Static and dynamic elasticity of some thermotropic liquid-crystalline copolyesters

Two series of thermotropic liquid crystalline copolyesters were studied in static and dynamic tensile deformation. Static loading generally does not lead to any peculiar phenomena: the stress-strain diagrams are linear or convex, except one copolyester fibre, the stress-strain diagram of which is concave at high temperatures. On the contrary, in dynamic loading the elasticity modulus increased during deformation for all the samples investigated. So it can be assumed that formation of a more rigid and stable structure takes place due to the dynamic deformation.Published in Mekhanika Kompozitnykh Materialov, Vol. 33, No. 5, pp. 704–709, September–October, 1997.     

Fatigue strength of glass fiber/epoxy angle-ply laminates with different matrix ductility

Conclusion Angle ply laminates made up of glass fiber/epoxy plies have a good static strength for laminate angles up to about 45 °. This is due to the limitation of transverse strain in the plies due to constraints exerted by neighboring plies. At laminate angles of 50 ° and above the transverse and shear strains in the plies are not sufficiently constrained and cause failure in the matrix material. The constraint effect is present in fatigue but since the polymeric matrix material is sensitive to fatigue loading fatigue failure will occur at much lower strain levels than in static loading. This effect cannot be offset by increasing the ductility of the matrix material. Thus, strain to failure under fatigue loading will be only a small fraction of the static strain to failure for angle ply laminates with ply angles up to around 45 ° where the static strength is due to the constraint effect.Published in Mekhanika Kompozitnykh Materialov, No. 5, pp. 632–638, September–October, 1992.     

Nonaxisymmetric thermal stress state of laminated rotational bodies of orthotropic materials under nonisothermic loading

A procedure for numerical investigation of nonaxisymmetric temperature fields and the elastic stress-strain state of laminated rotational bodies of cylindrically and rectilinearly orthotropic materials under nonisothermal loading is proposed. The deformation of orthotropic materials is described by the equations of anisotropic elasticity theory. The equations of state are written in the form of Hookes law for homogeneous materials, with additional terms which take into account the thermal deformation, changes in the mechanical properties of materials in the circumferential direction, and their dependence on temperature. A semianalytic finite-element method in combination with the method of successive approximations is used. An algorithm for numerical solution of the corresponding nonlinear boundary problem is elaborated, which is realized as a package of applied FORTRAN programs. Some numerical results are presented.Translated from Mekhanika Kompozitnykh Materialov, Vol. 40, No. 6, pp. 731–752, November–December, 2004.     

Wave triplets in two-phase composites

The possibility of an appearance of wave triplets in composites is studied. It is shown that a resonant interaction in industrial composites is possible. It is stated graphically only in two classic examples of layered and fiber reinforced composites that dispersion curves of the materials allow a resonant wave interaction. The quadratic nonlinear theory of two-phase mixtures is built as the variant of a microstructure theory of composite materials. Triplets of harmonic waves are studied in the framework of this theory.Published in Mekhanika Kompozitnykh Materialov, Vol. 31, No. 5, pp. 660–670, September–October, 1995.     

Nonlinear dynamics of inelastic failure of damaged layered systems

A method of solution is proposed and processes of the formation and development of a failure zone in piecewise-homogeneous rod during impact loading were analyzed. Such properties as being damageable and healing of defects as well as compliance of a barrier are taken into account for the first time. Deformation of the barrier is taken into account by means of a model of a Winkler foundation. The fulfillment of the known failure criterion at the failure front is assumed. The solution is reduced to a system of quasi-linear equations which are hyperbolic in the narrow sense, the right side of which contains integral terms of the hereditary type. As a result of numerical realization for the case of a bilayer rod, when the region contacting the barrier is damaged, it was found that the place and time of formation of the failure zone and time of complete failure depend substantially on compliance of the barrier as well as nonlinearity of the material.Presented at the Ninth International Conference on Mechanics of Composite Materials (Riga, October 1995).Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 5, pp. 640–645, September–October, 1995.     

The effect of interphase on residual thermal stresses. 1. Single fiber composite materials

Conclusions The residual thermal stresses in the constituents of a fiber-reinforced epoxy have been predicted using a concentric three-cylindrical (fiber-interphase-matrix) assemblage analysis. The interphase has been treated as a region with a variable Young's modulus — a direct consequence of the changes in the microstructure of the matrix near the fiber surface. The Navier equations of elasticity have been solved in series form solutions for each type of property variation.A parametric study is used to demonstrate the fact that changes in the interphase properties can drastically affect the residual stresses in the interphase.Published in Mekhanika Kompozitnykh Materialov, Vol. 30, No. 5, pp. 579–589, September–October, 1994.     

