PSI - Issue 12

Lorenzo Bergonzi et al. / Procedia Structural Integrity 12 (2018) 392–403 Lorenzo Bergonzi / Structural Integrity Procedia 00 (2018) 000 – 000

8

399

Table 4. Mechanical characteristic of materials used to model specimen (ABS) and fixture (AISI 304).

E

R s

R m

Materials

A s %

A m %

[MPa]

[MPa]

[MPa]

ABS

1250

32

32

3

49 70

AISI 304

193000

215

504

0.2

There remains a slight stress concentration at the end of the contact region which is however due to the specimen geometry and is also found in the ASTM specimen. Imposing the same displacement, it is noted that stress in the correspondence of the middle section and at the end of the radius differs with respect to the values determined from the previous model, where only the specimen is considered (see Table 3) presenting one relative variance of 4.06% and 0.93% compared to ASTM. The phenomenon had already been foreseen and is essentially due to the sliding of the specimen in the fixture seat while loading.

Figure 10.  XX distribution in specimen undergoing tensile loading.

Table 5. Stress in correspondence of the end of radius and on the middle section. ASTM D638 Equivalent

 XX

 XX

Extraction point

[MPa] 10.01

[MPa] 10.10

End of radius Middle section

9.83

9.43

4. Experimental campaign In order to verify FEM results and evaluate the specimen-fixtures interaction, a series of experimental tests with ASTM and equivalent specimens has been carried out using an MTS 810 tensile testing machine with a 25 kN load cell. From Figure 11 it is possible to see how, through suitable adapters, the fixtures have been installed into the MTS hydraulic fixtures. The two test campaigns were conducted using an extensometer in order to detect the corresponding deformation of the gage length. The specimens were obtained by water jet cutting from 3 mm thick PVC and ABS sheets in a parallel (0°) and orthogonal (90°) direction with respect to the extrusion direction: these materials have been chosen because usually their mechanical properties are determined using ASTM D638. As for the specimens printed using FDM (Fused Deposition Modeling), a Witbox 1 printer was used, with direction of the printing pattern parallel to the longitudinal specimen axis and 100% infill, in order to obtain the best mechanical properties, as described in the work of Rodríguez, et al., (2003); the material used is PLA. The first test campaign was conducted at a crosshead speed of 2 mm/min, while the second and at 5 mm/min, as indicated by ASTM standard.