial tensile, compression, and shear tests at different strain rates, temperatures, and aging times, combined with microscopic testing techniques such as nanoindentation to calibrate a finite strain VEVP material model that predicts its behavior. Results show that PA12 exhibits a viscoelastic-viscoplastic response, moderate strain rate sensitivity in yield stress, and anisotropy under tension.

In the study of the mechanical behavior of printed PA12 single struts, unit cells, and lattices of different sizes, it was found that smaller lattice cells exhibit a lower effective elastic modulus, a “smaller means softer” trend typical of SLS polymers. Compression tests, nanoindentation, X-ray tomography, DSC, WAXS, SAXS, and numerical simulations determined that this modulus reduction is not only due to defects like porosity or roughness. Instead, but is also results from a combination of incomplete particle melting and nanostructural changes from the printing process, as confirmed by micro- and nanostructural characterizations.

Experimental and numerical studies on the fracture and deformation of PA12 lattice structures revealed that defects from porosity, surface roughness, and strut geometry significantly affect mechanical properties. In-situ tensile and compressive tests on single struts and unit cells showed that lattice stiffness is lower than in bulk PA12. Fractures in single struts were initiated at random surface or internal defects, with crack propagation influenced by printing direction, while fractures in unit cells were mainly influenced by geometry. Numerical simulations using voxelized RVEs from X-ray tomographies, applying FFT and phase-field fracture models, confirmed that surface defects, print orientation, and structural geometry impact the stiffness and failure behavior of PA12 lattices in a complex way. The non-linear response was predicted with FFT using a VEVP material model with calibrated parameters for the bulk and strut scale, closely predicting the material’s behavior.

Beyond build orientation in SLS, other printing parameters like contour and hatching settings strongly influence the mechanical properties of AM components, especially near the machines resolution limit. These factors influence particle coalescence and thermal history, affecting the annealing effect in printed parts. These findings apply to commercial machines optimized for larger samples. Optimizing the parameters for microscale components could minimize defects and coalescence issues, reducing size effects. This study helps understand the critical features for designing lattice meta-materials by linking mechanical behavior to component size, and the methods used could also characterize other polymers in bulk and lattice forms.