13th International Conference on Fracture June 16–21, 2013, Beijing, China -6- a) b) crack front are extracted. Fig 4: example of mesh of a crack surface used for level-sets computation The G-θ method proposed by Destuynder et al. [11] was used to compute the SIFs. Due to corner point singularities, the values computed at surface points were not included in the analysis. 4.1. c The structural mesh. Half of the CCP specimen was modelled with linear hexahedral (50µm*50µm*50µm) elements in a parallalepidal zone centered at the crack tip, linked to tetrahedral elements, via pyramidal elements, out of the process zone. The boundary conditions are indicated on Fig. 5. Fig 5: Boundary conditions and mesh of the CCP specimen in the X-FEM model 4.2. Elastic-plastic finite element analysis This part focuses on the onset of crack twisting and thus considers flat, normal cracks, but takes tunnelling into account. Only a quarter of the CCP specimen was modelled, taking advantage of the symmetries. A 3D mesh with a straight crack front was first prepared, using linear elements, 30*30*150μm-wide near the crack front, which was then deformed, using the polynomial equation describing best the observed tunnelling front (Fig 7). Figure 7: a) F.E. mesh of a quarter CCP specimen and boundary conditions, b) tunnelling crack front
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