Identification of mechanical properties of composites based on design of experiments

A numerical-experimental method for the identification of mechanical properties of laminated polymeric composites from the experimental results is being developed. For the first time, it is proposed to use the method of experiment planning to solve the identification (inverse) problems. The basic idea of the approach is that simple mathematical models are determined only from information on the response of a structure in reference points of the design. Therefore, a significant reduction in the calculation of the identification functional (about 50–100 times) can be achieved in comparison with the conventional methods of minimization. Examples of the numerical identification of the elastic properties of the laminated composites from the measured eigenfrequencies of plates are discussed.Submitted to the 10th International Conference on Mechanics of Composite Materials, April 20–23, 1998, Riga, Latvia.Institute of Computer Analysis of Structures, Riga Technical University, Riga LV-1058 Latvia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 1, pp. 3–16, January–February, 1998.     

Structural approach to calculation of the effect of moisture on elastic characteristics of organoplastics

Data have been obtained for the structural calculation of the effect of moisture on the elastic characteristics of organoplastics from the properties of components. The distribution of moisture between the fiber and matrix — the components of a unidirectional composite — is considered. The elastic properties of the fiber are determined by an inverse calculation using the experimental dependences of the composite and matrix on moisture. The moisture effect on the properties of the materials is taken into account with influence functions, which differ by more than 25% for various characteristics. The results can be used for calculating the elastic properties of composites with various reinforcement schemes and at the nonequilibrium distribution of the moisture concentration in an actual environment.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Institute of Polymer Mechanics, Riga, LV-1006, Latvia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 4, pp. 525–530, July–August, 1998.     

Generalized self-consistent method: Modeling and computation of effective elastic properties of composites with composite or hollow inclusions

A new approach to the generalized self-consistent method [1,2] has been developed for problems of the statistical mechanics of composites with composite or hollow inclusions. The approach can reduce the problem of predicting the effective elastic properties of composites to a simpler averaged problem of a single, composite, or hollow inclusion with inhomogeneous elastic surrounding in a homogeneous effective elastic medium. The problem of predicting the effective elastic properties of composites with unidirectional hollow fibers or hollow spherical inclusions are studied by using the new approach.Submitted to the 10th International Conference on Mechanics of Composite Materials, April 20–23, 1998, Riga, Latvia.Perm' State Technical University, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 2, pp. 173–183, March–April, 1998.     

Creep prediction of a layered fiberglass plastic

The results of short-term creep tests of a layered glass fiber/polyester resin plastic in tension at angles of 90, 70, and 45° to the direction of the principal fiber orientation are presented. The applicability of the principle of time-temperature analogy for the prediction of long-term creep of the composite and its structural components is revealed. The possibility of evaluating the viscoelastic properties of the composite from the properties of structural components is shown.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 3, pp. 295–306, May–June, 1998.     

Concentration of electroelastic fields in a composite piezoceramic plate with defects crossing the interface

The characteristics of fracture and strength of a composite piezoceramic plate with defects in the form of cracks and holes situated in both of the plate components are investigated. The corresponding boundary-value problems of electroelasticity are reduced to systems of singular integral equations by constructing integral representations of the complex potentials. The results of numerical realization of the constructed algorithms are reported.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 6, pp. 777–786, November–December, 1998.     

Dependence of elastic and strength characteristics of high-modulus composites on schemes of reinforcement

The elastic and strength characteristics of high-modulus composites with 12 different reinforcement schemes have been studied thoroughly. The effect of reinforcement schemes on change in elastic and strength characteristics of composites has been evaluated. A calculation of the elastic characteristics of high-modulus composites has been performed from the properties of the reinforcement and binder, and a comparison of the results with experimental data is given. Diagrams of deformation upon extension of the composites studied are given.All-Union Scientific-Research Institute of Aviation Materials, Moscow. Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 6, pp. 1019–1027, November–December, 1974.     

Creep and damage accumulation in orthotropic composites under cyclic loading

Experimental results and theoretical prediction of the response of glassfiber-reinforced polyester under quasi-static, static (creep), and cyclic (fatigue) loading are presented. The nonlinear strain component at static loading and the strain amplitude rate at cyclic off-axis loading of an orthotropic composite are shown to follow the associated flow rule with a single-parameter quadratic potential function. The influence of fatigue damage on deformation is considerable due to the reduction in the elastic modulus of the composite and is apparently negligible with respect to its effect on the parameters of the creep kernel.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 4, pp. 447–460, July–August, 1998.     

