PSI - Issue 34

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

209

5

a

b

S-N curve - Fatigue torsion tests

10 12 14

Stress amplitude, S a (MPa)

0 2 4 6 8

c

c

S a = 63.31 (2N f ) R² = 0.9069

-0.204

100

1000

10000

100000

1000000

Number of reversals to failure, 2N f

Fig. 4. (a) S-N curve for the fatigue torsion tests of the chosen configuration set of specimens (zigzag infill pattern with two walls), obtained from monotonic torsion tests and image of the fractured surface of the lower (b) and higher (c) stress levels assessed, 4 and 12 MPa, respectively. It was observed that at lower stress levels crack initiates in a 90º plane with specimen longitudinal axis, but also originating secondary cracks which results in fracture surface morphology very irregular, Fig. 4 (b). When the intensity of the load is increased (above 5 MPa) specimens tend to present a flat fracture surface, as shown in Fig. 4 (c). Almost all specimens failed in the critical section next to the diameter change where stress concentrations are found. 3.3. Fractography From the microscopic images in Fig. 5, three types of pores were identified, one related to the physical gap between layers generated by voids (a); one, smaller and spherical due to entrapped gas caused by melting of filament during extrusion (b); and another type of pore due to bad fusion between deposited adjacent rasters (c). Stress whitening denotes the region where final fracture occurred suddenly, shown by the higher amount of plastic deformation on that location. The other half of the fractured surface was confined in a horizontal plane.

a

b

c

Fig. 5. Pores due to voids between printed layers (a), entrapped gas during extrusion (b), and lack of fusion between deposited adjacent rasters (c).

4. Conclusions and further work In this work, the fatigue response of an improved set of FFF ABS solid cylinder specimens under monotonic torsion was evaluated. From monotonic tests it was concluded that the zigzag infill pattern leads to better mechanical properties under torsion loading when compared to the concentric pattern. Increasing the number of walls proved to be beneficial for the concentric pattern, while it reduced zigzag pattern ’s performance. Both pattern specimens were characterized by a ductile fracture type in torsion, with fracture occurring in a 90º plane with longitudinal axis, whereas zigzag pattern showed a more pronounced brittle fracture type with a helix inclined 45º with the axis, concerning final fracture stage. From torsion fatigue tests it was observed that the fatigue mechanism between layers governed the