Investigation of mechanical and fracture behaviors of additively manufactured polymeric lattice materials

This thesis aims to investigate the mechanical and fracture properties of polymeric lattice materials fabricated by additive manufacturing (AM) based on experiments and numerical analyses. For this purpose, material and fracture properties are characterized for the PLA material manufactured by the material extrusion AM technique for solid and lattice specimens. A lattice cell is proposed to eliminate the problems of existing lattice cells, including support requirements due to overhanging issues during the AM process and inferior fracture properties. Lattice materials, composed of a strut element network that provides lightweight and high strength, currently draw attention in producing final products since AM techniques can fabricate them. However, due to the layer-wise nature of the fabrication technique, additively manufactured lattice materials include variations in the material and fracture properties. Hence, the mechanical performance of materials obtained from numerical investigations differs from that of fabricated materials. In the material extrusion AM technique called fused filament fabrication (FFF), which enables the fabrication of environmentally friendly PLA material that gains interest in biomedical, automotive, and aerospace, it is essential to predict the fracture behavior of materials to estimate damage comprehensively. However, existing studies mainly investigated solid materials' mechanical and fracture properties. Studies concerning the fracture behavior of lattice materials do not investigate the effect of variations introduced by AM on the prediction of the fracture behavior of lattice materials. To fill this critical gap, mode I, II, and III fracture behavior of PLA lattice materials fabricated by the FFF technique are characterized based on experiments, and effective properties were determined to use in numerical fracture simulations based on the extended Finite Element Method (XFEM) and to obtain the material-property relationships. For this purpose, the fracture behavior is investigated at two levels: (1) Solid material and (2) lattice material (macro) level. At the solid material level, the fracture behavior of the solid specimens manufactured by FFF is determined by fracture tests (mode I, II, III) for different build orientations. The material and fracture parameters were determined, including their variations based on build orientations, for use in the fracture simulations based on XFEM. At the lattice material level, mode I, II, and III fracture test specimens were fabricated in different build orientations and then tested. XFEM-based numerical models are developed by representing lattice cells with solid elements using effective properties calibrated based on the experimental results to include variations due to repeated tests as well as different build orientations. A lattice cell type called face- and simple-centered cubic (FSCC) is proposed for fabrication by the FFF technique without overhang issues and with high fracture properties. Fracture behaviors and their variations of the FSCC lattice and one existing lattice cell type called body-centered cubic (BCC) are determined experimentally. It was shown that the proposed FSCC lattice cell type can be fabricated without support requirements and has higher fracture properties than the existing BCC lattice. After the extraction of the material and fracture properties of the PLA in use, a calibration process was implemented to extract the calibrated material properties to be used in the modes I/II/III numerical analyses conducted using the XFEM technique. To consider the effects of build orientation in the numerical analyses, a transversely isotropic material model was utilized. For the lattice specimens, another calibration procedure was employed to extract the equivalent solid material model, which was utilized to model the fracture behavior of the FSCC lattice specimens using the XFEM technique.