## 3 Fatigue in Polymers and FFF Prints  
Fatigue in polymers occurs both as thermal and as mechanical failure. Thermal failure occurs due to hysteretic heating and consequent thermal softening of polymers, leading to ductile fractures [27]. In polymers, the dissipated energy (plastic work energy) is converted to heat within the material, resulting in the gradual thermal degradation of polymeric chains [28]. When subjected to fatigue loading, the induced plastic deformation in each mechanical cycle is converted to thermal energy, which cannot be transferred to the surrounding environment due to the low thermal conductivity of polymers. Thus, the generated heat is stored within the material, leading to overheating and stiffness loss. Consequently, stiffness loss leads to an increase in specimen deformation and failure. If the generated heat reaches the glass transition temperature of the polymer, mechanical properties degrade quickly. Therefore, using materials with a higher glass transition temperature in load-bearing applications is beneficial. For instance, poly(_para_-phenylene) possesses high fatigue properties due to its relatively high glass transition temperature (~177 \({}^{\circ}\)C) [29]. Polyacrylamide, polyurethane, and PTFE are among the polymeric materials with a high glass transition temperature [30]. Figure 6 illustrates the fatigue limit of some common filament materials. Adding ceramic or metal fillers can be a solution to increase the fatigue strength when thermal failure is dominant. This is because filler materials  
Figure 5: Illustration of raster orientation and loading direction in (**a**) compressive (**b**) flexural, and (**c**) rotating bending specimens. Since extruded filaments and the loading direction are not co-planar in the pure compression state, the raster angle is not defined for compressive specimens. contribute to fatigue strength enhancement through the energy dissipation mechanism in which a part of the induced energy is absorbed by the filler materials and is dissipated as the deformation of the reinforcement phase. Thus, less energy is converted to heat within the polymeric matrix [31]. Another failure mode in polymers is the mechanical failure that occurs under fatigue stresses and is greatly affected by stress concentration. Generally, fatigue failure occurs in high-stress-concentration areas. In FFF prints, internal holes (printing defects) serve as stress concentration zones, where the fatigue failure typically starts. These holes are formed due to the round shape of extruded filaments. Figure 7 illustrates the internal holes in a cross-sectional view of an ABS print subjected to fatigue loading. The bright-white lines in this figure show microcracks initiated from internal holes and extended along the loading direction. In a homogenous tensile specimen, fatigue failure occurs in the middle section (necking area) where the stress is highest. However, in FFF prints, failure can appear at different areas. In many cases, failure occurs around the shoulders. According to classical lamination theory, this can be attributed to the change in cross section and fibre diversion, which causes the local stresses to be focused near the shoulder area [36]. Figure 8 illustrates the  
Figure 6: Endurance limit of some plastics used as 3D printing filaments [32; 33; 34]. Figure 7: Formation of internal holes (microoids) during FFF printing of an ABS specimen. Bright lines show the path of microcracks initiated from internal holes (stress concentration areas) [35]. CC BY 4.0.  
initiation and propagation of tensile fatigue cracks around the shoulder area in an Ultem 9085 print [36]. Since 3D printing parameters affect the quantity and shape of internal holes [35], they also influence the fatigue performance of fabricated specimens. For instance, Figure 9a-c shows the tensile fatigue failure of PLA specimens fabricated at raster angles of 0\({}^{\circ}\), 90\({}^{\circ}\), and 45\({}^{\circ}\), respectively [37].