Evgeny Deryugin

The paper presents the results of tensile and impact bending tests of 17%Cr-19%Mn-0.53%N high-nitrogen austenitic stainless steel in temperatures ranging from −196 to 20 °C. The steel microstructure and fracture surfaces were investigated using transmission and scanning electron microscopes, as well as X-ray diffraction analysis. The steel experiences a ductile-to-brittle transition (DBT); however, it possessed high tensile and impact strength characteristics, as well as the ductile fracture behavior at temperatures down to −114 °C. The correspondence between γ–ε microstructure and fracture surface morphologies was revealed after the tensile test at the temperature of −196 °C. In this case, the transgranular brittle and layered fracture surface was induced by ε-martensite formation. Under the impact bending test at −196 °C, the brittle intergranular fracture occurred at the elastic deflection stage without significant plastic strains, which preceded a failure due to the high internal stresses localized at the boundaries of the austenite grains. The stresses were induced by: (i) segregation of nitrogen atoms at the grain boundaries and in the near-boundary regions, (ii) quenching stresses, and (iii) reducing fcc lattice volume with the test temperature decrease and incorporation of nitrogen atoms into fcc austenite lattice. Anisotropy of residual stresses was revealed. This was manifested in the localization of elastic deformations of the fcc lattice and, consequently, the stress localization in -oriented grains; this is suggested to be the reason of brittle cleavage fracture.

This paper provides a brief overview of the issues concerning the stress field singularity in a solid with a Griffith crack. A new model of the crack is proposed which takes into account the effect of the plastic deformation zone around the crack on the stress field in the solid. The stress field singularity arises as a special case when the curvature radius at the crack tip and the size of the plastic deformation zone around the crack reduce to zero. The model allows analyzing the influence of plastic strain gradients on the concentration and distribution of stresses at the crack tip. It is shown that the plastic strain gradient at the free surface of the crack is crucial for stress and strain calculations for a solid with a crack. An energy analysis is carried out for the fracture parameters of ductile materials with cracks. The derived equations can be used to evaluate the crack resistance of sufficiently ductile materials.