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Bykov D.L.,Central Scientific Research Institute for Engineering TsNIIMash | Konovalov D.N.,OT Kontakt Ltd. | Mel'nikov V.P.,The Federal Center for Dual use Technologies Soyuz | Osavchuk A.N.,The Federal Center for Dual use Technologies Soyuz
Mechanics of Solids | Year: 2010

The filled polymer materials exhibit viscoelastic properties in a wide time range including the millisecond range (~10-2-10 ms) characteristic of different shock loadings of structures made of these materials. We propose a method for the identification of the filled polymer material relaxation kernel in the millisecond time range; this method is based on a shock loading test of a cylindrical sample made of this material. In this test, the disk indenter acceleration is measured by using a piezotransducer. The test scheme does not impose any rigid constraints on the sample dimensions. In particular, it is possible to use samples of typical dimensions of the order of 10 cm, for which the conditions that the sample material is representative of the structure material are necessarily satisfied. The relaxation kernel parameters are identified by numerical minimization of the theoretically predicted indenter velocity deviation from the velocity-time dependence obtained by integrating the acceleration transducer readings. The minimization problem is solved by using a genetic algorithm. The problem of theoretical prediction of the indenter velocity is solved numerically by using a reduced computational scheme whose parameters are chosen from the minimum condition for the deviation from the prediction obtained in the framework of the detailed computational scheme. The use of the reduced computational scheme permits decreasing the computational costs by 3-4 orders of magnitude compared with the detailed computational scheme, which is a necessary condition for the practical applicability of the genetic algorithm in identification problems. We present examples of relaxation kernel identification in the range of 0.1-10ms from the results of the test where the disk indenter raised to the height of 1m falls on the sample end surface. © 2010 Allerton Press, Inc.

Bykov D.L.,Central Scientific Research Institute for Engineering TsNIIMash | Kazakov A.V.,Central Scientific Research Institute for Engineering TsNIIMash | Konovalov D.N.,OT Kontakt Ltd. | Mel'nikov V.P.,The Federal Center for Dual Use Technologies Soyuz | And 3 more authors.
Mechanics of Solids | Year: 2014

We present the results of a large series of experiments aimed at the study of laws of damage accumulation and fracture in highly filled polymer materials under loading conditions of various types: monotone, repeated, low- and high-cycle, with varying type of stress state, dynamic (in general, more than 50 programs implemented on specimens from one lot of material). The data obtained in these test allow one to make conclusions about the constitutive role of the attained maximum of strain intensity when estimating the accumulated damage in the process of uniaxial tension by various programs (in particular, an additional cyclic deformation below the preliminary attained strain maximum does not affect the limit values of strain and stress in the subsequent active extension), about the strong influence of the stress state on the deformation and fracture, about the specific features of the nonlinear behavior of the material under the shock loading conditions and its influence on the repeated deformation. All tests are described (with an accuracy acceptable in practical calculations, both with respect to stresses and strains in the process of loading and at the moment of fracture) in the framework of the same model of nonlinear viscoelasticity with the same set of constants. The constants of the proposed model are calculated according to a relatively simple algorithm by using the results of standard uniaxial tension tests with constant values of the strain rate and hydrostatic pressure (each test for 2–3 levels of these parameters chosen from the ranges proposed in applications, each loading lasts until the fracture occurs, and one of the tests contains an intermediate interval of total loading and repeated loading) and one axial shock compression test if there are dynamic problems in the applications. The model is based on the use of the criterion fracture parameter which, in the class of proportional loading processes, is the sum of partial increments of the strain intensity on active segments of the process (where the strain intensity is at its historical maximum) with the form of the stress state and the intensity of strain rates taken into account. © 2014, Allerton Press, Inc.

Bykov D.L.,Central Scientific Research Institute for Engineering TsNIIMash | Kazakov A.V.,Central Scientific Research Institute for Engineering TsNIIMash | Konovalov D.N.,OT Kontakt Ltd. | Mel'nikov V.P.,The Federal Center for Dual use Technologies Soyuz | And 2 more authors.
Mechanics of Solids | Year: 2012

Determination of mechanical characteristics of filled polymer materials in shock wave processes is of interest in calculations of the strength of these materials. The standard computation methods are based on the use of the linear theory of viscoelasticity, where there is no distinction between the active and passive deformation processes. In the present paper, dynamical experiment and theoretical modeling are used to illustrate the important role played by the sharp decrease in the resistance of a filled polymer material in unloading (in the millisecond time range). The higher the degree of filling of this material, the more significant this effect is. © 2012 Allerton Press, Inc.

Voronkov V.N.,Central Scientific Research Institute for Engineering TsNIIMash
Mechanics of Solids | Year: 2016

Complex systems whose subsystems interact at finitely many points are considered. The couplings are given by linear homogeneous differential relations. The problem of determining the coupling parameters is solved. To this end, the system oscillations are represented as linear combinations of harmonic responses of the subsystems. For each point of coupling, one can construct a system of linear algebraic equations for the parameters (rigidities) of this coupling. The method is intended for determining the values of rigidities of the couplings between blocks of spacecraft carriers. The analytic model of a simplest structure is carried out as an example. © 2016, Allerton Press, Inc.

