Mechanical behaviour of additive manufactured lattice structures

ENGLISH ABSTRACT: Lattice structures are open-cell, strut-based structures made up of unit cells that are tessellated in 3 orthogonal directions. Depending on the physical design of the unit cell, the lattice structure can be designed for customized stiffness, strength, and specific strain energy absorption. This allows for the design of lightweight, load-bearing structures, suitable for functional engineering applications. 5 X 5 X 5 octet-truss and diamond lattice structures of a specified relative density were designed using equations that relate the relative density to the strut dimensions. Computer-aided design (CAD) models of the structures were created and used to produce these lattice structures of Ti6AL4V using the additive manufacturing (AM) technique: laser powder bed fusion (L-PBF). Finite element analysis (FEA) was used to simulate the uniaxial compression of the lattice structures, yielding predictions for the stress distribution in the lattice struts, and allowing for the prediction of the deformation and failure modes. 3D solid and 1D beam elements were used for this purpose. A prediction of the global mechanical properties and deformation mechanism of the lattice structures was established for both types and compared. A comprehensive flowchart describing the FEA approach taken in order to predict the mechanical behavior of L-PBF lattice structures is provided. Mechanical uniaxial compression of the as-built lattice structures was conducted. Global mechanical properties were determined from the load-deformation data. The progressive collapse of struts under the load was analyzed in order to categorize the failure as stretch- or bending-based. Micro-computed tomographic (μCT) analysis of the as-built structures showed that the struts were thicker as compared to the CAD structures. Thus new CAD models were created with the strut thickness correlating to the actual average dimension. This resulted in improved prediction of the mechanical response of the as-built structures. A comparison of the FEA predictions and the experimentally measured mechanical properties of the lattice structures was carried out. It was determined that both the 3D solid and 1D beam structures predicted the actual mechanical properties of the octet-truss lattice structure within an error margin of less than 25 %. However, for the diamond lattice structure, only the 3D solid structure predicted the actual mechanical properties with an error margin of less than 20 %. A study of the stress distribution across individual struts in the structure was used to explain the deformation mechanisms observed in the respective lattice structures. The octet-truss structure was found to deform by a combination of 45° and 135° shear bands caused by the stretching of the horizontal struts in those planes, whereas the diamond lattice structure was found to deform by 45° shear bands caused by strut bending.