13th International Conference on Fracture June 16–21, 2013, Beijing, China -8- Experimental results at the strain rates 8 -1 10 s ≤ correspond to the regime of heterogeneous nucleation then the formation of micro-cracks occurs on inhomogeneities (defects of the exponential spectrum). MD results and experiments [3,20] for the strain rates 8 -1 10 s > correspond to the mode of homogeneous nucleation when the micro-cracks are typically formed in homogeneous parts of material (zones of perfect crystal or zones with a bundle of identical defects). The term «homogeneous nucleation» is conditional for polycrystals because in this case the fracture begins in the grain boundaries [7]. Therefore, we use here this term in the sense of presence of a quantity of identical uniformly distributed defects, for example, grain boundaries (in contrast with the defects of exponential spectrum). Calculations show, that plasticity is a dominant factor influencing on the voids growth and on the dynamic strength at the strain rates ≤ 104 s–1. On the contrary, at the strain rates ≥ 107 s–1, the plasticity is negligible in the dynamic strength calculations, but it can influences on the resulting form of voids and on the picture of destructed material. 4. Conclusions The micro-cracks nucleation and growth have been analyzed for the purpose of the dynamical fracture description. The model was formulated, which agrees well with experimental and MD data on the strain rate dependence of the dynamic strength for a number of metals. This model can be practically used in the material dynamics simulations. The strain rate dependence of the dynamic strength has two different regions corresponding to regimes of the heterogeneous nucleation (at the strain rates < 108 s–1) and the homogeneous nucleation (at the strain rates > 108 s–1). In the first case, the weakened zones of the material play an important role in fracture. In the second mode, the number of weakened zones is insufficient, voids are nucleated in undefected material, and the strength growth is decelerated. It contradicts with the commonly used uniform power dependence of spσ on / d dt ε [3,10] in the whole region of the strain rate, therefore, the ideal strength [3] can be achieved at the higher strain rates (above 1011–1012 s–1), than it follows from the power dependence. Acknowledgements The study was supported by The Ministry of education and science of Russian Federation, projects 14.B37.21.0384 and 2.2320.2011. References [1] G.I. Kanel’, S.V. Razorenov, K. Baumung, J. Singer, Dynamic yield and tensile strength of aluminum single crystals at temperatures up to the melting point. J Appl Phys, 90 (2001) 136-143. [2] G.I. Kanel’, V.E. Fortov, S.V. Razorenov, Shock waves in condensed-state physics. Phys Usp, 50 (2007) 771-791. [3] S.I. Ashitkov, M.B. Agranat, G.I. Kanel, P.S. Komarov, V.E.Fortov, Behavior of aluminum near an ultimate theoretical strength in experiments with femtosecond laser pulses. JETP Letters, 92 (2010) 516-520. [4] E. Moshe, S. Eliezer, E. Dekel, A. Ludmirsky, Z. Henis, M. Werdiger, I.B. Goldberg, An increase of the spall strength in aluminum, copper, and Metglas at strain rates larger than 107 s–1. J Appl Phys, 83 (1998) 4004–4011. [5] V.H. Whitley, S.D. McGrane, D.E. Eakins, C.A. Bolme, D.S. Moore, J.F. Bingert, The elastic-plastic response of aluminum films to ultrafast laser-generated shocks. J Appl Phys, 109 (2011) 013505. [6] V. Dremov, A. Petrovtsev, Ph. Sapozhnikov, M. Smirnova, Molecular dynamics simulations of
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