PSI - Issue 12

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

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on the loadcell side. The specimen is inserted into fixtures seat and the load is transmitted by shape constraint between the sides of the specimen and the steel seat of the fixture: the lid, screwed after the specimen is placed, serves only to keep it in position. Various geometries of fixtures have been evaluated, some shown in Figure 8, in order to avoid stress concentrations outside the reduced section, especially at the end of the seat. All the geometries are modeled in the same conditions, so that the only difference between the models is the geometry configuration of the fixture.

(a)

(b)

Figure 8. Two different fixtures alternatives.

Starting from the geometry of the specimen, a numerical model of the specimen - fixture assembly is created, as shown in Figure 9 using the materials indicated in Table 4. The mesh is mainly composed of parabolic brick elements: in particular, that of the specimen is the same of the previous model (see paragraph 2.2). Symmetry constraints are imposed as in Figure 4 but the displacement of 0.5 mm is imposed to the steel fixture in correspondence of the load cell mounting hole using rigid elements (RBE2). The analysis is non-linear in order to correctly model the contact between specimen and seat where, in the absence of experimental data, a friction coefficient of 0.2 has been imposed as typical for the steel-ABS contact. The specimen material, due to the small displacement imposed, maintains an elastic behavior. 3.1. Results and discussion The results reported in Table 5 are those related to the solution (b), which is the best among the proposed alternatives, while in Figure 10 it is possible to evaluate  XX stress distribution. During the analysis of solution (b), different lengths of the fixture seat were evaluated, in order to reduce the difference in stress values towards ASTM specimen.

Figure 9. Scheme of specimen and fixture FEM model.