## 1 Introduction  
In recent years, additive manufacturing (AM) or 3D printing has got the highest interest from industries, including aerospace [1], automotive [2], biomedical [3], construction [4], food [5], and other industries [6]. AM offers the freedom of design, implying that complex geometries can be manufactured in one step, eliminating the tooling cost and post-processing steps, such as machining [7]. Further, several components can be manufactured in one step via AM, reducing the weight of the final product and therefore saving up to 33% fuel in transport industries [8]. The economic benefits of AM on supply chains include lower material waste, simplified production processes, and improved flexibility because the finished product can be manufactured near the customer. Among the various AM techniques, fused filament fabrication (FFF), also referred to as fused deposition modelling (FDM) or material extrusion (MEX), is considered one of the most widely used and cost-effective manufacturing methods for plastic parts. In FFF, 3D parts are created using a continuous thermoplastic or composite material thread in filament form. Figure 1 schematically illustrates the 3D printing of a typical tensile specimen. This method includes an extruder that feeds the plastic filament through an extruding nozzle, which is selectively deposited layer by layer onto the build platform after melting in a predetermined automated path. Depending on the specimen orientation, supporting structures are 3D-printed through a different nozzle to support the structure of prints. Supporting structures, as sacrificial materials, are necessary to secure overhanging sections to the previous layers when the crucial inclination angle (often 45\({}^{\circ}\)) is reached and to assist in removing the part from the build plate without damaging the main part. These structures also favour maintaining the part's structural integrity during metal printing by reducing distortions caused by thermal stresses [9]. A review of different support structures and their designs for AM parts is provided in [10]. Supporting filaments are usually extruded thorough a different extruder. The second extruder can also be used to print materials with different colours or mechanical or electrical properties [11; 12]. The extruders (nozzles) are attached to the print head that can move in the X and Y directions. However, using two extruders implies high inertia to the print head, decreasing the printing accuracy. Furthermore, while the active extruder is operating, the remaining molten plastic in the inactive extruder might leak into the component [12]. Like other parts, FFF prints (referred to as \"print\" hereafter in this paper) are susceptible to mechanical failures during their service life. Of known failures, fatigue is structural damage caused by cyclic loading that might result in cracking or rupturing of the part at stresses lower than its yield strength. Therefore, it is vital to understand the resistance of FFF prints to cyclic loading and unloading [13]. Generally, FFF prints possess weaker strength than their injected-moulded (IM) counterparts due to the inherent flaws, such as void spaces, improper adhesion between extruded filaments, and material degradation during printing. Further, thermal stresses between extruded filaments may cause the formation of internal cracks, which can progress during the loading stage [14]. [PERSON] et al. [15] characterized the mechanical properties of tensile acrylonitrile butadiene styrene (ABS) specimens fabricated via injection moulding and FFF printing. The specimens were subjected to quasi-static and fatigue tensile loads at stresses equal to 40%, 60%, and 80% of their ultimate tensile strength (UTS). The authors concluded that the FFF prints had considerably inferior performance, showing approximately 63% and 41% lower tensile and fatigue strength, respectively, compared to that observed in IM specimens.