Vol 22, No 3 (2018)

The article presents models of superelastic hardening of materials with unstable phase structure at a constant temperature. The kinetic equation of the process of formation and growth of spherical nuclei of a new phase is formulated depending on the level of development of inelastic structural deformations, according to which the new phase first represents separate inclusions from embryos, developing it forms the structures of the matrix mixture in the form of interpenetrating skeletons, and finally the new phase is transformed in a matrix with separate inclusions from the material of the remains of the old phase. The influence of structural deformations on the features of phase transformations and nonlinear hardening of inhomogeneous unstable materials with different degree of connectivity of the constituent phases is studied. Various variants of the microstructure material formed in the conditions of the phase transition in the form of separate inclusions and in the form of interpenetrating components are considered. New macroscopic determining relationships for unstable microinhomogeneous materials are established and their effective elastic moduli are calculated. Macroscopic conditions of direct and inverse phase transitions are obtained, their effective limits and hardening coefficients are calculated. It is shown that the values of the macroscopic elasticity moduli of the obtained models lie inside the fork of the lower and upper Hashin-Shtrikman boundaries. Numerical analysis of the developed models has shown good agreement with known experimental data.

Methods of theoretical analysis and experimental verification of conditions of limit deformation are considered for a reasonable choice of the constitutive equations for mathematical modeling of processes of hot and warm treatment by pressure of sheet metal products with a deep drawing. Attention is focused on the forming limit curve of sheet metal on the plane of the principal strains (one of that corresponds to stretching, and the second can specify stretching or compression), the characteristic of the local state of the material corresponding to the critical growth of strain localization. Localization here is understood as a local thinning of the sheet and corresponds to diffuse form of localization. Other defects (shear bands, crack formation) develop from this limiting state or (formation of folds and wrinkles) are not local and require complete formulation of the problem. The forming limit curve (FLC) defines the conditions of realization of a technological process and can be theoretically predicted depending on the constitutive equations of plasticity, indicator of critical state and initial imperfections. The Marciniak-Kuczyński scheme is considered for getting FLC, where the sample has two zones of homogeneous strains and allows analytical reduction of the problem to the system of several ordinary differential equations solved numerically. The experimental methods assume testing by pressing a punch with a spherical or cylindrical tip into a specimen cut from a sheet. Depending on the depth of the lateral cutouts from the specimen, it can be provided tension or compression of the specimen in the transverse direction in these tests. Both approaches are analyzed as tools for selection and experimental verification of the constitutive model and the limit state indicator. They solve methodological problem of identification of mathematical models on a quite non-standard experiments involving strain localization. With the use of Marciniak-Kuczyński scheme the effect of a number of yield criteria for anisotropic sheet metal, hardening laws, damage accumulation models and criteria of viscous failure on qualitative and quantitative features of the FLC. To do this a proprietary algorithm has been developed. Experimental standard test methods of Hasek, Marciniak and Nakajima were implemented numerically in the software package LS-DYNA. The numerical FLD obtained were compared with theoretical and experimental ones. Possibilities of integration into Marciniak-Kuczyński scheme the dependence on temperature, strain rate and microstructure parameters for each basic rigid-plastic (scleronomous) model were discussed. It is noted this scheme is significantly limited by proportional changes of the main deformations in the sample outside and inside the strain localization zone. It is revealed this scheme is not adapted for determination of limit properties of the metals deformable in the conditions of deformation softening (aluminum, titanium alloys and some steels at temperatures of dynamic recrystallization). For a wider range of material deformation conditions, there is no alternative to the above-mentioned numerical method for predicting FLC. An open and relevant question is the description of the evolution of anisotropic plastic and fracture properties due to the anisotropic damage accumulation.

Linear model of micropolar elastic continuum (known also as the Cosserat continuum) is considered. Kinematics and strain measures are discussed. The symmetric small strains tensor, relative microrotation vector and spatial gradient of the total microrotation vector (the wryness tensor) are then employed for a covariant formulation of the micropolar theory. By means of the principle of virtual displacements much simplified by the lack of internal forces and couples contributions to the virtual work and the Lagrange multipliers method the micropolar theory of elasticity is developed. Hemitropic micropolar continuum model is investigated in further details. The paper is to be considered as a universal covariant script of equations of the linear micropolar theory of elasticity derived from the virtual displacements principle.

