13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- equaling 9.64Å. The length of crack is 70Å in the two models. Non-periodic and shrink-wrapped boundary conditions were used in the x and y directions, while it was periodic boundary in the z-direction. Before loading, the whole system was made relax freely till the valley of energy status. During MD simulations of tensile crack propagation, the boundaries were stretched by moving different distance increment on different regions of boundaries in the Y-direction at each time step. Molecular dynamics simulation time step was given with t=0.005ps. 204 Å 200 Å 70 Å 204 Å 200 Å 70 Å Figure 4. MD model with loading direction [0110] Figure 5. MD model with loading direction [0001] For the loading condition shown in Fig.4, the phenomenon observed in model by means of CNA was shown in Fig.6. Step=0 indicated that the relaxation was finished. The energy of the system reached minimal. With the increment of loading, crack constantly opened. When the loading increased to Step=3600，it was obvious that crack tip successively emitted dislocations along crystal orientation [2110]and[2110]respectively[13]. The phenomenon continued in the whole process of loading. With the emission of dislocation, crack tip moved forward slowly and the crack blunting phenomenon is obvious at crack tip. When Step=8400, we also found that HCP→BCC phase transformation in the vicinity of the crack tip for α-Ti via CNA. The values of CNA corresponding to different atomic structure were displayed in Fig.6. BCC Structure（CNA=3） Disordered Structure（CNA=5） HCP Structure （CNA=2） Phase Transition Step=0 Step=3600 Step=8400 [2110] [2110] Figure 6. Deformed mechanisms of α-Ti on the condition of loading direction[0110] When Step=2200, there was region where atoms arranged disorderedly near the crack tip. With the increment of loading, the deformed region expanded in the vicinity of the crack tip. When Step=3000, it was obvious that some red atoms (CNA=5) surrounded a lump of blue atoms (CNA=2) in Fig.7. The structure of atoms is identical in the inside and outside of the region that the
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