## No 1 (2014)

**Year:**2014**Articles:**9**URL:**https://ered.pstu.ru/index.php/mechanics/issue/view/34**DOI:**https://doi.org/10.15593/perm.mech/2014.1

Superposition of power-logarithmic and power singular solutions in two-dimensional elasticity problems

#### Abstract

A comparison of the results obtained recently for power and power-logarithmic singular asymptotics of solution associated with a class of singular integral equations of the two-dimensional elasticity is performed. It is noted that leading parts of the integral equation contain similar terms for these singular solutions. In this connection, transcendental equations in regard to singularity exponents for additive form (superposition) of power and power-logarithmic solution asymptotics were constructed. It was established that superposition of the mentioned singular solutions has the singularity exponent which is known for the classical power asymptotics of elastic stress. The general nature of the obtained results is discussed that is related to the description of numerous boundary value problems of the two-dimensional elasticity by means of systems of singular integral equations belonging to the class under consideration. Based on theory of the Kolosov-Muskhelishvili complex potentials power-logarithmic singular solution of a boundary value problem is constructed. This solution represents the obtained results from point of view of direct asymptotic analysis of the boundary value problems. The parametric approach for equations on real singularity exponent is suggested to extend domain where non-oscillatory asymptotic is implemented. Numerical results on leading power-logarithmic singularity exponent for the two-dimensional problem of the elasticity theory for the crack terminating an interface are presented. The efficiency of the developed parametric approach for examined crack problem is demonstrated.

**PNRPU Mechanics Bulletin**. 2014;(1):5-30

Mathematical modelling of vegetable oil plunger extraction

#### Abstract

The research relates to studying a mixture of grained oilseeds saturated with oil and deformed with plunger molding. Mathematical simulations aim was determination of oil extraction velocity under the set loading conditions. Current statement treats processed material as biphasic mixture. Oilseed cake presented the first phase, which also was material’s porous skeleton. Vegetable oil filling the porous skeleton was the second phase of the mixture. Multiphase dynamics approach was applied in current research for material behavior description. Balance equations were set up for each mixture components. Interfacial volumetric force introduction modelled filtration of liquid. According to former researches, viscous liquid model described properties of porous skeleton as well as properties of vegetable oil. Porous skeleton viscosity assumed to be pressure dependent. Numerical solution of problem was carried out in two-dimensional statement for expression chamber middlesection using finite element approach. The primary variables were constituent’s velocity and pressure fields. Current study used cake pressure dependent porosity model that is common in porous media mechanics. Computational domain discretization was carried out using nine-node rectangular finite elements with linear and quadratic approximation for pressure and velocity fields respectively. Oil saturation distribution along expression chamber height obtained in numerical experiments demonstrates nonlinearity under high external loads. Moreover, the study investigated porosity changes influence on vegetable oil flow during expression.

**PNRPU Mechanics Bulletin**. 2014;(1):31-56

Investigation of panel flutter of functionally graded circular cylindrical shells

#### Abstract

The paper is devoted to the analysis of panel flutter of functionally graded cylindrical shells in a supersonic gas flow. The aerodynamic pressure is calculated based on the quasi-static aerodynamic theory. The inner surface of the structure is made of aluminum and the outer surface is made of zirconium dioxide. The effective properties of the material continuously changes through the shell thickness with radial coordinate according to the power law. The geometric and physical relations and the equations of motion written in the framework of the classical shell theory are reduced to the system of eight ordinary differential equations for new unknown quantities. A solution of the problem is found by integrating the obtained system of equations by the Godunov’s orthogonal marching method at each step of the iterative procedure generally used in Muller’s method to evaluate complex eigenvalues. The reliability of the method was assessed by comparing the obtained results with the available experimental and theoretical data. The paper presents the results of numerical experiments carried out to estimate the effect of the properties of functionally graded materials on the stability boundary of circular cylindrical shells for different combinations of boundary conditions and linear dimensions. It has been found that the type of loss of stability is defined not only by geometrical characteristics of the structure and boundary conditions but also by given composition of the functionally graded material. It has been shown that an effective control of critical aerodynamic loading can be executed only for shells with certain geometrical dimensions.

