Keyword: «crack»

One of the little-studied problems of modern mechanics and the physics of fracture is the branching of the crack, which is observed in materials of a different nature. To experimentally determine the rate of crack branching velocity in steels, a series of tests were carried out on the fracture by internal pressure of steel thin-walled cylindrical shells (vessels) with measuring of the crack velocity by the method of breaking the conductive strips. A measurement setup was created on the basis of a precision converter “Tercon” signals of resistance thermometers and thermocouples, connected to a computer. Measurements of the crack velocity were carried out with a rectilinear crack propagation and crack branching.

Oil and gas production and processing facilities and the trunk pipeline system of the Republic of Sakha (Yakutia) have been built and are operating in the Arctic, where the elimination of the consequences of accidents requires significant material and time resources. The work considered, the fracture of the main gas pipelines in violation of the technology of manufacturing pipes (manufacturing defects), during long-term operation (corrosion defects), during the degradation of perennial frozen soils with the strain of the pipeline and shut-off reinforcement; the fracture of tanks for petroleum products with an uneven base precipitate from defects; the fracture of the wind-energy tower TW-250 from the fatigue crack. It is shown that the causes of accidents on the metal structures of the Arctic during long-term operation are corrosion damage on underground pipelines, fatigue cracks on tanks and gas pipelines, uneven base precipitate of the reservoirs, the formation of areas with the non-projective position of pipelines due to the degradation of perennial frozen soils.

Examples of destruction of welded joints of sections of main gas pipelines in Yakutia under the influence of fields of residual welding stresses are considered. The results of experimental studies of residual stress fields of welded joints of plates made by arc welding under conditions of low climatic ambient temperatures are discussed. The mechanism of the influence of residual stresses on the occurrence of additional plastic deformations in the zones of welded joints after welding under conditions of low climatic temperatures is presented.

The article investigates the influence of temperature on the stress triaxiality in the prefracture zone at the crack tip. Based on the asymptotic expansion of stress tensor components, the concept of an effective stress triaxiality coefficient, calculated at the boundary of the plastic zone, is proposed. An analytical relationship between the effective stress triaxiality coefficient and temperature has been established via the temperature dependence of the yield strength, described by the thermally activated flow model. It is shown that a decrease in temperature leads to a monotonic increase in the effective stress triaxiality coefficient. The obtained results provide a quantitative explanation for the physical mechanism of cold brittleness: an increase in stress triaxiality with decreasing temperature creates favorable conditions for the transition from a ductile to a brittle fracture mechanism.

The paper presents an investigation of the stress triaxiality distribution in the vicinity of a crack tip. It has been established that the stress triaxiality values remain finite at the crack tip despite the presence of stress field singularity. The study demonstrates that positive T-stresses lead to a more pronounced decrease in stress triaxiality with increasing distance from the crack tip compared to negative T-stresses. A significant difference in triaxiality levels between plane stress and plane strain conditions has been identified. Under plane strain conditions, the stress triaxiality exceeds that of plane stress conditions by a factor of three, which explains the increased susceptibility to brittle fracture when deformation is constrained in the thickness direction.