## 3 Fatigue in Polymers and FFF Prints
#### Fatigue Testing of Polymers  
Different standards are used for the fatigue testing of polymers, including ASTM D7774 [40], ASTM D3479 [41], ASTM D7791 and ISO 13003 [42,43], ASTM D6115 and ISO 15850 [44,45], ASTM E739 [46], ASTM E606 [47], and ASTM D4482 [48]. ASTM D3479 investigates the tension fatigue of polymer matrix composites for particular loading and environmental conditions. ASTM D7791 and ISO 13003 are for uniaxial fatigue in plastics, whereas ASTM D7774 is used for three-/four-point bending tests, and ISO 13003 is for all testing methods. ASTM D6115 and ISO 15850 are applicable for crack propagation, i.e., fatigue delamination in the interlaminar region of fibre composites. ASTM E739 can be used for the statistical analyses of fatigue data. ASTM D4482 examines the tensile fatigue resistance of rubber compounds. Finally, ASTM E606 [47] is used for the tensile  
Figure 11: Shearing and debonding of layers in PLA prints subjected to compression fatigue in (**a**) (Adapted with permission from [38]. Copyright 2021, Elsevier) and (**b**) (Adapted with permission from [17]. Copyright 2016, Elsevier) and (**c**) illustration of fatigue cracks at the layers’ interfaces (Adapted with permission from [39]. Copyright 2021, Elsevier). Cyclic compression caused the shear deformation of prints and cracking and debonding at the layers’ interfaces.  
Figure 10: Fracture surface of (**a**) \(-45^{\circ}/45^{\circ}\) and (**b**) \(0^{\circ}/90^{\circ}\) ABS specimens after failure under tensile fatigue load (Adapted with permission from [24]. Copyright 2019, Elsevier).  
fatigue testing of plastics under strain-controlled loading. Table 1 summarizes the existing standards for the fatigue testing of polymers.  
Although 3D-printed plastics can be tested using the mentioned standards, it is not clear whether they meet the requirements and assumptions of these standards. During 3D printing, layer deposition results in anisotropic properties and partly creates residual stress, making fatigue testing more challenging. Besides, the staircase configuration of EFF prints, internal pores, and extruded filaments' junctions cause stress concentration at numerous locations, resulting in invalid test results in some cases. For example, ASTM E606 is only used for homogenous materials, while FFF prints are intrinsically heterogeneous. Further, in notched fatigue specimens (e.g., in ASTM D671 [49]), cracks are expected to initiate around the notched zone, while this might not occur in FFF prints due to the presence of numerous stress concentration zones [50]. In addition, ASTM D4482 states that the tested material lacks any initial flaws or cracks, which is not a valid assumption in FFF prints where void gaps exist within the structure.  
Given that FFF prints are inherently different from conventionally manufactured plastics, specialized standards for testing FFF prints are to be developed. However, ASTM D7791 and ASTM D7774 are the most used standards for the uniaxial and flexural fatigue testing of FFF prints. In ASTM D7791, a tensile/compression specimen is subjected to uniaxial fatigue loading at the recommended loading frequency of 5 Hz. The endurance limit of the specimen is then recorded when the specimen fails or reaches \(1\times 10^{7}\) cycles. ASTM D7774 regulates the procedures for the three-point/four-point bending fatigue testing of plastics. Similar to ASTM D7791, the loading frequency can be 1-25 Hz; however, to avoid internal heating, 5 Hz is the recommended frequency. It has been demonstrated that increasing the loading frequency causes internal heating in plastics [51], thus affecting the fatigue limit of FFF prints [34; 52].