EFFECT OF LOCAL STRESSES ON THE FRACTURE OF METALLIC MATERIALS WITH DIFFERENT LATTICES UNDER SINGLE LOADS

Abstract

The study discusses the effect of the local stress field at the crack tip on the fracture behavior of coarseand ultrafine-grained materials with bcc, fcc, and hcp lattices under single loads (impact and static). The local stresses at the crack tip under impact and static loading were assessed by the hmax/t ratio, where hmax 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 followed by 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 field at the crack tip. Nevertheless, some particular relationships were established: (i) the cleavage, quasi-cleavage, or intergranular brittle fracture of materials, regardless of lattice type, is indicative of plane strain deformation, (ii) under plane stress, all materials, regardless of lattice type, exhibit ductile fracture with micropitting, and (iii) in the transition from plane strain to plane stress, most fcc materials fracture by a mixed mechanism.