3D printing facilitates the fabrication of industrial applications with different materials and complex structures. In terms of polymers, fused filament fabrication (FFF) is the most widespread 3D printing process. Due to the layer-wise deposition of the thermoplastic material, the manufacturing process induces defects in the parts, which significantly affect the resulting mechanical properties. Since the thereby formed weld lines mostly represent the weak spots of additively manufactured components, the resulting mechanical properties strongly depend on the bonding or welding quality between individual layers. Furthermore, components for industrial applications often have to withstand continuously loading and unloading cycles. Consequently, understanding the fatigue behaviour of process-related welds is vital.
Therefore, the behaviour of these weld lines under fatigue loads was investigated in this study. Hence, one strand thick boxes were printed with varying die temperature (Fig. 1a), as previous studies showed that this processing parameter significantly effects the interdiffusion depth between adjacent strands. Polylactide (PLA) was chosen as the material of choice because it is easy to process using 3D printing and is also a bio-based polymer. Fatigue test specimens were prepared from the boxes in a way that the load acts perpendicular to the strands. Fatigue tests were carried out in tension (R=0.1) on different stress levels (Fig. 1b). The stress levels are set to reach cycles to failure in the range of 10³ to 106. A proper loading frequency is selected in order to avoid excessive hysteretic heating. After fatigue testing, the respective fracture surfaces were measured under a microscope to evaluate the resulting stresses. Finally, correlations between the manufacturing process, fatigue result and fracture surface were made.