Characterization of the mechanical properties of additively manufactured hyperelastic lattice structures
Dror Raf & Dana Solav
Foot orthotics are devices that provide support for the foot by redistributing ground reaction forces acting on the foot while standing, walking, or running. To optimize the pressure distribution, tailored foot orthotics can be designed with different mechanical properties in different regions. However, fabricating an interface with locally variable materials is difficult with standard fabrication techniques. Additive manufacturing (AM) techniques enable the fabrication of complex geometric shapes such as lattices and other cellular structures, made up of certain internal structures arranged periodically or randomly.
The mechanical properties of lattice structures are influenced by parameters such as structure topologies, the orientation of cells with respect to the loading direction, and the mechanical properties of the base material. These parameters and their combination allow for designing structures with different bulk mechanical properties, such as stiffness, energy absorption capability, and structural strength.
In this study, we investigate the effects of various lattice parameters on the mechanical properties of foot orthotics manufactured by AM. Specifically, selective laser sintering (SLS) is used due to its ability to print relatively isotropic complex lattice structures without requiring the printing of support structures, with rubber-like thermoplastic polyurethane (TPU). To characterize the mechanical properties of the base material, we will perform dynamic uniaxial and biaxial tension and compression tests and fit the constitutive parameters using the force data and full-field deformations computed using Digital Image Correlation (DIC).
Next, we perform a nonlinear Finite Element Analysis (FEA) on models with simplified geometry to determine how changing parameter values impact the mechanical properties. Constraints related to the printing process and physical limitations in the design serve as constraints of our FEA. As part of the FEA, we use a nonlinear homogenization procedure to replace, on a macroscopic scale, the actual, complex geometry of the elementary unit cell with an equivalent homogeneous medium. Finally, we perform experiments on 3D-printed models to evaluate the numerical characterization's accuracy. This allows for a later expansion to more complex models.