Quasi-static Compressive Response of SS316L Strut-based Lattice Structures

The additive manufacturing (AM) technique allows the fabrication of periodic metallic lattice structures with complex geometries. The lattice structures exhibit exceptional properties such as high stiffness, strength-to-mass ratios and energy absorption which makes them an attractive candidate for various engineering applications including wave filters, blast and impact protection systems, thermal insulation, structural aircraft and vehicle components, and body implants. The mechanical characterisation of such structures is mostly done by performing mechanical testing. In recent years, Finite Element Analysis (FEA) of lattice structures have gained much research attention. The computationally efficient FE models of lattice structures are required to accurately evaluate the mechanical performance and deformation behaviour of such materials. This dissertation aims to investigate the quasi-static compressive response of SS316L strut-based lattice structures through quasi-static compression tests and explicit FE simulations. The explicit algorithm is used to capture the large deformations of lattices until the densification. Initially, FE models of strut-based lattice structures using solid and beam elements are established to perform simulations for quasi-static compression. In FE models, the lattice structures are designed to have a similar unit cell stacking order as the as-built lattice structures, and the strut components are modelled with perfect geometry. The capabilities of FE models to predict the quasi-static response of lattices is demonstrated using existing experimental studies performed on SS316L bending-dominated lattice structures. It is observed that FE models using solid elements accurately predict the experimental stress-strain curves and deformation behaviour of SS316L bending-dominated lattice structures. Then, the capability of FE solid models of single unit cells in predicting the stress-strain curves of bending-dominated (body centred cubic and rhombic dodecahedron) and stretch-dominated lattice structures (face centred cubic and octet) is investigated. The findings indicate that the stiffness and collapse strength of stretch-dominated lattice structures can be closely estimated using a single unit cell with free boundary conditions on the unit cell surfaces. However, for bending-dominated lattice structures, using free boundary conditions on the unit cell surfaces leads to underestimation in the mechanical properties of lattices such as stiffness, strength, and plateau stress in comparison to full-size FE models of such structures. Finally, the mechanical response of SS316L rhombic dodecahedron and octet lattice structures are investigated through quasi-static compression tests while monitoring the deformation behaviour using a high-resolution camera. Morphological analysis performed using scanning electron microscopy (SEM) reveals that strut diameters of the fabricated lattices are smaller than those intended in the design. It is also observed that manufacturing defects are related to the built angle of struts. FE models established with adjusted strut diameters based on as-built masses and SEM images can predict the stress-strain curves obtained from lattices with higher accuracy than the FE models based on design dimensions. However, capturing shear crushing band formation in stretch-dominated octet lattice structures requires the integration of geometric manufacturing defects of lattices into FE models. The deformation of stretch-dominated octet lattice structures can be better predicted using statistical geometrical defect parameters obtained from relevant literature.