Bykov D.L.,Central Scientific Research Institute for Engineering TsNIIMash | Martynova E.D.,Moscow State University
Mechanics of Solids | Year: 2013

A method for determining the material functions of nonlinear endochronic theory of aging viscoelastic materials (NETAVEM) with preliminary mechanical damage was developed. The proposed method is based on an analysis of the differences between two graphs of the stress dependence on time obtained in tension with the same constant speed of two specimens made of the same filled polymer material. One of the specimens was not preloaded, and the other was preloaded. The reduced time [1] contained in the NETAVEM constitutive relations and its dependence on the actual time are determined by the distances from the stress axis to two points corresponding to the same stress value and lying on the graphs for the damaged and undamaged specimens. The relaxation kernel is determined in the experiment with the undamaged specimen. These two material functions and the curve obtained for the damaged specimen are used to obtain the NETAVEM aging function, and then the function of viscosity can be calculated. As a result, all characteristics of the damaged material become known, and the strength of structures made of this material can be calculated. © 2013 Allerton Press, Inc.

Bykov D.L.,Central Scientific Research Institute for Engineering TsNIIMash | Konovalov D.N.,OT Kontakt Ltd. | Peleshko V.A.,Central Scientific Research Institute for Engineering TsNIIMash
Mechanics of Solids | Year: 2011

We study composite polymer materials with a high degree of dispersion filling (several tens of percent in volume). A tensor generalization of the previously developed variant of the geroendochronic theory of viscoelastic materials is obtained, which allows us to pose and solve initialboundary value problems using this model. A numerical solution algorithm is proposed, which is realized as the UMAT subroutine for the ABAQUS finite element software package. Finite element computations are performed for the process of tensile stretching of bodies having the shape of short wide strips made of a highly filled polymer material and the results are compared with the relevant experimental data published by K. Ha and R. A. Schapery (Int. J. Solids Struct. 35 (26-27), 3497-3517 (1998)). The computational results for the deformation and fracture of solids in which a weakly inhomogeneous stress-strain state (SSS) is realized show a quite satisfactory agreement with the experiments. It has been found that, for correct strength analysis of bodies with holes and cuts, one has to consider the influence of the SSS concentration in the model. To this end, we propose to generalize the constitutive relations as follows: in the equation for the damage and fracture parameters, introduce a material function of the concentration parameter, for which we take the ratio of some state variable (the fracture parameter from the model where the concentration effect is not taken into account) at the point in question to the average value of this variable in a neighborhood of a given radius. A method is suggested for reducing the initial-boundary value problem of the proposed nonlocal theory to a problem for a piecewise-homogeneous body composed of a set of layers described by local constitutive relations. The method was successfully tested in the calculations of bodies with a hole and an sharp internal cut (stress concentrators of moderate and high level, respectively). The obtained results show that the developed model has a high accuracy, including adequate prediction of the time and location when and where the fracture begins, which is the main objective of the strength analysis. © 2011 Allerton Press, Inc.

Peleshko V.A.,Central Scientific Research Institute for Engineering TsNIIMash
Mechanics of Solids | Year: 2016

The deviator constitutive relation of the proposed theory of plasticity has a three-term form (the stress, stress rate, and strain rate vectors formed from the deviators are collinear) and, in the specialized (applied) version, in addition to the simple loading function, contains four dimensionless constants of the material determined from experiments along a two-link strain trajectory with an orthogonal break. The proposed simple mechanism is used to calculate the constants of themodel for four metallic materials that significantly differ in the composition and in the mechanical properties; the obtained constants do not deviate much from their average values (over the four materials). The latter are taken as universal constants in the engineering version of the model, which thus requires only one basic experiment, i. e., a simple loading test. If the material exhibits the strengthening property in cyclic circular deformation, then the model contains an additional constant determined from the experiment along a strain trajectory of this type. (In the engineering version of the model, the cyclic strengthening effect is not taken into account, which imposes a certain upper bound on the difference between the length of the strain trajectory arc and the module of the strain vector.) We present the results of model verification using the experimental data available in the literature about the combined loading along two- and multi-link strain trajectories with various lengths of links and angles of breaks, with plane curvilinear segments of various constant and variable curvature, and with three-dimensional helical segments of various curvature and twist. (All in all, we use more than 80 strain programs; the materials are low- andmedium-carbon steels, brass, and stainless steel.) These results prove that the model can be used to describe the process of arbitrary active (in the sense of nonnegative capacity of the shear) combine loading and final unloading of originally quasi-isotropic elastoplastic materials. In practical calculations, in the absence of experimental data about the properties of a material under combined loading, the use of the engineering version of the model is quite acceptable. The simple identification, wide verifiability, and the availability of a software implementation of the method for solving initial–boundary value problems permit treating the proposed theory as an applied theory. © 2016, Allerton Press, Inc.

In the proposed theory of plasticity, the deviator constitutive relation has a trinomial form (the vectors of stresses, stress rates, and strain rates, which are formed form the deviators, are coplanar) and contains two material functions; one of these functions depends on the modulus of the stress vector, and the other, on the angle between the stress vector and the strain rate, the length of the deformation trajectory arc, and the moduli of the stress and strain vectors. The spherical parts of the stress and strain tensors satisfy the relations of elastic variation in the volume. We obtain conditions on the material functions of the model which ensure the mathematical wellposedness of the statement of the initial–boundary value problem (i.e., the existence and uniqueness of the generalized solution, and its continuous dependence on the external loads). We also describe the scheme for solving the initial–boundary value problem step by step using the model and present the expression for the Jacobian of the boundary value problem at the time step. These results are formalized as a subprogram for prescribing the mechanical properties of the user material in the finite-element complex ABAQUS, which allows one to calculate the structure deformations on the basis of the proposed theory. © 2015, Allerton Press, Inc.

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