13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- Dynamic fracture of metals in wide range of strain rates Alexander E. Mayer1,* 1 Department of Physics, Chelyabinsk State University, Chelyabinsk, 454001, Russia * Corresponding author: mayer.al.evg@gmail.com, mayer@csu.ru Abstract A model of dynamic tensile fracture is constructed, which is applicable for many metals in wide range of strain rate. It considers a stage of thermofluctuation nucleation of voids and stages of voids growth and aggregation. The model contains two fitted parameter for each substance: the first parameter is a specific free energy of metal surface; the second one is a distribution parameter for weakened zones of material. These parameters are found for copper, aluminum, iron, titanium, nickel and molybdenum by fitting with the experimental data and molecular-dynamics simulations. Calculations show that there are two regions with different slope in the strain rate dependence of strength: in the first one (at strain rate < 108 s–1) voids are nucleated in weakened zones, and strength grows up relatively fast with strain rate; in the second one (at strain rate > 108 s–1) number of weakened zones becomes insufficient, voids are nucleated predominantly in perfect material, and the strength growth is decelerated. Plasticity is not effect on voids nucleation and on the material strength at the strain rate > 107 s–1, but it becomes a dominant factor at the strain rate < 104 s–1. Keywords Fracture, High-rate tension, Metal, Micro-cracks, Two-level approach 1. Introduction Dynamical tensile strength is an important property of materials. A number of experimental works is devoted to its determination: a plate impact [1,2], a short-pulse laser irradiation [3-5] and a powerful ion irradiation [1] can be used to produce conditions of metal rupture. Very high strain rates (above 109 s–1) are available now in experiments on irradiation of thin foils by short laser pulses [3,5]. Molecular dynamics (MD) is also a useful method for the investigation of fracture at ultra-high strain rates [6-10]. The dynamical strength value substantially depends on the strain rate. Experiments and MD simulations can not cover all possible ranges of this parameter. Therefore, their data have to be supplemented by some approximation for using in simulations of dynamical processes in materials. For example, the strain rate dependence of the dynamic strength is commonly approximated by a simple power law [3,10]). Various physical models of fracture typically separate the stages of the voids nucleation and of the voids growth [11]. Nucleation of voids is strongly effected by the presence of inclusions, grain boundaries and other defects of the crystal structure [12]. A modification of the model [13] is presented here, which considers weakened zones of material with reduced threshold of the micro-voids generation. It allows describing the strain rate dependence of strength in the range of strain rates from 103 s–1 up to 1011 s–1. Kinetics of the dynamic fracture of metals is numerically investigated basing on the proposed model. 2. Model of fracture The model uses two-level approach for fracture description. Micro-level of modeling deals with individual micro-cracks in crystalline material and describes their thermo-fluctuation nucleation, growth and aggregation. Macro-level deals with averaged values through a length scale much large the distance between the cracks. Micro-cracks growth is described with use of simplified dynamics
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