Elastic constants of monotropic plastic foams. 2. Deformation parallel to foam rise direction. Numerical calculations

In order to perform numerical calculations of the elastic constants of monotropic plastic foams with a pronounced strut-like structure, a package of computing programs has been developed. Numerical values of the elastic constants were determined using the Simpson method for the calculation of triple averaging integrals and stepwise unconditional minimization of the one-argument potential energy function. The parameters of the numerical calculation process providing acceptable accuracy of the results were evaluated. Dimensions of the structural elements in the model of isotropic plastic foams were determined and analyzed.Institute of Polymer Mechanics, Riga, LV-1006, Latvia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 34, No. 2, pp. 163–172, March–April, 1998.     

Effect of side branches on the properties of cross-linked polyurethanes

Two series of samples of cross-linked polyurethanes: based on diethylene glycol and polypropylene glycol having calculated M_c 1000, were investigated. In synthesis by partial substitution of glycols by fatty acid ester and triethanolamine, long side branches were created in the structure of the polymer. The effect of the mass concentration of these branches on the physicomechanical properties of polyurethanes was evaluated. The different character of the effect of these branches on the properties of two types of polyurethane was demonstrated.Submitted to the Xth International Conference on Mechanics of Composite Materials (Riga, April 20–23, 1998).Latvian State Institute of Wood Chemistry. Riga, LV-1006 Latvia. Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 840–844. November–December. 1997.     

Production and characterization of UHMWPE fibers/LDPE composites

The production of unidirectionally fiber-reinforced composites (tapes) consisting of high-strength/high-modulus UHMWPE fibers and LDPE matrix is described. Two production techniques are applied: impregnation by aqueous powder dispersion and dry powder impregnation. The produced composites exhibit excellent mechanical properties. Depending on the fiber volume content which can be obtained: tensile strength (in fiber direction) 460–1100 MPa, elastic modulus 11–22 GPa, and elongation at break 4.9–8.3%.Presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga, October, 1995.Technical University of Berlin. Institute of Nonmetallic Materials. Polymer Physics Group.-Englishe Str. 20, D-10587 Berlin. Germany. Published in Mekhanik Kompozitnykh Materialov, Vol. 32, No. 6. pp. 719–728, November–December, 1996.     

Stresses in an anisotropic ring with moduli of elasticity varying along the radius subjected to internal and external pressure

The effect of nonuniformity of the elastic properties on the state of stress of an orthotropic ring subjected to pressure has been theoretically investigated. The results of the numerical calculations show that the effect of the variability of the circumferential modulus is more important.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 6, pp. 1128–1130, November–December, 1971.     

Fatigue of glass laminates subjected to repetitive impact loading

Fatigue characteristics obtained for repetitive impact tensile loading in a DSO impact tester are presented for a glass-reinforced Textolite. There are differences in the fatigue life, temperature rise, and the variation of the elastic properties as compared with the case of repetitive sinusoidal stress cycles. The effect of the geometry of the test piece gage length on the measured parameters is examined and methods of predicting fatigue life are discussed.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 4, pp. 751–754, July–August, 1969.     

Friction and wear of polymer-based composites

Polymeric materials containing different fillers and/or reinforcements are frequently used for applications in which friction and wear are critical issues. This overview describes how to design high temperature-resistant thermoplastics, e.g., filled with carbon fibers and internal lubricants, for operation under low friction and wear at elevated temperatures as sliding elements in, e.g., textile drying machines. Further information will be given on the systematic development of continuous fiber/polymer composites with high wear resistance, and on attempts for the prediction of their load-bearing capacity using a finite element approach. Finally, the application of such composites in thermoplastic filament-wound journal-bearings is discussed.Presented at the 10th International Conference on the Mechanics of Composite Materials (Riga, April 20–23, 1998).Institute of Composite Materials (IVW), University of Kaiserslautern, D-67663 Kaiserslautern, Germany. Published in Mekhanika Kompozitnykh Materialov, Vol. 34, No. 6, pp. 717–732, November–December, 1998.     

