Figure 6: Normalized stress/strain function of z/a for r = 0. R = 3.5mm, a = 2mm. 3.3. Orientation distribution of voids For the sake of clarity, a zoom of figure 3b is discussed in this section (figure 7). The knowledge of the void morphology [1] enables to draw arrows indicating the orientation of these voids. By superimposing on figure 7 the geometrical construction in figure 1b, three points were considered with respective radii ρ1, ρ2, ρ3. Local voids orientations were symbolized by the corresponding angles β1, β2, β3 respectively measured between the arrows and the vertical z axis. Recall that in figure 1b, the orientation of the largest principal stress is symbolized by the angle α described in equation (8). In figure 7, it turns out that the evolution of angle α is in excellent agreement with the aforementioned β. Indeed, voids angle is null close to the z axis and gradually increases to coincide with the local notch root curvature near the surface. As a matter of fact, these observations were encountered when voids were located outside the minimum cross section (z ≠ 0). To the authors’ knowledge, such investigations dealing with the mechanical parameters state combined with tomographic observations constitute a novel experimental approach. At this stage, the main important conclusions applied to the PA6 under study consist of: - voids orientation parallel to the largest principal stress. Therefore this seems to indicate the component of the stress involved in void stretching; - no relevance of the largest principal strain with voids orientation (flat contour map); - quantification of voids characteristics allowing the local stress measurement provided a relevant stress scaling methodology (e.g. finite element analysis). 5. Conclusion PolyAmide 6 semi-crystalline polymer deformation mechanisms were studied thanks to Synchrotron Radiation Tomography (SRT) carried out at the European Synchrotron Radiation Facilities (ESRF). An initially notched round bar was tested under tension, up to the end of the stress softening. The sample was then released from the traction machine to be observed by SRT. Specific morphology of voids allowed identification of the voids distribution according to the axial/radial direction. Furthermore, voids orientation was observed to be dependent on their location within the notched region. By comparing these features with the theoretical stress/strain fields, it can be concluded that the largest principal stress is the key mechanical parameter that control voids growth. Data collected from image analysis of SRT would be of great importance to be utilised as input in the mechanical analyses (FEA). In particular, material model parameters governing damage evolution should be adjusted to match the experimentally measured void volume fraction distribution. 0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0.2 0.4 0.6 0.8 1 z/a 0 0.2 0.4 0.6 0.8 1 1.2 σzz/σeq σrr/σeq εzz/εeq σ/σeq ε/εeq 53
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