13th International Conference on Fracture June 16–21, 2013, Beijing, China -2- 3. Fracture surface analysis Figure 1 represents the microphotographs of spall fracture surfaces of two materials under study. A highly coarse-grained structure of vanadium samples allowed one to define boundaries between crystallites (figure 1, left), and study the fracture regularities inside separate crystallites. It was shown that fracture surfaces of such crystallites consist of cleavage facets, ad the fraction of plastic deformation is significantly low (figure 2, left). Evidences of low fraction of plasticity in fracture of vanadium samples were obtained also from measurements of free surface velocity. In the central part of each cleavage facet a crack formation occurs, different stages of such cracks growing were observed. With increasing of projectile velocity relief of cleavage facets becomes smoother, which reflects higher fracture velocity, and the fraction of plastic deformation is getting lower. Figure 1. Microphotographs of fracture surfaces of two materials: left - coarse-grained vanadium (projectile velocity 280 m/s), right – polycrystalline iron (projectile velocity 300 m/s). Fracture surfaces of polycrystalline iron-targets consist of intergranular cleavage facets with more inhomogeneous character because of much lower grain size (figure 1, right). In central part of facets, which size is corresponding to grain size scale, a formation of microvoid is usually occurs. Typical picture of such fracture surface elements is presented on figure 2, right. Increasing of projectile velocity in investigated interval didn’t reveal any qualitative changes in fracture surface relief. Figure 2. Elements of fracture surface relief on two materials: left picture – vanadium, right picture – polycrystalline iron.
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