PSI - Issue 34

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

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From Fig. 3, it can be observed that concentric pattern tends to fracture in a 90º plane with specimen longitudinal axis characterized by a ductile fracture type in torsion and with increasing the number of contours, close to the final instant specimens’ fracture are in 45º planes showing a more brittle fracture type in torsion. Zigzag pattern also shows similar behavior, i.e it is characterized by crack initiation and propagation in a 90º plane, but in this case, there is a bigger influence of the pattern resulting in more pronounced fracture surfaces in the 45º plane. In both patterns, an increase of contours from one to two walls proved to be beneficial for the improvement of surface quality of the specimens, with no relevant difference between two and three walls. It can therefore be concluded that zigzag infill pattern with one and two walls proved to be the configurations with best performance under monotonic torsion loads, with residual differences. Although one wall configuration had slightly better mechanical properties, zigzag pattern with two contours was selected for the fatigue tests because it showed better surface quality.

Fig. 2. Shear stress-strain average curves of ABS torsion specimens for the concentric and zigzag infill patterns with one, two and three walls. Each curve represents the average curve of the three repetitions per configuration.

Fig. 3. Surface fracture images of the concentric (a) and zigzag (b) infill patterns with one, two and three walls ABS torsion specimens.

3.2. Fatigue torsion tests

Load-controlled tests with fully reversed loading and a frequency of 5 Hz were performed with the chosen set from monotonic torsion tests. From the seven stress levels assessed, the S-N curve was constructed and is shown in Fig. 4 (a) along with the images of the fracture surface of the lower (b) and higher (c) stress levels assessed, 4 MPa and 12 MPa, respectively. From the S-N curve, with 2 = 0.9069, it was possible to estimate that the fatigue strength coefficient, ′ , is 63.31 MPa, and the fatigue strength exponent, , is – 0.204.