Permeability and Mechanical Properties of Triply Periodic Minimum Surface Shell Structures Produced by Additive Manufacturing

The additive manufacturing technique has developed rapidly in recent years, and the widespread application makes it accessible to create complex porous structures. The Triply Periodic Minimum Surface (TPMS) structures, which have smooth and connected channels and high geometric surface area, are now being used in many areas such as biomimetic materials, energy absorption systems, catalyst systems, and lightweight design. Meanwhile, those applications also give more requirements for better structural design with TPMS structures. This research aims to investigate the influence of the design of different TPMS structures on their permeability and mechanical properties. Specifically, an orthogonal experimental design was used to study how TPMS architectures, porosity, and cell size affect permeability and mechanical properties. Moreover, the effect of the weight function on the mechanical properties of a one cell hybrid TPMS structure was also investigated. The results of orthogonal experiments show that the Primitive TPMS structure has the highest permeability but the lowest mechanical properties. Permeability (k1) increases with porosity, and so do the Forchheimer coefficient β, Young’s modulus, and energy absorption. Porosity is the most important factor for all properties. For permeability, TPMS structure type is less important for k1 but more important for β, meaning it has a larger effect in high-speed flow than in slow flow. For mechanical properties, Young’s modulus and energy absorption show similar trends, suggesting they can be improved together by controlling the same factors. For hybrid TPMS structures, the Young’s modulus results are affected by the cell number and become stable when the cell number is 3*3*3 or larger. Structures made by combining the hybrid TPMS cell and its mirror symmetry structure cell by different sequences show similar mechanical properties, which means this assembly method is feasible. For hybrid TPMS structures with linear and nonlinear weight functions, mechanical properties differ in the x, y, and z directions, showing both are anisotropic. For the linear hybrid TPMS structures, the Young’s modulus in x and y directions is much lower than z direction, and for the nonlinear hybrid, which in y and z directions is similar and higher than x direction. Compression strength experiments show the linear hybrid improves all mechanical properties compared to basic Gyroid and Diamond TPMS, while the nonlinear hybrid mainly improves the Young's modulus. Three-point bending experiments show that both hybrid structure beams perform better than basic TPMS structure beams in most properties. The linear hybrid beam has higher energy absorption, while the nonlinear hybrid beam has higher flexural modulus. This means that the weight function can be used to affect the mechanical performance of hybrid TPMS structures.