PSI - Issue 34

D. Rigon et al. / Procedia Structural Integrity 34 (2021) 199–204 Rigon D. et al./ Structural Integrity Procedia 00 (2021) 000–000

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5

comparison purposes, the fatigue curve of 50% of survival probability of the recycled material processed by IM were reported in Figs (3a) and (3b).

100

100

rPPCC-GF_F0_5_-1 rPPCC-GF_F90_5_-1 rPPCC-GF_F±45_5_-1 recycled IM F0° F90° F±45° IM from Fig. (2a)

b) R= 0.05 rPPMF-GF AM

a) R= -1 rPPMF-GF AM

k= 16.9, T σ = 1.41, σ A,50% = 12.4 MPa k= 16.8, T σ = 2.82 σ A,50% = 5.52 MPa k= 24.3, T σ = 1.45. σ A,50% = 10.1 MPa

rPPCC-GF_F0_3.5_005 rPPCC-GF_F90_3.5_005 rPPCC-GF_F±45_3.5_005 recycled IM F0° F90° F±45° IM from Fig. (2b)

k= 15.6, T σ = 3.93, σ A,50% = 8.77 MPa k= 6.1, T σ = n.a, σ A,50% = 2.46 MPa k= 12.5, T σ = 2.21, σ A,50% = 4.6 MPa

σ A,50%

σ A,50%

σ a [MPa]

σ a [MPa]

σ A,50% σ A,50% σ A,50%

10

10

σ A,50% σ A,50%

σ A,50%

N A

N A

scatter band: 10-90% of survival probability

scatter band: 10-90% of survival probability

1

1

1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07

1.0E+01 1.0E+02 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07

Number of cycles to failure, N f

Number of cycles to failure, N f

Fig. 3. Fatigue test results and relevant fatigue curves of rPPMF-GF specimens produced by AM (a) for R=-1 and (b) for R=0.05.

The results relevant to AM specimens reported in Fig. (3a) and (3b) highlight the effects of the infill pattern on the fatigue strength. Regarding AM results, the F0° specimens exhibit the highest fatigue strength at N A equal to 12.4 MPa and 8.77 MPa for R=-1 and 0.05, respectively. On the contrary, the lowest fatigue strength pertained to F90° specimens ( σ A,50% =5.52 MPa (R=-1) and 2.46 MPa (R=0.05)), while the F±45° specimens showed an intermediate fatigue strength between F0° and F90° specimens. All AM specimens have lower fatigue properties than the same material produced by IM; more precisely, for R=-1, F0°, F90° and F±45° specimens have -45%, -55% and -75% lower σ A,50% than the IM ones, while for R=0.05 the percentage differences are equal to -43%, -70% and -84%. 3.3 Macroscopic damage analyses Fig. 4 reports one image of the fracture paths for each testing condition, technology and material. As shown in the first two columns of the figure, IM virgin and recycled samples are characterised by similar macroscopic failure modes with fracture surfaces almost perpendicular to the loading direction for both the load ratios. Similar fracture paths were obtained in specimens broken under quasi-static loads (Rigon et al. 2021). In the case of recycled F0° specimens, the failure initiated mostly at the starting point of the fillet due to the presence detrimental defects between the wall and the infill, which were introduced during the pellet AM process for the specific specimen’s geometry. Overall, the fracture propagated in a zigzag appearance due to interlayer delamination and debonding of adjacent filaments. Regarding the recycled F90° specimens, the fracture surfaces were planar, normal to the loading direction and located at the interface between adjacent filaments. The last column of Fig. 4 report an example of failure in F45° specimens. In these cases, a combination of interlayer delamination and debonding of adjacent filaments were observed. Finally, it is worth noticing that the IM samples did not show visible discontinuities in the polymer matrix before testing; conversely, the upper surface of the AM specimens (i.e. the last 3D printed side) presented visible and diffuse defects both between adjacent filaments and also in the filament. Such defects can be observed in regions far from the position of the fracture surface as reported in Fig. 4, which shows the last printed specimen’s surface. Further investigations regarding fibers orientation, distribution of defects and fatigue damage will be performed for a better understanding of the fatigue behaviour of the IM as well as AM specimens.