Vol 23, No 2 (2019)

On a 4-manifold of conformal torsion-free connection with zero signature (--++) we found conditions under which the conformal curvature matrix is dual (self-dual or anti-self-dual). These conditions are 5 partial differential equations of the 2nd order on 10 coefficients of the angular metric and 4 partial differential equations of the 1st order, containing also 3 coefficients of external 2-form of charge. (External 2-form of charge is one of the components of the conformal curvature matrix.) Duality equations for a metric of a diagonal type are composed. They form a system of five second-order differential equations on three unknown functions of all four variables. We found several series of solutions for this system. In particular, we obtained all solutions for a logarithmically polynomial diagonal metric, that is, for a metric whose coefficients are exponents of polynomials of four variables.

In this paper, new representations of three-dimensional asymmetric stress tensor and the corresponding form of the differential equilibrium equations are given. Asymmetric theories of solid mechanics continues to attract attention in connection with the necessity of mathematical modelling of the mechanical behaviour of the advanced materials. The study is restricted to such asymmetric second rank tensors, for which it is still possible to keep the notion of real eigenvalues, but not to accept the mutual orthogonality of the directors of the principal trihedron. The exact algebraic formulation of these asymmetry conditions is discussed. The study extends the dyadic tensor representations of the symmetric stress tensor based on the notion of asymptotic directions. The obtained results are a clear evidence in favor of algebraic hyperbolicity both the symmetric and asymmetric second rank tensors in three-dimensional space.

The purpose of the research is to investigate the processes of accumulation of irreversible deformations through different mechanisms: creep and plastic flow. Initially irreversible deformations are produced due to the viscous properties of solid material as creep deformations. The mechanism of the production of irreversible deformations changes to plastic when stress state reach the yield surface. On the contrary, this mechanism changes from fast plastic to slow viscous during unloading. The continuity in such increase in irreversible deformations is provided by the corresponding set of creep and plasticity potentials. These processes are considered in the framework of the mathematical theory of small deformations on the example of a one-dimensional problem of the deforming of a viscoelastoplastic hollow sphere under the influence of volumetric pressure changing with time. The processes of creep and plastic flow under increasing pressure, plastic flow at constant pressure, the medium unloading at decreasing pressure and the repeated plastic flow at the unloading were considered. The regularities of the motion of elastic-plastic boundaries in the material of the hollow sphere were established. The parameters of the stress-strain state of the medium were calculated, stress relaxation after the unloading was investigated.

The Boltzmann-Volterra linear constitutive equation for isotropic non-aging visco-elastic materials is studied analytically in order to examine its capabilities to provide an adequate qualitative description of rheological phenomena related to creep under uni-axial loading combined with constant hydrostatic pressure and of evolution types of the Poisson's ratio (lateral contraction ratio) in creep. The constitutive equation does not involve the third invariants of stress and strain tensors and implies that their hydrostatic and deviatoric parts do not depend on each other. It is governed by two material functions of a positive real argument (that is shear and bulk creep compliances); they are implied to be positive, differentiable, increasing and convex up functions. General properties and characteristic features of the creep curves for volumetric, longitudinal and lateral strain produced by the linear theory (with an arbitrary shear and bulk creep functions) under constant tensile load and constant hydrostatic pressure are investigated. Conditions for creep curves monotonicity and for existence of extrema and sign changes of strains are studied. The Poisson's ratio evolution in time and its dependences on pressure and tensile stress levels and on qualitative characteristics of two creep functions are analyzed. Taking into account compressibility, volumetric creep and pressure influence (governed by the bulk creep function) affects strongly the qualitative behavior of longitudinal creep curves and the Poisson's ratio evolution and its range. In particular, it is proved that the linear theory can simulate non-monotone behavior and sign changes of lateral strain and Poisson’s ratio under constant tensile load (even if the pressure is zero) and the longitudinal strain may start to decrease provided the pressure level is high enough. The expressions for Poisson’s ratio via the strain triaxiality ratio (which is equal to volumetric strain divided by deviatoric strain) and in terms of pressure ratio to axial stress and the creep functions ratio are derived. Assuming creep functions are arbitrary (permissible), general accurate two-sided bounds for the Poisson's ratio range are obtained and the influence of pressure level on the range is studied. Additional restrictions on material functions and loading parameters are derived to provide negative values of Poisson’s ratio. Criteria for the Poisson’s ratio increase or decrease and for its non-dependence on time are found. The analysis revealed the set of immanent features and quantitative characteristics of the theoretic creep curves families and the Poisson's ratio dependence on time and pressure to axial stress ratio which are convenient to check in creep tests (with various levels of pressure and tensile stress) and can be employed as indicators of the linear viscoelasticity theory applicability (or non-applicability) for simulation of a material behavior. The specific properties and restrictions of the model with constant bulk creep compliance which simulates a material exhibiting purely elastic volumetric deformation are considered.

The article discusses the computational procedure of the alternance optimization method as applied to the problem of semi-infinite programming. These problems are reduced numerous applied problems of optimization of objects with distributed and lumped parameters: robust parametric optimization of dynamic systems, parametric synthesis of control systems, etc. Since the calculation of alternance optimization method is rather difficult, as reduced to the solution, as a rule, the transcendental system of constitutive equations is proposed for efficient computational complexity of an embodiment of a computational procedure. To reduce the complexity of the computational procedure, the properties of the extremum points of the optimality criterion established in the alternance method are used in the region of permissible values of variables. These properties allow creating a topology of this area and thereby minimizing the number of references to it during the search procedure. The proposed computational method is especially effective for non-convex and nonsmooth optimality criteria, to which the technologically sound statements of semiinfinite optimization result. A step-by-step algorithm for preparing data and performing calculations, suitable for implementation in most programming languages, has been developed. The efficiency of the algorithm, which is higher, the larger the number of parameters included in the control vector and the higher the dimension of the optimization domain, is investigated. An estimate of the computational complexity of the computational procedure of the alternance optimization method is proposed, which makes it possible to determine the effectiveness of the application of the proposed algorithm for solving the problem of optimal control of the technological control object.

In the theory of ordinary differential equations, the Clairaut equation is well known. This equation is a non-linear differential equation unresolved with respect to the derivative. Finding the general solution of the Clairaut equation is described in detail in the literature and is known to be a family of integral lines. However, along with the general solution, for such equations there exists a singular (special) solution representing the envelope of the given family of integral lines. Note that the singular solution of the Clairaut equation is of particular interest in a number of applied problems.In addition to the ordinary Clairaut differential equation, a differential equation of the first order in partial derivatives of the Clairaut type is known. This equation is a multidimensional generalization of the ordinary differential Clairaut equation, in the case when the sought function depends on many variables. The problem of finding a general solution for partial differential equations of the Clairaut is known to be. It is known that the complete integral of the equation is a family of integral (hyper) planes. In addition to the general solution, there may be partial solutions, and, in some cases, it is possible to find a singular solution. Generally speaking, there is no general algorithm for finding a singular solution, since the problem is reduced to solving a system of nonlinear algebraic equations.The article is devoted to the problem of finding a singular solution of Clairaut type differential equation in partial derivatives for the particular choice of a function from the derivatives in the right-hand side. The work is organized as follows. The introduction provides a brief overview of some of the current results relating to the study of Clairaut-type equations in field theory and classical mechanics. The first part provides general information about differential equations of the Clairaut-type in partial derivatives and the structure of its general solution. In the main part of the paper, we discuss the method for finding singular solutions of the Clairaut-type equations. The main result of the work is to find singular solutions of equations containing power and exponential functions.