**PNRPU Mechanics Bulletin**. 2014;(1):57-75

Experimental and theoretical investigations of visco-plastic deformation in structural steels considering mutual influence of plasticity and creep effects

#### Abstract

This paper considers the developed mathematical model of inelastic deformation in structural steels, describing thermoviscoplastic deformation taking into account mutual influence of plasticity and creep effects. An integration algorithm for constitutive relations of thermoviscoplasticity has been developed. It consists in the formulation of constitutive relations in increments, depending on the selected time step. In difficult areas of deformation paths, time step can be adjusted throughout the whole estimation time in case of stability calculations. Stresses, plastic deformations and creep deformations are determined by integrating the defining relations of thermal creep by Runge-Kutta method with the correction of stress deviator and subsequent determination of stress according to thermal plasticity equations with regard to the average creep strain at the next sampling time. Experimental studies of influence between creep processes and plasticity under high temperature using 12H18N9 steel have been conducted. By numerical computer simulation of stress-strain state (SSS) kinetics in laboratory samples and by comparing the obtained results with field experiments, the authors carried out certification of the developed thermoviscoplastic model and integration algorithm of constitutive relations. All of these led to the conclusion about the reliability of model concepts and methods for determining material parameters under joint actions of fatigue and creep mechanisms. The authors have compared computer and physical tensile experiments of laboratory 12H18N9 steel samples with different histories of changes in temperature and mechanical deformation. It is shown that the developed thermoviscoplastic model qualitatively and quantitatively describes main effects of inelastic deformation in structural steels with different histories of mechanical deformation and changes in temperature. It is concluded that the defining relations of thermoviscoplasticity are reliable, and the above methods of integration are accurate.

**PNRPU Mechanics Bulletin**. 2014;(1):76-107

Influence of circular hole on the shell stress state for arbitrary Gaussian curvature

#### Abstract

The work is devoted to determining isotropic shell of stress-strain state for arbitrary Gaussian curvature with a circular hole, located in the center of the structure. An axial tension or an internal pressure is applied to the surface of the shell. The isotropic shallow shell theory equations were used, which coincide with the isotropic shell theory equations with a large measure of variability. The integral Fourier transformation and the theory of generalized functions were applied. As a result the problem was reduced to solving the system of boundary integral equations. One benefit of using the method of boundary integral equations for the study of shell stress-strain state weakened by a hole is the ability to define the unknown quantities directly on the contour of the hole, not evaluating them on the whole surface of the shell. To obtain the kernels of the singular integral equations, integral representations of displacements and shallow isotropic shells static equations fundamental solutions were used. As the unknown functions, a combination of displacements, rotation angles and their derivatives were used. Analytical calculations are considerably simplified if it is assumed to take into account not four unknown functions on the contour, as it is customary, but five. In this paper it is chosen to use the differential equation which relates the unknown functions as the fifth equation of the boundary integral equations system. In order to obtain numerical solution of the problem, the method of mechanical quadratures for systems of integral equations and finite difference method for the fifth differential equation were used to reduce a problem to a system of linear algebraic equations. The stress concentration factors values depending on the isotropic shell curvature are given. Also the results were compared with other researchers.

**PNRPU Mechanics Bulletin**. 2014;(1):108-125

Mathematical models of growth deformation

#### Abstract

Currently biology and medicine become one of the most attractive areas of applied mathematics. To fix certain pathologies of children, growth modelling for living tissue and growth management are the issues of major importance. In the process of growth a growing body itself experiences deformation that proves a fundamental difference of mechanics of growing bodies from the classical mechanics of bodies of constant composition. This paper presents an analysis of publications related to various models of the mechanism of living tissues growth and a brief analysis of biological growth concept. The authors considered basic principles of growth modelling and specified major areas for developing certain models of body-growing tissue. The following classification of growth models for living tissue has been given: models based on the hypothesis about the influence of intracellular pressure on tissue growth as a stimulating factor; models of multiphase media, the so-called “mixture theory”; model based on the hypothesis about the influence of residual stresses on tissue growth as a stimulating factor; models connecting the rate of growth from the deformations known from observations and experiments. The analysis resulted in specifying factors influencing the growth of living tissue. These are the chemical composition, concentration, transport and stresses in the material body. Stress is a significant factor affecting growth. The practical importance of growth model for mechanical deformation is based on its wide application for describing normal and pathological growth of hard tissues in the human body. In this case, from mechanical point of view, it becomes possible to model and control growth.

