# Research on Basic Properties of Polymers for Fused Deposition Modelling Technology
## 5 Conclusions  
The experimental and numerical studies demonstrated that the Johnson-Cook material model effectively predicts the mechanical behaviors of certain commercially available filaments used in additive manufacturing. By comparing the stress-strain curves from simulations and experiments, we quantitatively assessed the model's accuracy across different materials. A strong agreement was found for monotonic materials like FiberFlex 40D, PETG, HIPS, TPU, and PA+1 SCF Rosa 3D, with maximum fitting errors not exceeding 2.6%. This correlation is due to their monotonic stress-strain relationships, which the Johnson-Cook model captures well through its power-law formulation. The parameters determined in this study can be utilized in simulations to enhance predictive capabilities. Minor discrepancies in materials like TPU and PET-G Carbon arose from challenges in modeling the smooth transition from elastic to plastic behavior, but the overall errors remained acceptable. However, the model was inadequate for non-monotonic materials like PLA 3D and PET-G, resulting in higher errors and incompatible numerical and experimental curves. Cytotoxicity tests, which were conducted under ISO 10993-5 and 10993-12 standards, confirmed that materials like PETG Carbon, PETG, HIPS, TPU, PA+1 SCF, and PLA are non-cytotoxic and potentially safe for biomedical applications. In contrast, FiberFlex 40D exhibited cytotoxic properties, making it unsuitable for contact with biological tissues without modification. Scanning electron microscopy provided insights into the fracture mechanisms of various filament materials used in additive manufacturing. The material composition significantly influenced fracture behavior. Unreinforced materials like PLA 3 DActive, PET-G Sunlu, PET-G 3 DActive, and HIPS Devil Design exhibited fracture surfaces with fault systems indicative of combined ductile and brittle mechanisms, forming patterns resembling rivers and basins. This suggests that both brittle and ductile failure modes contribute to their mechanical performance. In contrast, fiber-reinforced composites such as PET-G Carbon and PA+1 SCF displayed distinct fracture characteristics. PET-G Carbon samples showed broken carbon fibers embedded in the matrix, indicating strong fiber-matrix adhesion and effective load transfer, enhancing the mechanical strength and stiffness. PA+1 SCF samples exhibited fiber pull-out from the matrix, pointing to weaker fiber-matrix adhesion; while this absorbs energy during fracture, it may reduce overall strength compared to composites with stronger interfacial bonding. The quality of 3D printing significantly affected fracture behavior. Well-bonded filament paths improved the mechanical properties due to efficient stress distribution and resistance to crack initiation, whereas poor layer adhesion and voids acted as stress concentrators, reducing the strength and facilitating crack growth along weak interfaces. The print path directionality influenced crack propagation patterns. In unreinforced materials, cracks followed stepped patterns along poorly bonded layers, reflecting a combination of brittle fractures at weak interfaces and ductile deformation in well-bonded regions. For fiber-reinforced composites, crack propagation was more linear, with embedded fibers bridging cracks and reinforcing the matrix, contributing to higher strength and stiffness. The SEM observations correlated with the mechanical test results. Materials exhibiting ductile fracture features, such as material pull-outs and plastic deformation, demonstrated higher elongation and toughness, making them suitable for applications requiring energy absorption and flexibility. Fiber-reinforced materials achieved higher tensile and bending strengths, consistent with the effective fiber reinforcement and strong fiber-matrix adhesion observed in fracture surfaces. Future studies should focus on accelerated aging tests and environmental simulations to better understand the degradation pathways and mechanical fatigue of these materials over time.