PSI - Issue 24

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Lorenzo Bergonzi et al. / Procedia Structural Integrity 24 (2019) 213–224 Lorenzo Bergonzi et al. / Structural Integrity Procedia 00 (2019) 000–000

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3.4. Preliminary experimental campaign using FFF Since the work is still in progress, it was not possible to have experimental data regarding mechanical behavior of different specimens fabricated with metal AM. To understand if any macroscopic size effect would have been present, a preliminary test campaign was realized using Free Filament Fabrication (FFF). The miniature specimens geometry (including the ones described in Appendix A) were compared in terms of maximum tensile strength with full-size MaCh3D specimen. All specimens were printed using polylactide (PLA) with 100% infill and concentric raster orientation, thus allowing to have filament deposition parallel to specimen axis in the reduced region avoiding any stress concentration due to infill orientation. Tensile testing was performed using MaCh3D, with a constant strain rate of 0.04 mm/mm/min corresponding to a maximum crosshead speed of 2mm/min while testing full-size specimen; 5 specimens were tested for each geometry. In Figure 8 maximum stress is reported for each configuration:

30 32 34 36 38 40 42 44 46 48 50

Maximum Stress [MPa]

Full-Size

L-Size 45.98

M-Size

S-Size 46.34

TS [MPa]

43.60

42.68

Figure 8. MaCh3D different specimen geometry configurations maximum tensile stress values. Red lines indicate standard deviation. Average maximum tensile stress is " = 44.87 MPa with a standard deviation of 2.27 MPa, corresponding to a ± 5% variation in respect to the average value. Due to the nature of the material used and the size of defects typical of this production technology, the results are quite good, indicating low dispersion in terms of maximum stress. 4. Conclusions Three different miniaturized specimen geometries have been developed and numerically validated in terms of stress distribution in the parallel section and at the end of the spline fillet. A G2 cubic interpolation spline has been used as transition curve, allowing to decrease stress concentration factor at the end of the fillet, without altering specimen working principle based on droplet heads interference with the custom designed machine fixture. Geometry were partially validated using FFF production method, determining that maximum stress levels reached for different geometries are comparable. The validation process is still an ongoing process, having to produce and tests metallic samples fabricated with AM as well as cut from wrought steel in order to assess effective specimen compliance with ASTM E8 and ISO 6892. Appendix A. Two more miniaturized geometries have been developed, to be used with less performing material or in the case such small section as the one of S-Size specimen can’t be produced. Having larger cross sections, the two other geometries are defined as “M-Size” and “L-Size” and overall dimensions are reported in Figure 9 and Figure 10, respectively.