Comparative Study of Uniform and Graded TPMS FRD Lattices Fabricated via Additive Manufacturing: Experimental Tests, CT and SEM Characterization, and Finite Element Simulation

With emphasis on the Face-Centered Rhombic Dodecahedron (FRD) topology, this thesis examines the mechanical behavior and structural performance of triply periodic minimal surface (TPMS) lattice structures made using Laser Powder Bed Fusion (L-PBF). For comparison, the uniform and functionally graded variations (FRD-30, FRD-40, and FRD-45) were evaluated against Gyroid and Fischer–Koch–S structures. They were developed with a constant average volume percentage of 45%. The relationship between relative density and the mechanical performance of these designed materials was further validated by Ashby-Gibson modeling. The evaluation of internal porosity, defect morphology, and geometric integrity was done using high-resolution X-ray computed tomography (XCT). According to the results, the graded FRD-30 structure had the most uniform defect distribution and the lowest pore volume, but the geometric complexity of the FRD-40 structure resulted in higher porosity variability. In comparison to the uniform FRD-45, Gyroid, and Fischer-Koch-S, the graded lattices showed better deformation control, smoother stress-strain responses, and higher energy absorption properties, according to mechanical tests conducted under quasi-static compression in accordance with ISO 13314 standards. Particularly, among the structures examined, FRD-40 had the highest elastic modulus, indicating higher stiffness and load-bearing capacity. The Fischer–Koch–S lattice demonstrated its efficiency for energy-dissipating applications by achieving the highest specific energy absorption among all studied configurations. The predictive accuracy of the numerical models was confirmed by finite element simulations, which replicated the experimentally observed deformation behavior and stress localization. Additionally, the Ashby-Gibson model's application demonstrated how relative density affects mechanical performance, with variations due to design complexity and defects in the process. To further investigate the failure mechanisms, fracture surfaces of the tested specimens were analyzed using Scanning Electron Microscopy (SEM). The micrographs revealed key fracture features such as delamination zones, and the presence of unfused powder particles. These observations highlighted the role of process-induced defects and local structural anisotropy in crack initiation and propagation. Overall, this work confirms that functional grading and topology optimization significantly enhance the mechanical efficiency of TPMS lattice structures. The integrated experimental, analytical, and numerical approach provides valuable insights for the design of advanced architected materials in aerospace, automotive, and biomedical applications.