A. I. Dmitriev

Movable cellular automata method for simulating materials with mesostructure

Mathematical formalism and applications of the movable cellular automata (MCA) method are presented for solving problems of physical mesomechanics. Since the MCA is a discrete approach, it has advantages over that of the finite element method (FEM). Simulation results agree closely with the experimental data. The MCA approach cab solves mechanical engineering problems ranging from those in material science to those in structures and constructions. Computer simulation using MCA can also provide useful information in situations where direct measurements are not possible.

The features of fracture of heterogeneous materials and frame structures. Potentialities of MCA design

A new promising numerical method named movable cellular automata (MCA) is described. Because this approach is based on the discrete concept, in contradistinction to FEM-based software, the software based on the MCA concept has a few clear advantages. The main one is connected with modeling of real fracture process. The MCA method has been successfully used for modeling dynamic loading of heterogeneous materials and structures. The results of simulations agree closely with the experimental data. The results show that the MCA approach could be really useful to solve a lot of civil engineering problems from materials to constructions. Special software has been developed on the basis of this method. Due to its potentially unique abilities, the MCA method could be considered as a breakthrough in numerical techniques and a new tool of engineering mechanics.

Computer-aided examination and forecast of strength properties of heterogeneous coal-beds

One of the modern and perspective applications of computational mechanics is investigation of complex multiphase media containing components in different aggregative states. Striking examples of multiphase media are coal-beds, soils, geological medium and so on. In the present paper the new method for simulation of response and fracture of multiphase media, the hybrid cellular automaton method, is proposed. Developed approach was applied for computer-aided simulation of response and fracture of lignite under complex boundary conditions imitating real-life environment in coal beds. Simulation results show that presence of constrained boundary conditions can lead to change of lignite fracture regime in the range from brittle to quasi-viscous. In the case of hard boundary conditions increase of surrounding material hardness leads to transition from brittle fracture of fined detritus to degradation regime, which is characterized by generation and accumulation of numerous damages. As a result at slump of lateral pressure (in practice this situation is realized during mining or digging) fine detritus can demonstrate explosion-like fracture. In the case of boundary conditions realized by means of applied force (this type of boundary conditions is the most close to real conditions in coal beds) the vice versa dependency for fined detritus is observed. Here explosion-like regime is observed at lateral pressure slump in the case of soft surrounding material. Proposed hybrid cellular automata concept can be efficiently used for solving various geo-mechanical, biological, engineering and materials science tasks, which consider heterogeneous multiphase objects.

Molecular-Dynamics Study of Nanofragmentation during Relaxation in After-Loaded Solids

The possibility of nanofragmentation development during the initial relaxation stage in the subsurface layer of a solid after preliminary loading is studied by computer simulation using the molecular dynamics method. It is established that disoriented nanoblocks can form in the initial stage of relaxation. This fragmentation arises in the region of strain localization in the vicinity of stress concentrators and then spreads in depth of the material. In the region of strain localization, the radial distribution function (RDF) of atomic density exhibits smeared maxima corresponding to peaks in the RDF of the ideal fcc crystal structure. In the crystal region free of strain localization, the RDF peaks exhibit splitting caused by the strain-induced symmetry breakage. The obtained results suggest that fragmentation is the possible mechanism of relaxation of internal stresses in loaded solids.

Molecular-dynamics study of the initial stage of nanoscale deformation localization in the surface layers of a loaded solid

The nucleation and development of plastic deformation in the surface layers of a solid under dynamical loading conditions have been studied on a nanoscale level by computer simulation using molecular dynamics method. It is established that the onset of deformation localization is directly related to the loss of structural stability in the surface layers of a loaded solid, the appearance of surface local straining zones, and their spreading in the bulk of the material. This is preceded by uncorrelated atomic displacements in the near-surface region of the solid. The obtained results agree with the well-known experimental data and confirm the special role of the surface layers of solids in the onset and development of plastic deformation.

A possible method of computer-aided design of materials with a highly porous matrix structure based on the method of moving cellular automata

The method of moving cellular automata was used to model a sample of ZrO2 ceramic with a matrix structure under conditions of uniaxial compression. Studies were made of the strength and type of damage to the initial structure and its modifications. Particular attention was devoted to studying the influence of characteristics of the matrix structure on the formation and suppression of internal stress macroconcentrators. It is shown that the method of moving cellular automata can be used for the computer-aided design of matrix materials with a complex structure by specifically influencing the formation and evolution of stress macroconcentrators.

Size effect on the kinematic parameters of nanometer-thick bilayer films

The behavior of nanodimensional bilayer structures (plates) of finite length consisting of nanometer-thick crystalline Ni and Cu films has been studied by means of molecular dynamics simulation. The inter-atomic interactions were described within the framework of the embedded atom method. It is shown that, in the absence of an external action, the nanostructures perform mechanical oscillations with the amplitude and frequency determined by the length and thickness of the plate. The dependence of the parameters of oscillations of the nanodimensional structures on their dimensions is established. The results can be used in designing components of nanodevices for various applications.

The role of excess volume in the stage of plastic deformation initiation in near-surface regions of a loaded crystal

The initial stage of the process of atomic displacement localization in the near-surface region of a loaded crystal is studied by means of analysis of the excess volume redistribution based on the results of molecular dynamics simulation. It is established that the excess volume is initially concentrated in the regions where structural changes are observed in the subsequent stages. In these regions, the excess volume can reach 5% of the specific volume of atoms occurring outside of the displacement localization zone. The obtained results lead to reassessment of the role of excess volume in the process of plastic deformation initiation and development on the atomic level.

Molecular-dynamics study of the features of dynamic vortex structure formation in a material with micropores under high-rate deformation conditions

The response of a material containing a system of micropores subjected to high-rate shear deformation has been computer-simulated by methods of molecular dynamics. Deformation of such materials is accompanied by the formation of vortexlike dynamic defects. This process can be divided into three stages. The first stage is characterized by a predominating laminar character of atomic displacements in the regions adjacent to the loaded layers. A special feature of the second stage is the development of a correlated vortexlike motion of atoms in the regions between micropores, with periodic formation and breakage of vortices in a period on the order of several picoseconds. The third stage is related to the loss stability of the atomic structure and the formation of deformation localization bands. This is accompanied by the loss of correlation of the vortexlike motion of atoms in the regions between micropores. The results can be used in analyzing the behavior of materials under conditions involving irradiation.

Molecular-Dynamics Study of a Possible Structural Instability during Relaxation of a Deformed Crystal

The development of elastic and plastic deformation in a crystal immediately upon termination of the stage of active loading has been computer-simulated by methods of molecular dynamics. Depending on the level of deformation reached at the active loading stage, the subsequent relaxation process may follow different scenarios. In particular, an interval of deformations is found for which the crystal lattice occurs in a state of unstable equilibrium, whereby small variations in the degree of compression may lead to significant changes in the character of formation of the bands of localization of atomic displacements. The obtained results may be important for an analysis of the influence of the inertial character of structural changes on the development of plastic deformation in crystals.