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.     

Elastic and Sorption Characteristics of an Epoxy Binder in a Composite During Its Moistening

Results of an experimental investigation into the elastic and sorption characteristics of a model composite material (CM) — epoxy resin filled with LiF crystals — during its moistening are presented. Properties of the binder in the CM with different filler contents (v _f = 0, 0.05, 0.11, 0.23, 0.28, 0.33, 0.38, and 0.46) were evaluated indirectly by using known micromechanical models of CMs. It was revealed that, for the CM in a conditionally initial state, the elastic modulus of the binder in it and the filler microstrain (change in the interplanar distance in the crystals, measured by the X-ray method) as functions of filler content had the same character. The elastic modulus of the binder in the CM with a low filler content was equal to that for the binder in a block; the elastic modulus of the binder in the CM decreased with increasing filler content. The maximum (corresponding to water saturation of the CM) stresses in the binder and the filler microstresses as functions of filler content were of the same character. Moreover, the absolute values of maximum stresses in the binder and of filler microstresses coincided for high and low contents of the filler. At v _f = 0.2–0. 3, the filler microstrains exceeded the stresses in the binder. The effect of moisture on the epoxy binder in the CM with a high filler content was not entirely reversible: the elastic characteristics of the binder increased, the diffusivity decreased, and the ultimate water content increased after a moistening-drying cycle.__________Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 4, pp. 499–511, July–August, 2005.     

Resistance to interlayer shear of plastics on a carbon filament base

Conclusions 1. An equation has been derived for the strength during interlayer shear along reinforcing fibers of a composite material of unidirectional texture which allows calculation of this index as a function of composition of the material and state of the interphase boundary. Experimental confirmation of this equation has shown that the effectiveness of surface treatment of the reinforcing filler can be estimated by its use.2. It has been shown that the strength during interlayer shear of an epoxy-phenol plastic can be increased by a factor of more than 3, and during bending by a factor of 1.3, as the result of treatment of high-modulus carbon fibers in nitric acid.3. A correlation has been established between changes in electrical resistivity of the carbon fiber and the molecular component of adhesion to it of consolidated epoxy-phenol binder.Moscow. Translated from Mekhanika Polimerov, No. 3, pp. 445–451, May–June, 1977.     

The effect of interphase on residual thermal stresses. 2. Unidirectional fiber composite materials

In real composite materials an additional phase may exist between the fiber and the matrix. This phase, commonly known as the interphase, is a local region that results from the matrix bonds with the fiber surface or the fiber sizing. The differing thermal expansions or contractions of the fiber and matrix cause thermally induced stresses in composite materials. In the present study, a four-cylinder model is proposed for the determination of residual thermal stresses in unidirectional composite materials. The elastic modulus of the interphase is a function of the interphase radius and thickness. The governing equations in terms of displacements are solved in the form of expansion into a series [1]. The effective elastic characteristics are obtained using the finite element approach. The effect of the interphase thickness and different distributions of the interphase Young's modulus on the thermal residual stress field in unidirectional composite materials is investigated.For Pt. 1, see [1].Published in Mekhanika Kompozitnykh Materialov, Vol. 33, No. 2, pp. 200–214, March–April, 1997.     

Effect of fillers on the structure and properties of cured resins

The elastic (modulus of elasticity and equilibrium high-elastic modulus) and thermal (volume coefficients of thermal expansion below and above the glass transition temperature) properties of compositions based on ÉD-5 epoxy resin cured with polyethylenepolyamine have been investigated. Quartz powder and aluminoborosilicate glass powder were employed as fillers at concentrations from 0 to 0.413. The thermal expansion coefficients of the compositions were studied in a dilatometer, in which the specimen is free of mechanical loads. The Young's modulus at 25°C and the equilibrium high-elastic modulus at 125°C of the compositions were determined in the compression regime in an instrument based on the IZV-2 optical length gage. The thermal expansion coefficients of the polymer matrix were calculated with allowance for the elastic properties of the resin and the filler. It is shown that, as the filler concentration increases, the thermal and elastic properties of the resin in the filled system change. This can be interpreted as a change in the properties of the resin as it approaches the surface of the filler particles. Increased interaction between the filler surface and the epoxy resin tends to stiffen the polymer network.Scientific Research Institute of Precision Technology, Moscow. Translated from Mekhanika Polimerov, No. 6, pp. 1018–1022, November–December, 1969.     

