14 parameters affect the quality and the mechanical performances of 3D-printed parts by fused filament fabrication. This study analyzes the impact of infill density on fatigue behavior in PLA specimens using rotary bending tests. Dog samples are printed vertically at a 90° raster angle. Various peak bending stresses (14, 35, 49, and 70 MPa) are applied to different dog-bone specimens with infill densities of 25%, 50%, 75%, and 100%. S-N curves are then plotted to visualize the relationship between the number of cycles until failure and the experimental conditions. To account for inherent result variability, ANOVA analysis is employed to quantify the influence of applied stresses and infill density on specimen service life. The analysis reveals that stress is the primary contributor, accounting for 23% of the ANOVA response model. Furthermore, fractography analysis is conducted to examine fracture modes under different test conditions. Fractures align with the direction of filament deposition, with smoother surfaces indicating strong filament bonding. Notably, voids between layers act as stress concentrators, adversely affecting mechanical properties. Fatigue crack initiation primarily stems from these voids rather than surface defects, leading to progressive crack propagation. The process leaves characteristic beach marks and fatigue striations on fracture surfaces. Additionally, the plastic nature of PLA is evident in the formation of craze structures, indicating a combination of brittle and plastic failure mechanisms.