## 4 Influence of 3D Printing Parameters on the Fatigue Life of FFF Prints
### Layer Height  
It has been shown that the layer height has different effects on the static and fatigue strength of FFF prints. In the case of static loading, reducing the layer height increases the mechanical strength through two mechanisms: First, it decreases the gaps between extruded filaments at adjacent layers, resulting in a less porous structure and higher stiffness. Second, the contact area between the extruded filaments increases, favouring the heat transmission between layers and better layer adhesion [73]. Further, the cross-sectional shape of the void spaces between the deposited filaments changes with the layer height [81], affecting the anisotropy and mechanical strength of the final product. Figure 13 schematically illustrates the effect of the layer height on the porosity and shape of cavities in FFF prints.  
Figure 13: Change in the porosity and cross-sectional shape of void spaces by changing the layer height.  
In a study conducted on PLA prints with different layer heights, a 24.5% improvement was observed in the tensile strength of prints by reducing the layer height from 0.4 mm to 0.2 mm [82]. The same effect was reported for the extrusion width when it increased [83]. However, the influence of the extrusion width on the fatigue performance of FFF prints has not been studied yet.  
In contrast to static loading, the influence of the layer height on the fatigue strength of FFF prints is the opposite, showing a direct correlation with fatigue life. The flexural fatigue strength of PLA [84] and ABS [35] specimens was found to be directly proportional to the layer height. However, the impact of the layer height on flexural fatigue strength has been reported to be low in comparison to other parameters. The results achieved for the layer heights of 0.1 and 0.3 mm showed a slight improvement in the flexural fatigue life of PLA prints produced with the higher layer height [84]. This is while a lower layer height led to higher stiffness and elastic modulus in the latter study. There was also a 37% increase in the fatigue life of ABS prints when the layer height increased from 0.05 to 0.15 mm [35].  
The same effect can be seen when prints are subjected to rotating bending fatigue. In another study, the rotating bending fatigue performance of ABS prints at different layer heights was investigated according to ASTM D7774 [64; 67]. It was found that a higher layer height leads to a higher number of cycles to failure. [PERSON] et al. [66] introduced layer height as the most influential factor in the rotating bending fatigue strength of PLA prints. Specimens with the honeycomb infill pattern, 75% density, 0.5 mm nozzle diameter, and 0.3 mm layer height sustained approximately 4700 cycles before failure. This was nearly 2.3 times that observed in specimens 3D-printed in the same condition but with a layer height of 0.1 mm. In a similar study conducted by [PERSON] et al. [18], the layer height was reported as the most influential factor on the rotating bending fatigue strength of PLA-wood composite FFF prints. The authors of this study examined three layer heights (0.2, 0.3, and 0.4 mm) and concluded that higher fatigue life is obtained when a higher layer height is implemented in the designs.  
The contradicting effect of the layer height on the static and fatigue strength of FFF prints is believed to be due to their underlying failure mechanisms. Reducing the layer height increases the number of layers within the object, resulting in more stress concentration zones and reduced fatigue strength. However, it enhances static strength by reducing void spaces and improving the stiffness of the structure. Since studies in this area are scarce, there is a need to investigate the effect of the layer height on the performance of FFF prints under different fatigue loads.