PSI - Issue 24

5

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

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

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In order to determine the parallel section area needed to perform mechanical characterization of AM fabricated metals on MaCh3D, given the limited maximum applicable load, it was necessary to determine average maximum strength of such materials. From the literature review of Gu et al. (2012) it has been found that material such as Ti 6Al-4V or Inconel 718 can easily reach and exceed 1000MPa. Hence, given the previous described geometry constraints, S 0 must be equal (or lower) 5mm 2 . 2.3. Fillet geometry Specimen geometry must be compatible with MaCh3D fixture. In particular, reduced section trait has to be connected to specimen droplet-shape heads, the latter being unmodified in respect to full size MaCh3D specimen, reported in Figure 1 (a). As first attempt, different circular radiuses were adopted to connect the rectilinear trait to sample extremities, starting from the same radius as the one found for round ASTM E8M Type V (2mm) and gradually increasing its value up to 30mm, as shown in Figure 1 (b).

(a)

(b)

Figure 1. MaCh3D full-size specimen (a) mini specimen configurations varying fillet radius (b). NOTE: reported geometries are out of scale.

Due to the geometry of the system, fillet radius could not be increased any further without risking specimen slippage outside the fixtures. To reduce stress intensity factor at the transition between heads and reduced section, an alternative geometry has been developed, using a G2 cubic interpolation spline as transition curve. G2 Continuity or Curvature continuity or Radial continuity implies two faces/surfaces meet along a common edge, are tangent, and the rate of curvature change at each point along the edge is equal for both faces/surfaces. The transition across the edge is therefore curvature continuous: smoother section reduction allows to obtain reduced stress concentration factor. 2.4. Finite Element model to assess stress concentration factor A Finite Element (FE) model has been created using Midas NFX 2019 to determine stress concentration factor as well as to evaluate stress distribution in specimen section. Due to assembly symmetries, only a quarter of the specimen and gripping system has been modelled and is reported in Figure 2 (a). The seat of the specimen remains unchanged whilst specimen fillet radius was varied trough different simulations. An imposed displacement u=0.005mm is applied on the fixture through rigid elements RBE2, resulting in a 0.01mm displacement on the entire geometry. Non-linear contact between specimen head and fixture seat has been carefully modeled in order to reproduce real behavior. First order, hexahedron elements were predominantly used: element type and size were chosen after convergence analysis and trade-off evaluation in terms of solution accuracy and time needed to perform calculations. To avoid any related mesh-dependent results, mesh size in rectilinear section has been kept fixed for the