Vol 2016, No 10 (2016)

The conditions of possible microcrack nucleation and stability in the elastic field of disclinations screened by a free surface, disclinations of the opposite sense, or a distributed ensemble of moving dislocations are considered. The plastic deformation screening the elastic field of a disclination is shown to effectively stabilize microcrack growth at a sufficiently large grain size.

The evolution of the dislocation substructure in polycrystalline Cu–Al alloys with various grain sizes is studied during deformation to failure. A relation between the fracture of the alloys and the forming misorientation dislocation substructures is revealed. Microcracks in the alloy are found to form along grain boundaries and the boundaries of misoriented dislocation cells and microtwins.

It is shown that the elongation to failure δ and ultimate tensile strength σ_u in polymers are related as σ_u(1 + δ) = s_u = const at the temperatures of non-Hookean entropy viscoelasticity. Different drawing from different initial states brings the structure of polymer to the limiting uniaxial structure with the same true stress of breaking carbon chains s_u.

The oxidation kinetics of a composite material, which consists of an Al₂O₃–Al₅Y₃O₁₂ matrix and molybdenum fibers and has a high cracking resistance, is studied. The mass loss of the composite material during oxidation is shown to be several orders of magnitude lower than that of molybdenum. Oxidation in quiet air at 1250°C for several hours weakly changes the strength of the composite material at temperatures from room temperature to 1300°C. It is also shown that the strength of the composite material as a function of the oxide matrix composition (Al: Y ratio) changes nonmonotonically. The maximum strength shifts from the Al₂O₃–Al₅Y₃O₁₂ eutectic point toward garnet.

The defect structure of the electrolytic copper coatings formed upon mechanical activation of a cathode is described. These coatings are shown to have a fragmented structure containing disclination-type defects, namely, terminating dislocation, disclination and twin boundaries; partial disclinations, misorientation bands; and twin layers. They have both growth and deformation origins. The mechanisms of formation of the structural defects are discussed. It is experimentally proved that part of the elastic energy stored in the crystal volume during electrocrystallization can be converted into surface energy. As a result, catalytically active materials with a large developed surface can be synthesized.

The fatigue properties and the fracture mechanisms of the Ti–6Al–4V alloy produced by selective laser melting (SLM) from a powder of an CL41TiELI titanium alloy have been studied. Cylindrical blanks were grown at angles of 90° and 45° to a platform. The best fatigue strength is observed in the samples the blanks of which were grown at an angle of 45°. It is found that the structure of the SLM material can contain portions with unmelted powder particles, which are the places of initiation of fatigue cracks.

The mechanical properties of austenitic–martensitic VNS9-Sh (23Kh15N5AM3-Sh) steel are studied at a static strain rate from 4.1 × 10^–5 to 17 × 10^–3 s^–1 (0.05–20 mm/min). It is found that, as the strain rate increases, the ultimate tensile strength decreases and the physical yield strength remains unchanged (≈1400 MPa). As the strain rate increases, the yield plateau remains almost unchanged and the relative elongation decreases continuously. Because of high microplastic deformation, the conventional yield strength is lower than the physical yield strength over the entire strain rate range under study. The influence of the TRIP effect on the changes in the mechanical properties of VNS9-Sh steel at various strain rates is discussed.

A technique is developed to predict the strain and the stress in a fiber under combined relaxation and creep conditions during long-term loading. Stepwise exponential approximations of the characteristic segments in a relaxation curve are suggested for the initial stage of stress relaxation, and they are generalized by an adaptive exponential approximation containing at most two approximation parameters.

The structural evolution in bcc metals (molybdenum, niobium) with a high stacking fault energy (300 and 200 mJ/m^–2, respectively) is studied during high pressure torsion in Bridgman anvils at temperatures of 290 and 80 K. It is established that cryogenic deformation of these metals does not result in twinning; however, banded structures are formed at the initial stage of deformation. Misoriented kink bands, which inhibit the formation of a homogeneous submicrocrystalline structure similarly to twins, form in molybdenum. The banded structures in niobium are characterized by low-angle misorientations; they do not suppress the formation of a submicrocrystalline structure and the refinement of microcrystallites to nanosizes.

A computer model is developed to describe the pressing of a beryllium lens using the Deform software package. This model takes into account the rheological properties of beryllium and the deforming tool. The state of stress of a workpiece is determined at various stages of pressing, and the probability of fracture of nanocrystalline beryllium is estimated using the normalized Cockcroft–Latham criterion. The temperature dependence of the limiting values of the fracture criterion is found and used to choose the pressing conditions that exclude fracture of lenses.

The fracture strength and the sensitivity to delayed fracture of the pipes in oil-trunk pipelines that are made of ferritic–bainitic and bainitic steels are studied. The results of modeling of the delayed brittle fracture of pipe steel during a simultaneous action of mechanical stresses and a corrosive medium are presented.

The structure and mechanical properties of fragments from freight bogie solebars made of 20GL steel are determined in the initial normalized state and after thermal hardening by volume–surface quenching. This quenching is shown to increase the strength of a solebar fragment with a wall thickness of at most 20 mm almost twofold as compared to the normalized state and to increase its bearing capacity by a factor of approximately 1.5.

An approach to estimating the residual stresses in rails using acoustoelasticity and electromagnetic–acoustic conversion is described. The results of ultrasonic control of the residual stresses in rail segments are presented.

To test a mock-up of a hand-held X-ray diffractometer intended for determining the surface stresses in the important parts of rail transport, we propose to use control samples subjected to elastic deformation by three-point bending. The results obtained with hand-held and stationary X-ray diffractometers and the calculated stresses are compared. The stresses to be compared agree well within the limits of an acceptable error.