## 4 Influence of 3D Printing Parameters on the Fatigue Life of FFF Prints  
Apart from the material itself, the mechanical strength of FFF prints is greatly affected by the structural stiffness and bonding strength of extruded filaments [53]. The infill density and infill pattern directly affect the print stiffness, while a higher nozzle temperature favours the efficient fusion of filaments, resulting in stronger bonding. The bonding strength is also influenced by the contact area between the extruded filaments, which is impacted by the extrusion width and layer height. However, it should be noted that the effect of 3D printing parameters on different mechanical properties is not the same and varies according to the direction and type of the applied loads. For instance, [PERSON] et al. [54] showed that the air gap and raster orientation have a remarkable impact on the tensile strength of prints, while compressive strength is not influenced significantly by these factors. Further, the loading type (static or dynamic) should be taken into account when assessing the influence of 3D printing parameters. This is because fatigue strength is \begin{table}
\begin{tabular}{c c c} \hline
**Standard** & **Application** & **Ref.** \\ \hline ASTM D7774 & Flexural fatigue of plastics & [40] \\ ASTM D3479 & Tension–tension fatigue of polymer matrix composites & [41] \\ ASTM D7791 & Uniaxial (tension or compression) fatigue of plastics & [42] \\ ISO 13003 & Fatigue properties of fibre-reinforced plastics & [43] \\ ASTM D6115 & Crack propagation of fibre-reinforced polymer composites under fatigue (Mode I) & [44] \\ ISO 15850 & Tension–tension fatigue crack propagation & [45] \\ ASTM E739 & Statistical analysis of linear or linearized stress–life (S–N) & [46] \\  & and strain–life (\(\varepsilon\)–N) fatigue data & \\ ASTM E606 & Strain-controlled fatigue testing & [47] \\ ASTM D4482 & Tensile (extension) fatigue of rubber compounds & [48] \\ \hline \end{tabular}
\end{table} 
Table 1: Existing standards for the fatigue testing of polymers. greatly influenced by stress concentration, while stiffness is the most determining factor for static loading [29]. Thus, those 3D printing parameters that play a contributing role in the fatigue strength of a print may have a neutral or deteriorating effect on its static strength and vice versa. There are a handful of studies that have investigated this matter in the literature as the concept of fatigue has not been given as much attention as the static strength in FFF prints so far. Table 2 provides a summary of the works carried out on the effect of 3D printing parameters on the fatigue performance of FFF prints. The reviewed works were categorized based on the fatigue type and the tested material. The fixed parameters, variable parameters, testing conditions, standards used for specimen fabrication, and results were tabulated separately, as provided in Table 2. In the following sections, the effect of 3D printing parameters on the fatigue behaviour of FFF prints is reviewed in detail. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Type** & **Mat** & **Variable** & **Fixed Parameters ** **\#** & **Specimen** & **Testing Conditions** & **Results** & **Ref** \\ \hline \multirow{9}{*}{ \begin{tabular}{c} Build \\ orientation: X, \\ Y, Z \\ Raster angle: 0, \\ 45, 90\({}^{\circ}\) \\ \end{tabular}  } & \multirow{9}{*}{ \begin{tabular}{c} \\ \end{tabular}  } & \multirow{9}{*}{ \begin{tabular}{c} \\ \end{tabular}  } & \multirow{9}{*}{\ \begin{tabular}{c} Loading type: stress \\ controlled \\ Stress level: 40, 60, \\ 80\% UTS \\ Dimension \\ \end{tabular}  } & \multirow{9}{*}{ \begin{tabular}{c} Fatigue performance (at \\ similar stress): Y prints \\ controlled \\ 80\% UTS \\ \end{tabular}