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We consider fretting wear in elastic frictional contact under influence of oscillations of small amplitude and investigate the question, how wear damage can be influenced by the introduction of material gradients. To achieve a general understanding we restrict our consideration to media with a power-law dependency of the elastic modulus on depth. In this case, a complete analytical solution can be found for the final worn shape. In the limiting case of small fretting oscillations we obtain a simple, closed-form asymptotic solution of the problem. We find that the optimum grading depends on the oscillation amplitude: for large amplitudes, the use of materials with a positive exponent decreases the wear volume whilst for very small amplitudes the use of graded materials with slightly negative exponent is beneficial. Especially interesting is the case of the Gibson-medium which may help avoiding both fretting wear and fretting fatigue.

Problems associated with a qualitative increase in the selectivity of fluid filtration remain the major challenge in a variety of areas such as fluid transport through porous materials and media, ion separation, water desalination and purification, and many others. A promising way to solve these problems is to design and develop membranes with slit-shaped nanopores. In the paper, we studied the selectivity and permeability of slit-shaped nanosized pores in the natural mineral (hydroxyapatite) with the use of the nonequilibrium molecular dynamics approach with all-atom models. We showed that the subnanometer-wide slit-shaped pores in hydroxyapatite are able to demonstrate both good salt rejection and relatively high water permeability. In particular, the numerically predicted water permeability of hydroxyapatite with 0.4 nm thick slit-shaped nanopores reaches about 200 L/(m2 h bar) that is higher than that of commercial membranes and has the same order of magnitude as the theoretically predicted water permeability through single-layer MoS2 nanoporous membranes. An increase in the nanopore thickness is accompanied by a multiple growth in permeability, which is comparable with advanced 2D-CAP (2D-conjugated aromatic polymer) membranes, but in so doing the filtration selectivity is lost. The results show that nanoporous materials with the connected network of slit-shaped nanopores is a promising filter material for water treatment including seawater desalination and other important technical and environmental applications., Проблемы, связанные с недостаточной селективностью при фильтрации жидкостей, в настоящее время являются серьезными вызовами в таких ключевых областях, как транспорт флюидов в пористых средах, разделение ионов, опреснение, очистка воды и многих других. Перспективным способом решения этих проблем является проектирование и разработка мембран с щелевидными нанопорами. Настоящая работа посвящена теоретическому изучению селективности и проницаемости щелевидных наноразмерных пор в природном минерале (гидроксиапатит) с использованием неравновесного молекулярно-динамического подхода и полно-атомных моделей. Показано, что субнанометровые щелевидные поры в гидроксиапатите способны демонстрировать одновременно хорошую задержку ионов соли и сравнительно высокую водопроводимость. В частности, численно прогнозируемая водопроводимость щелевидной нанопоры в гидроксиапатите с раскрытостью 0.4 нм достигает величины порядка 200 л/(м2 ⋅ ч ⋅ бар), что выше чем у коммерческих мембран и сравнимо по порядку величины с теоретически предсказанной проводимостью однослойной нанопористой мембраны дисульфида молибдена MoS2. Увеличение раскрытости нанопоры сопровождается многократным увеличением ее проводимости, которая становится сопоставимой с характеристиками передовых 2D-CAP мембран, однако при этом наблюдается значительная потеря в селективности фильтрации. Полученные результаты свидетельствуют, что нанопористые материалы со связанной системой щелевидных нанопор являются перспективным фильтрующим материалом для очистки воды, включая опреснение морской воды и другие важные технические и экологические применения.

Within a discrete-element model of a porous permeable elastic-plastic rock, filled with a fluid, we have studied the shear strength of a fractured interface zone (a shear band) between blocks of a geological medium subject to compression and shear. Under these conditions, a fluid pore pressure is controlled by interplay of dilation of the elastic-plastic shear band and fluid transport between the blocks and the interface. We have found that the shear strength is a unique function of a combination of parameters, which includes viscosity of a fluid, permeability of the medium, shear rate and a characteristic size of the system. Based on the simulation results we have constructed the generalized binomial dependence of the shear strength of samples on the obtained combination of parameters.

The paper is devoted to the theoretical investigation of the peculiarities of material flow taking place in friction stir welding (FSW). The investigation was based on 3D computer simulation by the movable cellular automaton (MCA) method, which is a representative of the particle methods in mechanics of materials. Usually, material flow in FSW is simulated based on computational fluid mechanics, which assumes that the material is a continuum and does not take into account the material structure. MCA considers a material as an ensemble of bonded particles. Breaking of inter-particle bonds and formation of new bonds enables simulation of crack nucleation and healing, as well as mas mixing and microwelding. The simulation results showed that using pins of simple shape (cylinder, cone, pyramid) without shoulder results in small scattered displacements of the plasticised material in the workpiece thickness direction. Nevertheless, the optimal ratio of the longitudinal velocity to the rotational speed allows transporting of the welded material around the pin several times and producing the joint of good quality. Applying additional ultrasonic vibration to the pin may lead to better mixing of the plasticized material behind the pin.