In this paper, we study the steady creeping convective flow of a viscous incompressible fluid in the thin infinite layer. The study of the fluid flow is based on the exact solutions class for the Oberbeck-Boussinesq equations in the Stokes approximation using. The velocity field is described by the Hiemenz exact solution. The temperature field and the pressure field linearly depend on the horizontal (longitudinal) coordinate, it corresponds to the Ostroumov-Birich exact solutions class. The convective motion of a viscous incompressible fluid was induced by tangential stresses on the upper permeable (porous) boundary and thermal source definition at the lower boundary. In addition, the heat exchange according to the Newton-Richmann law takes into account at the upper boundary. The obtained exact solutions describe counterflows in fluids. The stagnant points number in the fluid layer does not exceed three. The formation of counterflows in the fluid is accompanied by sucking and injection of the fluid through the permeable boundary. The larger number of stagnant points presence forms a cellular structure of the streamlines. In addition, the velocity field, which obtained in the solution of the boundary value problem is characterized by localization of the flow near the boundary of the fluid layer (boundary layer). The exact solutions obtained in this paper can be used for the nonlinear Oberbeck-Boussinesq system solving. The Grashof number can take large values, which depends on the geometric anisotropy index for the linearized Oberbeck-Boussinesq system.

This paper is concerned with the dynamic behavior of an elastic cylindrical pipe with surface defects interacting with the internal flow of a compressible fluid. A defect in the form of a ring of rectangular cross-section is located on the inner or outer surface of an elastic body and characterized by its own set of physico-mechanical parameters. The behavior of an ideal compressible fluid is described using the potential theory, and the behavior of the pipe is considered in the framework of the linear theory of elasticity. The hydrodynamic pressure exerted by the fluid on the inner surface of the pipe (defect) is determined with the use of the Bernoulli equation. A mathematical formulation of the problem of the elastic body dynamics is based on the variational principle of virtual displacements, and the system of equations for a liquid medium is developed using the Bubnov-Galerkin method. For the numerical implementation of the algorithm, a semi-analytic version of the finite element method is used. The stability of the system is estimated based on the results of computation and analysis of complex eigenvalues for a coupled system of equations. Verification of the model is carried out for the case of an ideal pipe by comparing the obtained results with the known experimental and numerical data. The effect of the geometric and physicomechanical parameters of the defect on the critical fluid velocity responsible for the loss of stability is studied for a cylindrical pipe clamped at both ends. It is shown that defects reduce the boundary of hydroelastic stability. It has been found that the defect located on the outer surface of the pipe exerts a greater impact on the system stability than it does when located on the wetted surface of the pipe.

Using analytical solutions to analyze the state of the bodies at the research and engineering calculations provides computing resources. We propose a methodology for structuring full parametric solutions to the problems of mathematical physics, including the basic mixed problem of elastostatic. The tool is a relatively new energy method of boundary states based on computer algebra. The method is based on the concept of state of the medium, isomorphism of Hilbert spaces of internal and boundary states of the body. The method is self-sufficient in the sense that, in principle, does not require comparison of the solution of test problems with those constructed by other methods. For inclusion in the solution in an explicit form of the medium constants we recommend saving computing resources method of boundary states with perturbations in which the direct method is combined with approach to A. Poincare. To explicitly include in the decision parameters the boundary conditions we suggested the technology of the reference solutions. Its effectiveness is demonstrated on a concrete example the basic mixed problem of elastostatic. The object of research is a limited simply connected body whose boundary is divided into three sections. At each site held individual method of parameterization of the points of the border: polar, cylindrical, spherical coordinate systems. The calculations are made using the computer algebra of the system “Mathematica” and demonstrated the effectiveness of the developed methodology to achieve this goal. The sequence of steps leading to guaranteed achievement of goal is described. The decision of a concrete task is made. Its results are presented in explicit analytical form containing all the parameters of the boundary value problem of elasticity theory and illustrated graphically after calculation by the analytic solution for a concrete set of parameter values.