**PNRPU Mechanics Bulletin**. 2014;(1):126-141

A contact problem for bending of two-leaf spring with the leaves curved along the circular arc

#### Abstract

The unbonded contact problem for bending of two-leaf spring is considered; the leaves are curved along the circular arc in their natural states. The lengths of the leaves are different; each leaf has one end clamped and the other free. The angle formed by the long leaf is less than the right one. The cross-sections of leaves are the rectangles of the same width but of the different thickness. These thicknesses are considered to vanish while the elastic lines of the leaves are analyzed geometrically. The real thicknesses affect only the bending stiffness of the leaves. The given loading is applied transversely to the leaves. There is no friction between the leaves. The bending is described by Bernoulli - Euler model. The problem is reduced to finding the density of the leaves interacting forces. This density is the sum of the piecewise-continuous part and the concentrated forces. The rigorous problem statement is formulated, the uniqueness of solution is established and the complete analytical solution of the problem is provided. This construction also proves the existence of the solution. The substantiation of the solution includes proving of the non-negativity of the contact forces and contact distances and the proving of the existence of the root of transcendental equation that gives the length of the contact segment. The proving of the non-negativity of the contact distances uses a new approach that is based on the fact that these distances can be regarded as the solutions of some variational problems. It is shown that three patterns of the leaves contact are possible: the contact along the whole short leaf; the contact at the point on the end of the short leaf; the contact along the part of the short leaf and at the point. The pattern kind depends on the given loading. The obtained results generalize the known sufficient condition for the pointwise contact of the leaves.

**PNRPU Mechanics Bulletin**. 2014;(1):142-152

Study of the stress state in composite plate near the connecting line edge depending on the thickness and material parameters of the connecting interlayer

#### Abstract

The plane-stress state in the vicinity of singular point, in a plate composed of two identical elements with interlayer such as glue is studied. The purpose of this study is determining characteristics of this solid mechanics problem and influence of material properties and interlayer thickness on stress concentration near the edge of the combining the elements (at the singular point ). Analytical estimation of the restrictions count on the state parameters at the line edge of junction plate element and interlayer is conducted. It is shown that the count of independent restrictions depends on the material properties of the plate elements and is usually redundant (non-standard). Standard restrictions count is only an exceptional case, when certain combinations of plate elements material parameters take place. In this particular case the solution is named as a “ basic-solution ”. For considered case of stress-strained plate basic-solution has uniform stress and piecewise-homogeneous strain state solution. With material parameters near the basic-solution the nonstandard problem is considered by iterative numerical-analytical method based on minimizing the residual divergence of all the boundary conditions at the singular point vicinity. A set of stress state problems is calculated. It is shown that the solution is practically independent of the interlayer thickness, but it depends drastically on the material properties of the elements. The more rigid material has the highest value concentration ratio.

**PNRPU Mechanics Bulletin**. 2014;(1):153-166

The fatigue life-time propagation of the connection elements of long-term operated hydro turbines considering material degradation

#### Abstract

The work deals with development of a new approach for forecasting a high-cycle fatigue life-time of bolted connection of hydro turbines runner. Operation of hydro turbines on normal operation condition does not lead to high stresses rates in bolted connection. However the high cycle fatigue failures have been occurred. High rates stresses occur in bolted connection in transient (start/stop) regimes of hydro turbines operation. The frequency of transient regimes occurrence depends from many factors and defined in this paper as a random function of time. Long-time bolted connection operation lead to natural degradation of material (aging). The degradation process is also a random process of time. So, this work pays attention to developing stochastic mathematical model of damage accumulation that take into account stochastic nature of degradation process and frequency of transient regimes occurrence. Application of the developed models is shown on real engineering example. Degradation of properties has been modeled as a process of the reduction of fatigue (endurance) limit in time. Kinetics of damage accumulation is introduced in the context of the effective stress concept. Mathematical expectation, correlation function and the continuum damage parameter variance have been obtained as functions of time. Analysis of the influence of natural aging process on statistical parameters of damage accumulation as well as on the life-time has been carried out. The stress-strain state of bolted connection is determined by finite element method.

**PNRPU Mechanics Bulletin**. 2014;(1):167-193