Pore size and pore shape of polymer lattice materials : unexpected mechanical properties

Abstract

The overall aim of this study was to design and manufacture lattice materials with a constant porosity but different structures and subsequently investigate how the structure affects the mechanical properties (Young’s modulus, yield strength or plateau stress and the energy absorption). The investigated lattice materials had (i) different pore sizes, (ii) different pore shapes, and (iii) a pore size gradient. Lattice materials with defined pore sizes, pore shapes (cubic or spherical), and a pore size gradient were designed using computer-aided design (CAD) and manufactured via 3D printing. Uniaxial compression tests of the 3D-printed specimens showed that mechanical properties such as the Young’s modulus, the plateau stress, and the energy absorption depend on the structure. Flexible lattice materials with cubic pores have stronger mechanical properties compared to those with spherical pores. Further, the mechanical properties tend to increase with decreasing pore size of 3D-printed lattice materials. Pore size graded lattice materials, in turn, resembled the mechanical properties of their weakest part. The finite element method (FEM) visualized how the stress is distributed within the differently structured lattice materials, allowing a deeper understanding of their resulting properties. Experimental and CAD-modeled (FEM) results led to different trends of the mechanical properties as a function of the lattice materials’ pore size. To explain these differences, Microcomputed tomography (μCT) was used to perform a dimension deviation analysis between the 3D-printed structures and their CAD models. It was found that structural deviations decrease the mechanical properties of lattice materials. Furthermore, inhomogeneities of the bulk material, i.e. the polymer matrix originating from the manufacturing process were identified. Finally, a brittle instead of a flexible polymer was used, resulting in inverse trends of the mechanical properties of lattice materials with varying pore sizes. This study showed that for 3D-printed lattice materials with constant porosity the pore shape and pore size matter regarding the mechanical properties of lattice materials. Furthermore, the study points out that defects and deviations of the manufacturing method need to be considered when investigating the mechanical properties of 3D-printed lattice materials.