Effect of a filler on the strength properties of thermosets

The effect of a filler on the strength properties of polymers in tension is investigated. The thermostructural stresses that develop in the composite during cure are taken into account. Relations are given for the strength of the filled polymer as a function of the percentage filler content. In the process of analyzing the thermostructural stresses an analytic expression is obtained for the linear expansion coefficient of the composite with allowance for the structural distribution of the components. Calculated values of the strength and thermostructural stresses are presented for composites with different filler contents. The theoretical determination of the strength of filled polymers is compared with the results of experimental investigations of composites based on epoxy resin filled with quartz dust.Leningrad Mechanical Institute. Translated from Mekhanika Polimerov, No. 1, pp. 97–101, January–February, 1973.     

Effect of curvature of the reinforcement on the mechanical and thermophysical properties of a composite

The elastic constants and thermal expansion coefficients of a composite consisting of a matrix with low elastic properties and exceptionally stiff fibers have been experimentally determined. The curvature of the reinforcement in the specimens has been investigated. The results of the experiments are compared with the corresponding theoretical characteristics. The calculations are based on a model with an ideal arrangement of the fibers [4] and a model that takes the curvature of the reinforcement into account [5, 6].Moscow Power Engineering Institute. Translated from Mekhanika Polimerov, No. 6, pp. 1036–1039. November–December, 1971.     

Kinetics of absorption of water by filled epoxide polymers

The kinetics of the absorption of water by composite materials based on an epoxide binder and glass microspheres were investigated. It was shown that the absorption of water by these materials has the character of a stage mechanism due to destruction of the adhesion between the binder and the microspheres. Treatment of the glass spheres with a coupling agent not only reduces the water absorption but also changes the character of the kinetic curve for water absorption. It was shown that a thermomechanical method can be used to evaluate the stability of the adhesion bonds between the polymer and the filler under the influence of water.Vladimir Scientific-Research Institute of Synthetic Resins. Translated from Mekhanika Polimerov, No. 2, pp. 290–294, March–April, 1972.     

Thermal expansion of a polymer composite with an aggregating disperse filler

Conclusion The thermal deformation of HDPE with an aggregating disperse filler was experimentally studied in a wide range of temperatures. The effect of the filler on the characteristics of relaxation transitions in HDPE, determined from dilatometric tests, was analyzed. A method of calculating the effective thermal expansion coefficient of a composite with an aggregating filler was proposed. Satisfactory agreement between the calculated and experimental data was obtained. It was shown that the effect of aggregation results in a significant decrease in the thermal expansion coefficient of the composite.Translated from Mekhanika Kompozitnykh Materialov, No. 1, pp. 70–77, January–February, 1989.     

Influence of the concentration of carbon nanotubes (CNT) on the thermophysical properties of epoxy/CNT nanocomposites at low temperatures

The influence of the content of carbon nanofillers (multi-and single-wall nanotubes) on the thermophysical properties of epoxy nanocomposites was investigated on the temperature range from −150 to 150°C. A “plateau” was found to exist in the concentration dependence of thermal conductivity on the concentration interval from 0.1 to 1.0 wt.% carbon nanotubes (CNTs). The thermal conductivity of the CNT composites exceeded that of pure epoxy resin by about 40%. A further increase in CNT content de creased the conductivity, owing to increasing interfaces between the two phases and the additional thermal resistance caused by phonon scattering on them. It is found that the temperature interval of transition of the composite from a glassy to a viscoelastic state greatly depends on the filler type and concentration. There exists a critical concentration at which a drop in the glass-transition temperature by 30% can be observed. The reason is the undercure of binder as a result of interaction between CNTs and epoxy macromolecules, which reduces the cross-linking density of structure of the polymer. Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 5, pp. 697–708, September–October, 2008.     

Effect of a uniaxial load on moisture absorption by an epoxy binder

Conclusion The above-developed method of studying the effect of uniaxial compressive and tensile loads on moisture absorption in polymers was tested on epoxy binder EDT-10. It was established experimentally for this material that tensile stresses up to 50 MPa significantly increase the equilibrium moisture content (up to 20% of the initial value). There is a corresponding increase in the rate of moisture absorption during the initial stage of the sorption process — up to 80% of the initial value. No changes in the diffusion coefficient were seen in this case. The application of compressive stresses of up to 40 MPa also had no effect on the kinetics of moisture absorption.Deceased.Translated from Mekhanika Kompozitnykh Materialov, Vol. 29, No. 1, pp. 110–115, January–February, 1993.     

