13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- Figure 5 shows the development of void nucleation during crack propagation from the notch tip to the impact side. The maximum void density always occurs at the two sides of the fracture surface where fully ductile fracture is seen. The voids appear to peak faster near the plane of symmetry. Which implies that the plasticity at the two sides of the sample is larger than that near the plane of symmetry. Fig.5 Nucleation of voids in DWTT samples Fig.6 shows the void volume fraction distribution in the DWTT sample at different stages. It is seen that the void volume fraction in the pressed notch zone is slightly larger than in the other zone, suggesting that the process of notch pressing affects the initiation of fracture in the test sample. Hong et al. [18] concluded that the stress was less concentrated at the notch tip in the pressed-notch specimens compared to a Chevron notch, and this makes the initiation of fracture of pressed-notch specimens more difficult, and accordingly the deformation preceding the fracture initiation resulted in strain hardening in the hammer-impacted region increased. Fig. 6 Void volume fraction distribution in DWTT samples As seen in Fig.7, the effective strain rate shows a peak near the plane of symmetry until the specimen fractures. This is consistent with the higher crack growth rate due to stress concentration near the surface of symmetry. As expected, the shape of effective strain rate distribution is found to be similar to the shape of the voids distribution. In Fig. 7 (g), it was observed that the inverse
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