PSI - Issue 34

Carla M. Ferreira et al. / Procedia Structural Integrity 34 (2021) 205–210 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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conducted with a rate of angular displacement of 0.1º/sec. To study the mechanical properties of ABS torsional specimens, a set of six combinations with three repetitions per configuration was developed, and concentric and zigzag infill patterns were assessed, varying the number of contours between one, two and three. Table 1 shows this first set of combinations with the associated number and letter coding for each case. The remaining printing parameters were kept constant, infill density was set with a value of 113%, higher than 100% due to the use of a positive air gap and infill was printed first followed by the inner wall and so on until the last outer wall was printed. Table 1. Set of combinations for monotonic torsion tests with the respective number and letter coding. Infill pattern / Number of walls 1 2 3 Concentric C 1 W C 2 W C 3 W Zigzag Z 1 W Z 2 W Z 3 W From monotonic tests, shear stress-strain curves were obtained and the shear modulus , shear yield strength at 0.2% 0.2% , ultimate shear strength , fracture strain , resilience modulus , and toughness modulus , were calculated and used as response variables to assess ABS specimens’ mechanical properties. 2.3. Fatigue torsion tests The chosen set from monotonic torsion tests was tested under cyclic torsion loading conditions to study the fatigue performance of these components. Fully reversed (R=-1), load-controlled tests were performed to assess the lifespan and fatigue strength of ABS specimens. Seven stress levels were chosen and assessed at room temperature with only one repetition per level. The selected stress levels were chosen to be below the 0.2% , found from monotonic torsion tests, to make sure HCF regime was attained. A frequency of 5 Hz was applied since it accomplishes with standards ASTM D7791-17 and ASTM D7774-17. To avoid heating, temperature was controlled with thermographic camera Flir A300 9Hz. Specimens were subjected to constant amplitude loads and the corresponding reversals to failure were plotted on an S-N curve, obtained by equation 1, developed for the HCF regime (McKeen, 2010), where is the cyclic stress amplitude, ′ the fatigue strength coefficient, 2 the number of reversals to failure and the fatigue strength exponent. OPTIKA® SZM trinocular stereo microscope was used with several magnifications to evaluate specimen’s fractured surface. = ′ (2 ) (1) Concentric and zigzag infill patterns were assessed varying the number of contours between one, two and three walls delivering a total of six combinations with three repetitions per case. Fig. 2 shows the shear stress-strain average curves obtained for each configuration. From the presented curves, it is possible to conclude that the , , , and , are practically not affected by the number of contours in both infill patterns. While in concentric pattern the , the , and increase with increasing the number of walls the opposite occurs in the zigzag pattern leading to a decrease in both ductility and strength. Fig. 3 shows surface fracture images of all configurations. Literature reports (Cuan-Urquizo et al., 2019) state that increasing the number of contours can lead to an increase of strength and stiffness while an excessive number of walls can diminish the effect of the infill properties within the walls. In this case, when considering the cases of only one contour, C1W and Z1W, zigzag pattern has higher performance under torsion. When the number of walls is increased, the outside perimeter of the specimen, where maximum shear stresses occur, is approaching a concentric pattern and thus acquiring its characteristics and behavior. This tendency increases with the number of contours and thus degrading zigzag specimens’ performance. 3. Results and discussion 3.1. Monotonic torsion tests