Investigation of the state of water sorbed by an organic polymer by high-resolution1H nuclear magnetic resonance and differential thermal analysis

Conclusions The results of the experimental investigation of a moist organic composite by high-resolution NMR and DTA indicate a change in the structure of water, due to an interaction between the water molecules and the macromolecules when water is sorbed by the organic polymer. A part of the sorbed water molecules is strongly bound to the macromolecules; their mobility is very low and is not recorded by high-resolution NMR. As the moisture content of the composite increases, molecules of water appear, the mobility of which is intermediate between the strongly bound and free water molecules in the macrospace. This part of the molecules is represented in the NMR spectrum by a broadened and shifted line and a shift of the endothermic peak towards lower temperatures is observed on the thermal analysis diagrams. The weakly bound water can be detected in the composite starting at a moisture content of 5–8%.Translated from Mekhanika Kompozitnykh Materialov, No. 2, pp. 317–320, March–April, 1990.     

Taking account of heat transfer in locally heated thermoelastic plates made out of composite materials

Conclusions The proposed approach to the determination of the thermal stress state of thin plates made out of composite materials permits taking into account the effect of the coefficient of heat transfer from a region of local heating outside of it. The significant effect of heat transfer from the heating zone on the temperature field and the stresses indicates the necessity of taking it into account in connection with calculations on the strength of composite plates subjected to local heating.Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 1027–1030, November–December, 1979.     

Aging of polymer composite materials exposed to the conditions in outer space

A comprehensive investigation is made of glass, carbon, organic fiber-reinforced plastics, and epoxy-based hybrid composite materials employed in Salyut-type spacecraft which remained in space for up to 1501 1501 days. In particular, the properties, aging mechanism, and strain-strength variations in these materials due to exposure to the conditions in outer space were studied. After a series of tests were performed in space the standard strain and strength parameters as well as the mass, density, and thickness changes in the composite materials were estimated. Electron-microscopic and dynamic-mechanical analyses were performed, and the thermal expansion was estimated for a wide range of temperatures. The principal, dominant process occurring due to the continuous presence in outer space was found to be post-curing of the resin materials, which in turn affected the mechanical characteristics of the composite materials. After 456–1501 days in space the room-temperature strength of the composite materials (except for organic plastics) did not decrease, while at high temperatures it even increased. The post-curing and restructuring of some composite materials lowered their dynamic shear moduli in the glassy state of the resin. Due to consolidation of the surface layer of hybrid composite materials irradiated and subjected to thermal cycles, failure during bending varied from transverse fracture to delamination. The negative effect of the post-curing process was expressed as higher internal tension in the hybrid composite materials with different linear thermal expansion coefficients. The magnitude of this effect depended on the amplitude of the thermal cycles. The unprotected surface of the composites bombarded by atomic oxygen, microparticles, and space garbage were subjected to pickling and microerosion, the maximum effect occurring at the initial stage of exposure. Desorption of moisture and low-molecular products during the first 100–200 days of thermal cycling in the vacuum of near-earth orbit must be considered when estimating the total mass loss of composite materials. Data from microscopic, dynamic-mechanical, and other types of analyses revealed that the outer-space factors improved the supermolecular order of the resin volume, while the subsurface layer structure of the composite materials had loosened. Microcracks formed in the plastic's surface during 1501 days in outer space did not, in general, affect the mechanical parameters of the composite materials. Most of the observed effects of exposure to conditions in outer space were less pronounced for plastics protected by aluminum foil or other plastic coatings. The data obtained can be used for designing external elements of spacecraft by selecting materials with specified and predictable properties for long-term service.Translated from Mekhanika Kompozitnykh Materialov, Vol. 29, No. 4, pp. 457–467, July–August, 1993.In conclusion we thank I. G. Zhigun and R. P. Shlits for assisting in determining the mechanical properties of PCM as well as the crew of the Salyut-6 and Salyut-7 space stations for setting up, monitoring, and delivering samples to earth.     