Predicting the transverse strength of unidirectional composites under combination loading

This article presents a mathematical model for predicting the transverse strength of unidirectional fiber composites subjected to combination transverse loading under different conditions. The behavior of the matrix is described by nonlinear physical equations consistent with the strain theory of plasticity for the active loading section. The fibers are assumed to be isotropic and elastic. The boundary-value problem of micromechanics that is formulated includes strength criteria for the matrix and fibers that mark the beginning of their possible failure. The modeling of the fracture process is taken farther through the use of a scheme that reduces the stiffness of the matrix and fibers in the failed regions in relation to the sign of the first invariant of the stress tensor. The method of local approximation is used together with the finite-element method to calculate the stress and strain fields in unidirectional composites with cylindrical fibers in a tetragonal layup. The model is used to study the behavior of an epoxy-based organic-fiber-reinforced plastic subjected to transverse loading in different simple paths — including simultaneous compressive and tensile loads, as well as transverse shear.Paper to be presented at the Ninth International Conference on the Mechanics of Composite Materials (Riga, October 1995).Translated from Mekhanika Kompozitnykh Materialov, Vol. 31, No. 4, pp. 473–481, July–August, 1995.     

Investigation of fatigue damage in polymer films by the method of light scattering

Conclusions In cyclic loading of the investigated polyurethane there originates a polydisperse system of microdefects with nonspherical shape, in consequence of which the light scattering increases irreversibly. The method of light scattering is sufficiently sensitive for determining the relative concentrations and dimensions of microdefects that are smaller than the wavelength of light. It was shown that the predominant mechanism of failure of polyurethane is the increased concentration of microdefects whereas their dimensions change only imperceptibly.Translated from Mekhanika Kompozitnykh Materialov, No. 5, pp. 910–917, September–October, 1984.     

Continual approximations of displacements in thick layered plates upon decomposition of thermomechanical loads into bilateral symmetric and antisymmetric components

Continual models of deformation of thick layered plates, where the general order of resolving equations does not depend on the number of layers, are constructed with account of the specific character of the unflexural and flexural deformation caused by the symmetric and antisymmetric components of bilateral loads and temperature. Account of the load decomposition during the process of refining a certain reasonable initial solution, consisting in the analytical iterative integration, makes it possible to obtain rather precise continual approximations of stresses and displacements over the transverse coordinate and thus to revise the known continual models.Ukrainian State Academy of Water Management, Rovno, the Ukraine. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 6, pp. 743–756, November–December, 1999.     

Elements of structural mechanics of discretely soaked composites with randomly distributed fibers

Conclusions It is evident that the proposed approach is one of the first attempts to construct structural models and carry out examination from the viewpoint of the mechanics of composites of discretely soaked fiber-reinforced filtering materials. The constructed computer structural model and the algorithms of simulations of the processes of loading and failure of these materials are in the stage of experimental verification. At the same time, the approach proposed to examining this new grade of materials for composite mechanics may be of specific interest for widening the possibilities of computer simulation of the processes of deformation and failure of materials on the basis of detailed macrostructural examination and analysis of the results of mechanical tests.Translated from Mekhanika Kompozitnykh Materialov, Vol. 29, No. 5, pp. 629–639, September–October, 1993.     

Modeling the nonlinear deformation of composite laminates based on plasticity theory

The plasticity theory has been successfully used for describing the nonlinear deformation of laminated composite materials under a monotonically increasing loading. Generally, several tests are needed to determine the parameters of the plastic potential for a laminate. We explore an alternative approach and obtain the plastic potential by using theoretical considerations based on a laminate analysis. The model is shown to provide an accurate prediction for the response of a cross-ply glass/epoxy laminate under uniaxial tensile loading at different angles to the material orthotropy axes. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 43, No. 3, pp. 309–318, May–June, 2007.     

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.     

Resistance of composite flywheel rim to radial tensile stresses from centrifugal forces based on results of pure bending of a rim segment

A method is proposed for evaluating the resistance of a flywheel rim to radial stresses in free rotation. The method is based on loading a rim segment in pure bending and calculating the limiting moment and the corresponding limiting angular velocity. Applicability of the method is substantiated theoretically by investigating the similarity of the radial stress diagrams in rotation and pure bending. The method is verified experimentally for the strained state of a rim segment in pure bending.Translated from Mekhanika Kompozitnykh Materialov, Vol. 29, No. 4, pp. 521–526, July–August, 1993.     

