Mechanics of Solids最新文献

The properties of metamaterials with negative Poisson’s ratio (with an auxetic structure based on a cell in the form of a concave hexagon) to resist penetration by a rigid spherical striker along the normal have been experimentally studied. Using a 3D printer, samples of the same mass with a chiral and non-chiral structure are made from flexible thermoplastic polyurethane (TPU 95A plastic) and rigid e-PLA plastic, which are compared by the ability to reduce the kinetic energy of strikers at a speed of about 190 m/s. It is found that the chirality of the sample structure (for both TPU and PLA plastics) leads to an increase in their protective properties. However, when the sample structure is rotated by 90°, the samples without chirality show the best resistance to penetration. Based on the results of a series of experiments with TPU and PLA samples, the most effective in terms of resistance to penetration by a striker are auxetics made of thermoplastic polyurethane, with a non-chiral structure turned by 90°.

The article considers the bending of a long concrete pipeline when its section is raised to the free surface of a reservoir. The initial horizontal position of the pipeline is rectilinear. Its static bending occurs under the action of concentrated forces, the weights of the pipe and the transported medium, and the lifting force of water. The minimum required value of the lifting force and the corresponding length of the raised section of a long pipeline are determined. Taking into account the large ratio of this length to the depth of the reservoir, a linear bending equation is used. An analysis of the bend is given depending on the controlled lifting force and the controlled rise of the pipeline.

A complex model of pore annihilation during hot isostatic pressing (HIP), which takes into account the simultaneous action of the mechanisms of material plastic flow and diffusive pore dissolution due to the emission of vacancies by the pore surface, has been proposed. The obtained mathematical equations are applied to analyze the kinetics of pore annihilation in single crystals of the nickel-based superalloy CMSX-4 during HIP used for this alloy in industry. It follows from the analysis that both mechanisms (plastic flow and vacancy diffusion) make comparable contributions to the reduction of pore volume under these conditions. As the HIP pressure increases, the contribution of plastic flow increases, while the contribution of vacancy diffusion decreases. Large pores shrink in volume mainly due to the mechanism of plastic flow, however, at the final stage of pore closure, the mechanism of vacancy diffusion is more active. To ensure reliable pore healing by the vacancy mechanism, HIP should be carried out at a moderate argon pressure in the HIP plant.

This article proposes and examines a method of forming heat-resistant silicon carbide coatings on graphite products. This coating forms through the simultaneous occurrence of several chemical reactions between silicon melt, carbon monoxide, and the near-surface region of graphite, at temperatures slightly above the melting point of silicon. The resulting coating is several millimeters thick and has high mechanical strength and hardness. Various methods were used to examine the samples, including Raman spectroscopy and SEM. The thermal stability of the coatings was studied by testing them in high-enthalpy subsonic air flows. The results show that the coatings can withstand exposure at temperatures up to 1750°C for 30 minutes. The self-healing mechanisms of the coating under the influence of oxygen at high temperatures were revealed.

In this paper, we study the ultimate deformed state of an isotropic rod under the influence of external pressure and varying linearly along its generatrix. It is assumed that the rod rotates around its axis. The general equations describing the limiting state of rods under the influence of external pressure in a cylindrical coordinate system are considered. Integrals of relations describing the ultimate deformed state of the rod are obtained. The characteristics of the studied ratios and the envelope of the family of characteristics are found. The deformed state of a cylindrical isotropic rod with a circular cross-section under external pressure is determined.

In this article, the dynamics of precession of the Thompson top has been studied. Test benches have been developed and experiments have been carried out. Based on the data obtained, the dependences of the angular velocity of precession at high initial values of up to 340 rad/s are shown. Analytical studies have been carried out and the dependences of energy changes during precession have been obtained. The relation between the roughness coefficient and the resulting friction moment is obtained. The relations between the work of friction and the roughness coefficient, that gives an understanding of the dynamics of energy indicators during the entire process, are obtained. The question of the use of a viscous friction model remains open and as does a more detailed study of the proportionality coefficient to establish refined dependencies for precession. The experimental data obtained show the influence of the roughness coefficient on precession and on the occurrence of the friction force, leading to the capsizing of the top. Further research in this field should show the applicability of the viscous model and the study of critical points, namely the rise to the leg and the passage of the highest value of the moment of inertia.

The dynamics of an isotropic thermoelastic medium during the formation of cracks with an arbitrary surface geometry and non-opening edges is considered. The shock thermoelastic waves arise in the medium during such a process. The energy conservation law for a thermoelastic medium is considered considering shock waves. For shock thermoelastic waves, using the method of generalized functions, conditions are obtained for jumps in stresses, velocities, heat fluxes and energy density on their fronts. The crack model determines the relationship between jumps in stresses and velocities of relative displacement of the crack edges. The problem is posed and solved in the space of generalized vector functions. The solution is presented as a tensor-functional convolution of the Green’s tensor of the equations of coupled thermoelasticity with a singular mass forces containing simple and double layers whose densities are determined by the jump in velocities, stresses, temperatures and heat fluxes on the crack edges. The latter determine the crack model and are assumed to be known.

The Ivlev–Sporykhin continuum model, which is a model of a hardening elastoviscoplastic solid, is considered in this study. The model takes into account both reversible and irreversible deformations to investigate evolutionary processes occurring in a hollow sphere under the influence of a time-dependent temperature field. During the solution of this problem, an analytical expression for the temperature distribution within the body was derived. A generalized tree of evolution of regions of elasticity, plastic flow, unloading, and re-plasticity was also constructed. Expressions for the radial components of stress and displacement in these regions were also developed. Four rheological models were compared, taking into account the various properties of the medium.

The paper studies the stress state of a heavy anisotropic space weakened by a horizontal cavity. The problem is solved using the small parameter method. In the initial zero approximation, the material is considered to be ideally plastic, not possessing anisotropy properties. Such problem parameters as hole ellipticity and mass forces, and material anisotropy are taken into account in the first approximation.

During the study, analytical expressions were obtained for the stress components in the elastic and plastic regions, and the boundary of the elastic-plastic zone was also determined. The influence of anisotropy and material mass forces on the stress state of the medium is shown.

The obtained solutions can be used to assess the stability of underground structures, calculate the stress state of rock massifs and other problems of continuum mechanics.

The work presents the experimental study results of the titanium alloy Ti–6Al–4V fatigue life obtained during additive manufacturing by wire-arc surfacing using the cold metal transfer welding. This additive manufacturing technology is used for fusing large-sized products in the Laboratory of methods for creating and designing systems “material-technology-construction” PNRPU. The quality of the resulting blank is confirmed by the results of chemical analysis, microstructural research and static tensile tests. Samples were cut from the deposited plate in the longitudinal and transverse direction with respect to the formation plane of the layers. Experimental studies of fatigue life were conducted in the Center of Experimental Mechanics PNRPU using Instron testing equipment. According to the test results, the dependences of cyclic durability on the level of applied stresses are obtained. It is noted that the direction of cutting samples from the deposited fragment significantly affect to the resistance characteristics of the low- and high-cycle fatigue of the additive titanium alloy VT6. It is concluded that there is a significant anisotropy of cyclic properties, which must be taken into account when designing and manufacturing products from additive materials.