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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al. 2021) Then, shorter specimens were machined from the gauge part of the previous one to avoid buckling in the tension-compression fatigue tests, according to the geometry reported in Fig. (1a).

Table 2. Comparisons between mean values of fiber length of each material. Material Number of fiber measured

L m,ad [μm]

L w,ad [μm]

vPPMF/GF_IM rPPMF/GF_IM vPPMF/GF_AM rPPMF/GF_AM

1063 1301 1399 960

327 297 435 345

421 352 566 428

Starting from the same pellets made of the recycled compound, the AM specimens were produced by using a Pollen® Series P 3D printer. The specimens were produced in the “flat” position (F) according to the geometry reported in Fig. (1b) for tension-compression and tension-tension fatigue tests, respectively. The relevant process parameters are the same of those reported in (Rigon et al. 2021) except for the printing speed, that in this work was set to 20 mm/s. The specimens were manufactured over a previously 3D printed base plate of the same material by setting an air gap of 0.19-0.20 mm between the 3D printed base plate and the first layer of the specimens to ease detachment. However, this generates a rougher surface than the one produced in the last layer. Three sets of samples characterized by three infill patterns were printed adopting 100% of infill density. The infill patterns are indicated as 0°, 90° and ±45° where the angle means the orientation of the deposited filaments with respect to load direction. Notably, the notation ±45° means that the filament orientation of subsequent layers changes from +45° to -45°.

a) IM

b) AM

build direction

Type I – ASTM D638-14

0°

platform

90°

±45°

for fatigue tests R=-1 and R=0.05

for fatigue tests R= 0.05

for fatigue tests R=-1

Fig. 1. (a) Geometry of the specimens produced by IM for fatigue testing with load ratios equal to -1 and 0.05. (b) Orientation, infill patterns and geometries of the specimens produced by AM for fatigue testing with load ratios equal to -1 and 0.05.

Force-controlled axial fatigue tests were performed by using an electromechanical testing machine StepLab EA05 controlled by a TestCenter StepLab digital controller and equipped by load cell of 5 kN. Sinusoidal cyclic load was applied with a load frequency ranging from 2 to 16 Hz depending on the load level of the test to avoid thermal failure due to self-heating generated by the material during the test. To monitor the temperature, copper-constantan thermocouples were fixed at the frontal surface of the specimens approximately in the middle of the gauge length by using duct tape. The temperature signal was acquired by using a data logger National Instruments NI-USB-9162 USB carrier operating at a sampling frequency of 0.1Hz. The fatigue tests were interrupted when the complete separation of the specimens occurred (i.e., 100% of the stiffness loss) which defined the number of cycles to failure N f .