Bending features of a sandwich panel with asymmetric structure under local loads

The bending characteristics of a composite panel with asymmetric layered structure under local surface loads are obtained. A refined version of the applied theory is developed using the analytical solution of the bending problem of a sandwich plate with arbitrary asymmetric structure under a point load. Local effects are investigated within the limits of a discrete model allowing for the specific character of elastic properties of a soft filler. The advantages of the solution are expressions of bending characteristics — layer curvatures, displacements, and stresses — in a closed form. It is shown that these characteristics can vary several times depending on the asymmetry parameters of the structure. Degeneration peculiarities of the solution, stemming from the slipping of layers or, otherwise, their rigid linking by the Kirchoff—Love hypothesis, as well as from account of the transverse shear and compression of the normal, are examined in line with the degeneration of geometric and physical parameters of the discrete model adopted. The results obtained are illustrated by curves and surfaces for the characteristics studied.Submitted for the 11th International Conference on the Mechanics of Composite Materials (Riga, June 11–15, 2000).Institute of Polymer Mechanics, Latvian University, Riga, LV-1006 Latvia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 6, pp. 717–742, November–December, 1999.     

A model for taking into account the nonlinearity of properties of a semifinished product in the force analysis of the winding of composite materials

Conclusions 1. The proposed method of accounting for the nonlinearity of properties of a semifinished product by a piecewise-linear approximation of its stress—strain diagram greatly refines the calculation of stresses induced during the process of winding a product.2. A numerical analysis has revealed the ranges of values of the winding parameters where replacing the nonlinear model by simpler linear models for the calculation is appropriate.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 2, pp. 231–240, March–April, 1977.     

Comparative micromechanical analysis of nonelastic behavior of unidirectional plastics with tetragonal and hexagonal structures

A mathematical model for determining the effective elastic properties and describing the processes of inelastic deformation and damage accumulation of unidirectional fiber-reinforced composites with tetragonal and hexagonal structures is developed. A comparative analysis of the effective elastic moduli of glass, boron, organic, and carbon unidirectional plastics shows that, if the fiber volume fraction does not exceed 0.5, the effective elastic properties calculated by the models presented give closely related results. The calculation results for nonlinear fields of deformation and failure are presented and the limiting strength surfaces of fibrous glass plastics with hexagonal and tetragonal structures are obtained for different transverse loading paths. It is found that the structure of a composite affects significantly its strength properties.Submitted to the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000).Perm' State Technical University, Perm', Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 4, pp. 455–464, July–August, 2000.     

Modeling of transverse shears of piecewise homogeneous composite bars using an iterative process with account of tangential loads

A nonclassical model for the stress-strain state of a piecewise homogeneous composite bar is proposed. The model is based on an iterative process and takes into account the deplanation of cross sections of the bar caused by transverse shears. Based on the shear strains of some particular approximation, higher approximation models are constructed. The model accounts for both the normal and tangential loads.Presented at the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000).Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 4, pp. 487–500, July–August, 2000.     

Dynamic compressive strength of epoxy composites

The strength of laminated and unidirectionally reinforced composite materials was investigated in conditions of dynamic uniaxial compression with a strain rate of 50–1000 sec^–1 using the split Hopkinson pressure bar method. It was shown that in conditions of dynamic compression, glass/epoxy, aramid/epoxy, and carbon/epoxy composites exhibit elastic-brittle behavior with anisotropy of the strength and elastic properties. The effect of the strain rate on the strength characteristics of fiberglass-reinforced plastics was demonstrated.Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 6, pp. 776–782, November–December, 1995.Presented at the Ninth International Conference on the Mechanics of Composite Materials (Riga, October, 1995).     

Elastic displacements of a cement-based composite beam reinforced with punched steel grids

Using Mohr's method, a procedure is proposed for calculating the elastic displacements of a cement-based composite beam reinforced with punched steel grid ribbons. The approach proposed takes into account a possible asymmetry of the structure, the geometry of grids, and the elastic properties of cement mortar and steel.Riga Technical University, Riga, LV-1047, Latvia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 2, pp. 181–186, March–April, 1999.     

Electromechanical strain gage for measuring the deformations of structural plastics

By equipping a multiple-use strain gage based on a metal elastic element, whose natural mechanical stiffness is unacceptably high, with a spring-loaded mechanism forming part of the strain gage itself it is possible to compensate the force developed by the elastic element in the process of deformation.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No.5, pp. 951–953, September–October, 1969.