13th International Conference on Fracture June 16–21, 2013, Beijing, China -4- Figure 2. Strain energy versus torsional angle curves for Cu NWs with GBs. For the NW with one GB, an extremely early yielding is observed, which only corresponds to a critical angle of 0.603 rad. The evident existence of intrinsic SF is observed at the torsional angle 0.848 rad (see Fig. 3b1), which is initiated from the location of the GB. With further loading, more SFs are generated which is associated with a sharp strain energy decrease as illustrated in Fig. 3b2. Interestingly, a local HCP structure is produced between the torsional angle around 1.319 and 2.652 rad (see Fig. 3b3), which results a considerable strain energy increase as pointed out in Fig. 2. Figure 3. Atomic configurations at different torsional angles. Perfect NW: (a1) 1.634 rad, (a2) 1.665 rad, (a3) 2.482 rad; NW with one GB: (b1) 0.848 rad, (b2) 1.1 rad, (b3) 2.199 rad; NW with three GBs: (c1) 0.691 rad, (c2) 2.042 rad, (c3) 2.796 rad; NW with four GBs: (d1) 0.817 rad, (d2) 2.356 rad, (d3) 2.89 rad; NW with nine GBs: (e1) 1.759 rad, (e2) 2.733 rad, (e3) 0.911 rad. Similar as the previous case, the NW with three GBs also exhibits a particularly early yielding (around 0.654 rad). However, in contrast to the large volume of SFs for the NW with one GB after yielding (see Fig. 3b1), only micro SFs are generated initially around the locations of GBs as shown in Fig. 3c1. Along with the increase of the torsional strain, more SFs are generated around the two outside GBs (i.e., GBs 1 and 3 in Fig. 1a) as revealed in Fig. 3c2. It is observed that, no large SFs nucleation is occurred around the middle GB (i.e., GB 2 in Fig. 1a) only until the torsional angle reaches 2.444 rad (see Fig 3c3). For the NW with four GBs, a low critical angle around 0.798 rad is observed. After yielding, SFs
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