## 4 Influence of 3D Printing Parameters on the Fatigue Life of FFF Prints
### Infill Pattern  
The pattern in which the filaments are deposited in each layer is called the infill pattern. The infill pattern also determines the inner structure and thus the overall stiffness of the print [74]. In a specified infill pattern, junctions of extruded filaments at each layer (interlayer) or between two successive layers (interlayer) are formed in a certain shape, acting as stress concentration zones [39, 62]. These concentration zones make the joint areas susceptible to debonding and cracking [75]. Stress concentration is a primary reason for the fatigue failure of metals and plastics caused by material or geometrical discontinuities. Due to the formation of air gaps and imperfect bonding between layers during the 3D printing process, the fatigue life of FFF prints can be 100 times lower than that of injection-moulded parts [24]. Infill or hole patterns in FFF prints are another source of stress concentration and reduced fatigue life. Therefore, fatigue strength can be affected by the infill pattern by altering the specifications of the joint areas. A comparison between the results reported in the literature shows that cross-over patterns (e.g., grid and honeycomb) produce samples with higher tensile and bending fatigue resistance compared to those produced with rectilinear/line patterns. For instance, in a study by [PERSON] et al. [57], the tensile fatigue resistance of the weakest grid pattern (\(0/-90\)) was higher than that of the strongest rectilinear pattern with a raster angle of \(0^{\circ}\). In a study conducted on ABS prints, it was observed that the grid pattern (\(45/-45^{\circ}\)) produced the highest tensile fatigue life among the other patterns, i.e., \(0^{\circ}\), \(45^{\circ}\), and \(90^{\circ}\) at the same normalized fatigue stress [55]. Similar behaviour has been reported for the rotating bending fatigue of FFF prints. The effect of rectilinear and honeycomb infill patterns on the rotating bending fatigue strength of ABS and PLA-wood composite prints was studied in [64, 67] and [18], respectively. Round specimens were fabricated according to ASTM D7774 and were subjected to stress-controlled fatigue cycles. The infill patterns were so printed that the load was aligned with the carrying direction. Figure 12 depicts the used infill patterns and their orientation with respect to the loading direction. The results showed the superiority of the honeycomb pattern in sustaining fatigue cycles. Among different grid patterns, some literature studies have suggested that \(45/-45^{\circ}\) grid patterns perform better than other configurations under pure tensile cycles. As reported by [PERSON] et al. [24], when subjected to the same tensile force, \(45/-45^{\circ}\) patterned ABS prints exhibited a higher fatigue life compared to their \(0/90^{\circ}\) counterparts. However, when normalized stress (applied stress divided by the ultimate tensile strength of the specimen) was considered, both patterns showed a similar fatigue strength. Given that the static strength of each part is different, the static strength (UTS) of each print can be taken into consideration when assessing fatigue strength. In fact, the UTS is the highest-possible fatigue stress that a part can withstand only for one cycle, representing the first point (corresponding to the N = 1) in the S-N curve. The remaining points can be normalized by this value so that the S-N curve is bounded from 0 to 1. This is done by dividing the fatigue stress by the UTS. [PERSON] and [PERSON] [76] compared the normalized fatigue strengths of PLA prints. It was concluded that the tensile fatigue performance of a print is highly correlated to its ultimate tensile strength. A similar result was reported by [PERSON] et al. [57], in which different patterns (\(0/-90^{\circ}\), \(15/-75^{\circ}\), \(30/-60^{\circ}\), \(45/-45^{\circ}\)) were used to 3D-print ABS tensile specimens.