Movable cellular automata (MCA) is an efficient numerical method in particle mechanics, which assumes that any material is composed of elementary objects interacting among each other according to many-particle forces. In this paper MCA method is applied to modeling deformation of 3D coating-substrate system under its contact loading by rigid indenter. Main attention of the research is focused on the role of vortex-like structures in the velocity fields in deformation of the strengthening coating and substrate. The mechanical properties of model coating correspond to multifunctional bioactive nanostructured film (TiCCaPON) and the properties of substrate, to nanostructured titanium. Loading is performed by hard conical indenter. The peculiarities of velocity vortex formation and propagation, as well as its interaction with structural elements are studied. One of possible application of the study is non-destructive technique for detecting nanoscale defects in surface layer of a material using frequency analysis of the force resisting to sliding of a small counter-body on the material surface, known as tribospectroscopy. Possibilities of this technique are studied based on 3D modeling by MCA method for the above mentioned coating with nano-pores. It is shown that specific peaks at the friction force spectrum correspond to different geometrical characteristics of the nano-pores.

We propose a numerical model of liquid-saturated porous material, based on a coupled approach combining a discrete elements method and finite difference method. An ensemble of discrete elements simulates processes of deformation of a porous solid and filtration of single-phase liquid in an interconnected network of "micropores". Mass transfer of a fluid between the "micropores" and "macropores" (the latter are considered as the areas between spatially separated and non-interacting discrete elements) is calculated on a finer grid superimposed on an ensemble of movable discrete elements. The developed model was applied to study a mechanical response of brittle samples with water-saturated pore volume. It has been shown that the strength of liquid-saturated samples is determined not only by strength properties of "dry" material and a pore pressure, but largely by sample geometry, deformation rate and characteristics of porosity of a material. We suggest a generalizing dependence of the uniaxial compressive strength of water-saturated permeable brittle material on the specific diameter of filtration channels, which is the ratio of the characteristic diameter of the filtration channels to the square root of the strain rate. Values of parameters of mentioned dependence are strongly connected with the character of the relation between pore volume and pressure of a liquid.

The paper is devoted to molecular dynamics simulation of the formation of bicomponent nanoparticles in simultaneous electric explosion of copper and nickel conductors. The influence of the distance between conductors on failure dynamics and nanoparticle formation is studied. It is shown that simultaneous electric explosion of metallic conductors makes possible the formation of bicomponent particles. Varying the distance between conductors allows one to control the structure of formed bicomponent particles. An increase in the viscosity of the environment in which conductors are exploded increases the size of formed particles.

In this paper, we carried out investigation of behavior of the material under loading condition identical those used in FSW using molecular dynamic method. The loading was modelled by a rigid rotating “tool” that movies along boundary between two grains. We considered pairing of two crystallites of copper, crystallites of copper and iron, and two crystallites of aluminum 2024. Analysis of the structure of the sample showed the intermixing and stirring of dissimilar atoms as a result the FSW tool pass at the inter-crystallite boundary. It was shown, that under certain condition of loading when tool passes there a region where atoms can occupying the original position of the crystal lattice. We also show influence of an additional oscillating impact applied to the moving tool on the structure of the resulting weld. The simulation results obtained can be used for understanding the processes realized under mechanically activated diffusion.

The computer simulation results on the atomic structure of the copper crystallite and its behavior in nanoindentation demonstrate the key role of local structural transformations in nucleation of plasticity. The generation of local structural transformations can be considered as an elementary event during the formation of higher scale defects, including partial dislocations and stacking faults. The cause for local structural transformations, both direct fcc-hcp and reverse hcp-fcc, is an abrupt local increase in atomic volume. A characteristic feature is that the values of local volume jumps in direct and reverse structural transformations are comparable with that in melting and lie in the range 5-7 %.

The paper is devoted to the numerical study of some fundamental aspects of longitudinal shear crack propagation in sub-Rayleigh and supershear regimes. The simulation was carried out with use of particle-based movable cellular automaton method. It is shown that a well-known phenomenon of shear crack acceleration towards longitudinal wave speed is explained by the formation and development of elastic vortex traveling ahead of the crack tip at a shear wave velocity. The stress concentration area ahead of the crack tip revealed by different authors is connected with the elastic vortex. Shear crack accelerates towards the longitudinal wave speed through the formation of a daughter crack by the mechanism of shearing (the daughter crack is formed in the center of elastic vortex). Analysis of sub-Raleigh-to-intersonic transition showed that the condition of this transition can be expressed in terms of dimensionless geometrical crack parameter.

The paper is devoted to development of a new approach to study of friction stir welding (FSW) process on the mesoscopic scale. This approach is based on computer-aided simulation by movable cellular automaton (MCA) method. In the framework of developed formalism of MCA method the dynamics of the friction stir welding process of duralumin plates was investigated. It was shown that ratio of rotation velocity to velocity of translation motion of rotating tool greatly influences the quality of welded joint. Optimal choice of the ratio of these parameters could significantly decrease volume content of pores and microcracks in the welded joint.

The method of symbiotic cellular automaton, which combines two approaches to describe the simulated media – the method of conventional cellular automaton and method of movable cellular automaton, has been developed for modeling and study of heterogeneous materials. Verification of the method has been carried out by comparing the simulation results of sorption of carbon dioxide in the lignite with experimental data. Possibilities of the method are exemplified by results of numerical study of the influence of the gas phase on strength and fracture of the lignite specimens.