13th International Conference on Fracture June 16–21, 2013, Beijing, China -3- 3. Inner structure changes Increasing of loading velocity for iron samples leads to qualitative changing of structure deformation: in the case of minimum developed projectile velocity there are some separately arranged highly deformed grains (fig. 3, left), whereas the number of such grains in target corresponding to maximum loading velocity is much bigger (fig. 3, right). Figure 3. Microstructure near the loaded surface of deformed iron samples corresponding to different projectile velocity: left picture – 230 m/s, right picture – 260 m/s. Shock-wave loading initiates dynamic recrystallization processes in iron targets. Near the loaded surface dynamic recrystallization usually takes place on grain boundaries, while near the spall area new grains can nucleate on microcracks (fig. 4). Besides, the higher projectile velocity the less deviation of grain size caused by intensive growing of recrystallized grains. Figure 4. Microstructure of deformed iron samples corresponding to different projectile velocity: left picture – 230 m/s, right picture – 260 m/s. Another feature of high-speed loading of targets is accumulation of misorientations in subgrain boundaries, resulting in fragmentation of initial grains. Far enough from spall region this transformation of low-angle boundaries to high-angle boundaries has crystallogeometrical character (fig. 5, left) whereas near the spall fracture surface presence of shear mode of deformation is observed (fig. 5, right).
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