13th International Conference on Fracture June 16–21, 2013, Beijing, China -11- (3) The critical stress intensity factor has different variation tendency depending on the disclination strength and the geometry of the two disclination dipoles. For each situation, the most probable emission angles for positive dislocation and negative dislocation are different. (4) The grain size has great effect on the dislocation emission from the crack tip. The critical stress intensity factors first increase then decrease with increasing of the grain size. And there is a critical grain size making the dislocation can emit from the crack tip without any external loadings. (5) The dislocation emission from the shorter crack tip is much easier. So, the shorter crack can be easily blunted, and the longer crack tends to grow. (6) The location and geometry of the cooperative grain boundary sliding and migration have great influence on the critical stress intensity factors. Acknowledgements The authors would like to deeply appreciate the support from the NNSFC (11172094 and 11172095) and the NCET-11-0122. The work was also supported by the Fundamental Research Funds for the Central Universities, Hunan University. References [1] P. Barai, G.J. Weng, Mechanics of a nanocrystalline coating and grain-size dependence of its plastic strength. Mech Mater, 43 (2011) 496-504. [2] S.V. Bobylev, A.K. Mukherjee, I.A. Ovid'ko, Emission of partial dislocations from amorphous intergranular boundaries in deformed nanocrystalline ceramics. Scripta Mater, 60(1) (2009) 36-39. [3] M. Dao, L. Lu, R.J. Asaro, J.T.M. De Hosson, E. Ma, Toward a quantitative understanding of mechanical behavior of nanocrystalline metals. Acta Mater, 55(12) (2007) 4041-4065. [4] C.C. Koch, I.A. Ovid'ko, S. Seal, S. Veprek, Structural nanocrystalline materials: Fundamentals and Applications, Cambridge University Press, Cambridge, 2007. [5] M.A. Meyers, A. Mishra, D.J. Benson, Mechanical properties of nanocrystalline materials. Prog Mater Sci, 51 (2006) 427-556. [6] S.H. Xia, J.T. Wang, A micromechanical model of toughening behavior in the dual-phase composite. Int J Plast, 26 (2010) 1442-1460. [7] K. Zhou, A.A. Nazarov, M.S. Wu, Competing relaxation mechanisms in a disclinated nanowire: temperature and size effects. Phys Rev Lett, 98 (2007) 035501. [8] K. Zhou, M.S. Wu, A.A. Nazarov, Relaxation of a disclinated tricrystalline nanowire. Acta Mater, 56 (2008) 5828-5836. [9] C.C. Koch, Structural nanocrystalline materials: an overview. J Mater Sci, 42(5) (2007) 1403-1414. [10] I.A. Ovid’ko, Deformation and diffusion modes in nanocrystalline materials. Int Mater Rev, 50 (2005) 65-82. [11] R.T. Zhu, J.Q. Zhou, H. Jiang, D.S. Zhang, Evolution of shear banding in fully dense nanocrystalline Ni sheet. Mech Mater, 51 (2012) 29-42. [12] S. Cheng, E. Ma, Y.M. Wang, L.J. Kecskes, K.M. Youssef, C.C. Koch, U.P. Trociewitz, K. Han, Tensile properties of in situ consolidated nanocrystalline Cu. Acta Mater, 53(5) (2005) 1521-1533. [13] K.M. Youssef, R.O. Scattergood, K.L. Murty, J.A. Horton, C.C. Koch, Ultrahigh strength and high ductility of bulk nanocrystalline copper. Appl Phys Lett, 87 (2005) 091904. [14] K.M. Youssef, R.O. Scattergood, K.L. Murty, C.C. Koch, Nanocrystalline Al-Mg alloy with ultrahigh strength and good ductility. Scripta Mater, 54(2) (2006) 251-256. [15] A.V. Sergueeva, N.A. Mara, A.K. Mukherjee, Grain boundary sliding in nanomaterials at elevated temperatures. J Mater Sci, 42(5) (2007) 1433-1438. [16] S. Bhaduri, S.B. Bhaduri, Enhanced low temperature toughness of Al2O3-ZnO2 nano/nano composites. Nanostruct Mater, 8(6) (1997) 755-763. [17] A.A. Kaminskii, M.S. Akchurin, R.V. Gainutdinov, K. Takaichi, A. Shirakava, H. Yagi, T. Yanagitani, K. Ueda, Microhardness and fracture toughness of Y2O3- and Y3Al5O12- based nanocrystalline laser ceramics. Crystallogr Rep, 50(5) (2005) 569-873. [18] R.A. Mirshams, C.H. Xiao, S.H. Whang, W.M. Yin, R-Curve characterization of the fracture toughness of nanocrystalline nickel thin sheets. Mater Sci Eng A, 315(1-2) (2001) 21-27. [19] Y. Zhao, J. Qian, L.L. Daemen, C. Pantea, J. Zhang, G.A. Voronin, T.W. Zerda, Enhancement of fracture toughness in nanostructured diamond-SiC composites. Appl Phys Lett, 84 (2004) 1356.
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