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

This thesis aims to determine the structure-property relationships with a multi-level method developed based on experimental and numerical analyses to efficiently model the fracture behavior of lattice materials produced from polylactic acid (PLA) material using the material extrusion technique. This method, which constitutes the original value of the thesis, will be used for the fracture analysis of a new cubic lattice cell proposed to eliminate the existing cubic lattice cell deficiencies. Since the lattice materials consisting of struts provide high strength and lightweight and can be produced by additive manufacturing (AM), they have now started to attract attention in the fabrication of final products. However, due to the stacking of the material layer by layer, there are uncertainties and defects in the lattice materials produced by AM. In the material extrusion method, which enables the fabrication of PLA material, which has come to the forefront due to its environmental friendliness, especially in automotive, biomedical, and aviation, it is extremely necessary to predict what kind of failure or defects can occur in order to predict the damages that may occur during applications. In engineering materials, three modes of fracture (or loading) can be experienced in a cracked body: Mode I is the opening mode, mode II is the shearing mode in the crack plane, and mode III is referred to as the tearing mode. Studies concerning the fracture behavior of polymers fabricated by material extrusion usually ignore investigating all three pure fracture modes. To fill this critical gap in the literature, in this thesis, experiments were done using solid fracture specimens as well as lattice specimens to obtain fracture properties for all three modes. Extended finite element method (XFEM) was used to efficiently model the fracture behavior of solid and lattice specimens using the fracture properties extracted from experiments by taking account of geometric variations arising from the AM process. The original contributions of the thesis can be categorized as follows: (1) Examination of the fracture behavior of lattice cells by XFEM (2) Obtaining effective fracture properties through XFEM analyses while considering the geometric uncertainties that occur in the struts in the lattice cells and caused by the material extrusion method. (3) A cubic lattice cell, which can be produced without any support and has high fracture strength, is proposed and the structure-property relations are determined under uncertainties with XFEM fracture analysis.