13th International Conference on Fracture June 16–21, 2013, Beijing, China -5- red atoms surrounded, but their orientations were different. The orientations of lattice deflected to form twin deformation. The area of twin band increased following the crack propagation. From the crack propagation deformation in Fig.6 and Fig.7, it was shown that twin crystal easily occurred for α-Ti under loading direction[0001], while phase transition and dislocation easily happened under loading direction[0110]. Dislocation and twin are two kinds of principal mechanisms for α-Ti. TwinBand HCP Structure （CNA=2） HCP Structure （CNA=2） Disordered Structure（CNA=5） Step=2200 Step=2600 Step=3000 Figure 7. Deformed mechanisms of α-Ti on the condition of loading direction[0001] 4 Traction-separation response To quantify the cohesive zone law, stresses and displacements at local positions are analyzed in detail. MD simulation is conducted for loading direction [0110] at temperature of 300K to develop traction-separation relationships for the CZM. Tensile (σyy) traction is calculated as the average atomic stress at a distance of ±20 Å adjacent to the crack. The region is called T-S region, as illustrated in Fig.8. As this stress is calculated locally near the crack surface it therefore relates to the traction in the traction-separation law[8,9,13,14]. The crack opening displacement (separation) was also calculated from the average displacement of the atoms in T-S region. The opening displacements in normal direction therefore are defined and measured as the average atom displacement in Up Region with respect to Down Region. The average traction and opening displacement associated with region were calculated using all the atoms in T-S region. This effectively reduces the scatter of the data. T-S Region Up Region Down Region x y z 20Å 20Å Figure 8. T-S region selected and loading form The simulation process applying NVE system tracking and in every time step, maintain the
RkJQdWJsaXNoZXIy MjM0NDE=