Cleavage anisotropy in tungsten single crystals

We have performed an extensive study of the cleavage fracture of tungsten single crystals between 77 K and room temperature. The dependence of the fracture toughness on temperature and on crack propagation direction as well as the river line patterns on the fracture surfaces clearly indicate that the intrinsic brittle fracture process is anisotropic with respect not only to the plane but also to the direction of crack propagation. This anisotropy cannot be explained within Griffith's thermodynamic theory of brittle fracture. However, it compares well with atomistic simulations, which suggest that it may be a consequence of anisotropic lattice trapping.