G. V. Stepanov

Bio: G. V. Stepanov is an academic researcher from National Academy of Sciences of Ukraine. The author has contributed to research in topics: Stress relaxation & Strain rate. The author has an hindex of 8, co-authored 9 publications receiving 143 citations.

High-Density Pulse Current-Induced Unsteady Stress-Strain State in a Long Rod

Abstract: The results of experimental investigations and numerical calculations of the unsteady stress-strain state in a thin steel rod (wire length) prestressed statically below the yield strength upon the passage of a high-density current pulse are presented. The current pulse gives rise to oscillations of axial stresses with a period corresponding to the period of natural longitudinal oscillations of the rod. Maximum compression and tensile stresses are determined by current pulse front duration and its amplitude. A high level of the above stresses in the central portion of the rod brings about local losses of longitudinal stability. The results of numerical simulation of the stress-strain state in the rod upon its heating with a current pulse correspond to experimentally observed effects. A current pulse-induced decrease in the level of initial tensile stresses (at constant strains) produced by static loading points to the development of relaxation processes that are associated not only with heating. To separately evaluate the effects stemming from elastic stress waves in the rod, heating, and immediate current action, further studies would be required.

Analysis of pulse current-induced tensile stress relaxation

Abstract: A procedure is offered and results of experimental evaluation of high-density pulse current effects on electrical resistance and relaxation of tensile elastic stresses are presented for a number of metallic materials. Based on analysis of experimental data, plastic strain rates are shown to be influenced by tensile stresses, current density, and temperature.

Experimental evaluation of pulse electric current effect on residual stresses in composite-to-copper joints

Abstract: Experimental method for evaluation of residual stress distribution in carbon fiber reinforced carbon composite jointed to copper is presented. Pulse electric current passed through composite-to-copper joint essentially reduces the level of residual stresses.

Stress-Strain State Evaluation of a Welded Joint of Hot Collector to Nozzle of NPP Steam Generator PGV-1000

Abstract: The paper describes a method and main results of a refined stress-strain state evaluation for a welded joint of the hot collector to steam generator nozzle of a nuclear power plant with a WWER-1000 reactor, which takes into account a fuller range of the actual loading conditions in operation. In calculations, we considered the stresses induced by uncompensated thermal expansion of the reactor elements. The results of 3D finite element calculations suggest that the level of local stresses in the area of the joint exceeds the metal yield stress. The calculated stresses for the outer nozzle surface agree satisfactorily with the in-situ strain measurement data.

I and i

Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

The spall strength of condensed matter

Abstract: Two conditions are proposed which place constraints on the processes of dynamic spall in condensed media, and determine inequalities which bound the spall strength, fragment size, and failure time. Spall is defined as rupture within a body due to stress states in excess of the tensile strength of the material. The first is a horizon condition which establishes a domain of communication, consistent with the time to failure, within which spall must be independent of the surrounding environment. The second is an energy condition which requires that the potential and kinetic energy associated with the tensile loading process exceed the fracture energy of the material. Equality in the relations established from these conditions corresponds to energy-limited spall and provides specific analytic expressions for the spall properties. Inequality implies flaw-limited spall and requires more detailed material property information before spall can be characterized. Energy-limited spall is determined by the material fracture toughness in brittle solids and the material flow stress in ductile solids. Calculated spall properties, assuming energy-limited spall, compare well with experimental spall data for various materials. Under certain conditions, a transition from brittle to ductile spall (definition in text) with increasing strain rate is predicted. Comparison is made with spall data on 6061-T6 aluminum for which a brittle-to-ductile transition is predicted to occur at a critical strain rate of approximately 4 × 105 s−1. Energy-limited spall in liquids within their range of Newtonian fluid behavior is governed by surface energy and viscosity. Spall is predicted to be dominated by surface energy at low strain rates and viscous dissipation at high rates. Examples of each appear to exist within the scant experimental spall data available for liquids.

High strain rate properties of metals and alloys

Abstract: The high strain rate dependence of the flow stress of metals and alloys is described from a dislocation mechanics viewpoint over a range beginning from conventional tension/compression testing through split Hopkinson pressure bar (SHPB) measurements to Charpy pendulum and Taylor solid cylinder impact tests and shock loading or isentropic compression experiment (ICE) results. Single crystal and polycrystal measurements are referenced in relation to influences of the crystal lattice structures and nanopolycrystal material behaviours. For body centred cubic (bcc) metals, the strain rate sensitivity (SRS) is in the yield stress dependence as compared with the face centred cubic (fcc) case of being in the strain hardening property. An important consequence is that an opposite ductility influence occurs for the tensile maximum load point strain that decreases with strain rate for the bcc case and increases with strain rate for the fcc case. Different hexagonal close packed (hcp) metals are shown to foll...

Hydrodynamic simulations of metal ablation by femtosecond laser irradiation

Abstract: Ablation of Cu and Al targets has been performed with 170 fs laser pulses in the intensity range of ${10}^{12}\text{--}{10}^{14}\phantom{\rule{0.3em}{0ex}}\mathrm{W}\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}2}$. We compare the measured removal depth with 1D hydrodynamic simulations. The electron-ion temperature decoupling is taken into account using the standard two-temperature model. The influence of the early heat transfer by electronic thermal conduction on hydrodynamic material expansion and mechanical behavior is investigated. A good agreement between experimental and numerical matter ablation rates shows the importance of including solid-to-vapor evolution of the metal in the current modeling of the laser matter interaction.