PSI - Issue 2_A

M. Wicke et al. / Procedia Structural Integrity 2 (2016) 2643–2649

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M. Wicke et al./ Structural Integrity Procedia 00 (2016) 000–000

As it can be observed in Fig. 2, the reconstructed pores are characterized by different sizes and shapes in terms of complexity. While the first shrinkage pore (Fig. 2a) is small and relative regular in shape exhibiting only a few arms, the latter one (Fig. 2c) is elongated with some kind of a dendritic structure. In the following, the focus lies only on the first two shrinkage pores in Fig. 2, which were tested for their structural influence by finite element method (FEM). 3.2. Generation of 3D volumetric models The exported models are not suitable for performing numerical computations directly since the STL files only describe the surface geometry of each pore without any representation of the common CAD attributes. Therefore, the local pore geometry reconstructed by μ-CT was first converted into a 3D volumetric model using the commercial software Geomagic Studio 14 (3D Systems). After importing the STL-triangulated surface, the polygon mesh was retriangulated to produce a more uniform tessellation (Fig. 3a). In addition, imperfections were automatically repaired and the coordinate system was centered in the pore center of gravity to facilitate the following numerical investigation. The subsequent generation of a Non-Uniform Rational B-Spline (NURBS) surface was developed by the decomposition of the polygonal model into quadrangular patches. Regions with relative low-curvature portions were detected, allowing a separate specification of the NURBS parameters in the flat and curved areas. Once the initial produced patch layout was locally optimized (Fig. 3b), an ordered grid was constructed within each patch and finally the NURBS surface, whose precision is affected by the density of the grid, was computed as shown in Fig. 3c.

Fig. 3. Volumetric model generation: (a) retriangulated polygon mesh; (b) quadrangular patch layout; (c) fitted NURBS surface

Before exporting the NURBS model in the IGES (International Graphics Exchange Standard) format, a color-coded mapping of the differences between the fitted NURBS surface and the underlying polygon mesh was generated to quantify the accuracy of the surface reconstruction. As shown exemplarily by the color error map in Fig 4, the NURBS deviations in the model of the second shrinkage pore are within 0.8 μm and -2.7 μm.

Fig. 4. Color-coded error map of the second shrinkage pore model