Effect of the Local Stress State on the Fracture Mechanism of Metallic Materials with Different Lattices under Single Loads

The study discusses the effect of the local stress state at the crack tip on the fracture behavior of coarse- and ultrafine-grained bcc, fcc and hcp materials under single impact and static loads. The local stress state of the materials at the crack tip under impact and static loading was evaluated by the h_max/t ratio, where h_max is the maximum depth of the plastic zone under the fracture surface and t is the specimen thickness. The depth of plastic zones under the fracture surface was determined using layer-by-layer etching of the surface with subsequent X-ray diffraction analysis. The study results showed that it is not always possible to establish an unambiguous relationship between the fracture mechanisms of metallic materials and the local stress state of a material at the crack tip. Nevertheless, some particular features were found: (i) the cleavage, quasi-cleavage or intergranular brittle fracture of materials, regardless of the lattice type, is indicative of plane strain, (ii) under plane stress, all materials, regardless of the lattice type, exhibit ductile fracture with the formation of a microdimple pattern, and (iii) most fcc materials fail by a mixed mechanism in the transition region from plane strain to plane stress.