Thermal conductivity of unidirectionally reinforced hybrid composites

Conclusion The thermal conductivity of organic-glass, organic-carbon, and carbon-glass plastic in dependence on the volumetric content of organic, glass, and carbon fibers was experimentally investigated. The solution for transverse thermal conductivity of unidirectional hybrid composite, obtained in [8] by generalizing the method of self-congruence to the case of a triphase model, is in satisfactory agreement with the experimental data.Translated from Mekhanika Kompozitnykh Materialov, No. 5, pp. 817–822, September–October, 1990.     

Nonsteady-state damage accumulation processes in composite flanges under cyclic loads

We consider two typical flanges: for a powerplant casing and for the casing of suspensions with a sound-absorbing duct. We model the behavior of flanges under the action of cyclic external loads based on solution of the nonlinear axisymmetric problem of elasticity theory. In order to take into account fatigue fracture of the layers of the structure, we use maximum stress criteria, a tensor damage function, the rule of linear summation of damage, and reduction schemes for the deformation characteristics of the layers, describing different fracture mechanisms. We indicate the possible nature of the development of fracture zones in composite flanges. We have established the spare load-bearing capacity of the flanges after fracture begins.Report presented at the Tenth International Conference on Mechanics of Composite Materials (Riga, April 20–23, 1998).Perm State Technical University, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 5, pp. 636–643, September–October, 1997.     

Effect of shear stiffness and concrete wedge cone failure on fracture of a fiber-reinforced plastic rod at an intersection with a concrete crack

Experimental results of model speciments in which FRP rods fractured due to local deformation at a crack intersection in a concrete member were analyzed by a 3D nonlinear finite element method in which orthogonal anisotropy of the FRP rod was considered. The analytical results indicated that accurate prediction of shear modulus of the FRP rod and size of concrete wedge cone failure around the FRP rod was significant to predict deformation and fracture of the FRP rod. FRP rods as reinforcement in concrete members, the small shear modulus, because of the orthogonal anisotropy and the wedge cone failure, may prevent the FRP rod from fracturing at a very low tensile stress due to the local deformation at the crack intersection.Presented at the Ninth International Conference on the Mechanics of Composite Materials, Riga, October, 1995.Published in Mekhanika Kompozitnykh Materialov, Vol. 21, No. 2, pp. 158–166, March–April, 1996.     

Estimation of laminate stiffness reduction due to cracking of a transverse ply by employing crack initiation-and propagation-based master curves

The applicability range of toughness-and strength-based criteria for progressive cracking of a transverse layer in a cross-ply composite laminate subjected to tensile loading is considered. Using a deterministic cracking model, approximate relations for the crack density as a function of stress are derived for initiation-and propagation-controlled types of cracking. The master-curve approach is applied to progressive cracking in glass/epoxy laminates. The accuracy of estimation of laminate stiffness reduction by using crack density master curves is evaluated. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 44, No. 5, pp. 633–646, September–October, 2008.     

Fatigue failure criteria and degradation rules for composites under multiaxial loadings

To develop design rules for dynamically loaded composite structures, extensive static and cyclic tension/compression-torsion tests were carried out on carbon-fibre-reinforced composites, in which especially the important influence of multiaxial loading conditions on their fatigue behaviour was investigated. Physically based failure criteria for static loadings are modified for multiaxial cyclic loadings, and a good agreement with experiments is achieved. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 5, pp. 631–641, September–October, 2006.     

Fatigue strength of certain materials used in airframe construction

Conclusions The completed tests show that the fatigue strength of carbon-plastic is 2.5 times greater than that of glass-plastic. Consequently, it is best to use carbon-plastic with ED-20 epoxy resin filled with 4–5m glass microspheres for adhesive purposes in load-bearing components. This makes it possible to significantly reduce the weight of airframes and improve their service properties. It was established that, for the composite materials investigated, stiffness is lost during cyclic loading mainly as a result of damage accumulation; the loss in stiffness may serve as a criterion for evaluating the progress of fatigue fracture.Translated from Mekhanika Kompozitnykh Materialov, No. 3, pp. 451–455, May–June, 1980.     

Transverse compression of PPTA fibers

Results of single transverse compression testing of PPTA and PIPD fibers, using a novel test device, are presented and discussed. In the tests, short lengths of single fibers are compressed between two parallel, stiff platens. The fiber elastic deformation is analyzed as a Hertzian contact problem. The inelastic deformation is analyzed by elastic-plastic FE simulation and by laser-scanning confocal microscopy of the compressed fibers ex post facto. The results obtained are compared to those in the literature and to the theoretical predictions of PPTA fiber transverse elasticity based on PPTA crystal elasticity.Presented at the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000).3TEX, Inc. 109, MacKenan Drive, Cary, North Carolina 27511, USA. Published in Mekhanika Kompozitnykh Materialov, Vol. 36, No. 4, pp. 533–544, March–April, 2000.