Effect of mineral fillers on properties of composite matrix material

The influence of mineral fillers on thermomechanical properties of matrix material of composites is investigated. Different methods to determine elastic properties and thermal expansion coefficients of composite materials have been considered and compared. Injection moulded polyester samples containing varying concentrations of talc filler are tested and properties such as Young 's modulus, thermal expansion coefficients, and volumetric shrinkage during cure are measured. Results obtained by theoretical models and from experiments are compared and discussed.To be presented at the Ninth International Conference on the Mechanics of Composite Materials (Riga, October 1995).Published in Mekhanika Kompozitnykh Materialov, Vol. 31, No. 4, pp. 435–445, July–August, 1995.     

Calculation of characteristics of the elastic and thermophysical properties of a multiphase composite containing composite or hollow spherical inclusions

Conclusion We generalized the self-consistent method of effective media to the case of a four-phase model consisting of a core, a shell, a binder, and the effective medium. We obtained analytic solutions for the elastic characteristics, coefficient of linear expansion, heat capacity, and thermal conductivity of a multiphase composite containing several types of composite (or hollow) spherical inclusions. In the special case of a composite containing inclusions of just one type, the solutions obtained for the bulk modulus of elasticity K, coefficient of linear expansion a, heat capacity c, and thermal conductivity agree (within the framework of the two-stage approach) with the values found using known solutions for a three-phase model [8]. The first stage entails calculation of the effective characteristics of a spherical composite inclusion, while the second stage involves calculating the analogous characteristics for the composite as a whole.The possibilities of the solutions that were found were illustrated in a calculation of the shear modulus of a composite containing spherical hollow inclusions. It was shown that by assuming a nonaxisymmetric Weibull distribution of the parameter (the ratio of the thickness of the wall of a particle to its radius) it is possible to reach better agreement between the calculations and the experimental data in [4] than when calculations are performed using only the mean value of .The solutions obtained here can be used to find optimum combinations of volume fractions of different types of fillers in multiphase composites.The work was sponsored at the University Iberoamericana in 1994 by the Mexican National Council of Science and Technology (CONACYT).Translated from Mekhanika Kompozitnykh Materialov, Vol. 30, No. 4, pp. 512–519, July–August, 1994.     

Solution of the stochastic boundary-value problem of elasticity theory for composites with disordered structures in the correlative approximation of the method of quasi-periodic components

The method of quasi-periodic components, a new method of statistical mechanics of composites, is presented. In correlative approximation, this method makes it possible to reduce the problem of solving the stochastic boundary-value problem of elasticity theory for composites with disordered structures to a simpler problem for an individual cell with one inclusion in a homogeneous elastic medium. The generalized volumetric forces on the cell boundary take into account the random distribution of inclusions in the composite fragment studied. The problem for one inclusion cell can be solved, for example, by the boundary element method. The numerical solution in the correlative approximation of the method of quasi-periodic components for inhomogeneous interphase stress fields in the matrix of an epoxy composite containing randomly distributed unidirectional fibers is given. A comparison with the known analytical solutions obtained by other authors confirms the high accuracy of the correlative approximation.Perm' State Technical University, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 4, pp. 465–478, July–August, 1999.     

Thermal expansion of a composite with a hybrid granular-fibrous filler

Conclusion The thermal deformation of LDPE with a granular-fibrous filler was studied experimentally within a broad temperature range. We analyzed the effect of fillers on characteristics of relaxational transitions in LDPE as determined from dilatometric tests. A method was proposed for calculating the effective CLE of a composite with allowance for aggregation of a powdered filler and the statistical distributions of the length and orientation of the anisotropic fibers. The method was substantiated experimentally. Results were also presented of a theoretical analysis of the effect of the parameters of the fiber distributions f(l.) and (, ) and the relative volume contents of granular and fibrous fillers on _ij for the composite. It was established that aggregation of the granular filler may produce a hybrid effect consisting of maximum suppression of thermal expansion of the binder by the combination filler relative to a two-component composite.Translated from Mekhanika Kompozitnykh Materialov, No. 2, pp. 237–246, March–April, 1989.     

Prediction of the thermal conductivity of oriented fiberglass plastics

An analytical equation has been derived for calculating the thermal conductivity of fiberglass plastic pipes in the axial direction. The pipes, which represent items with the most general type of reinforcement, were manufactured by uninterrupted slanting and longitudinal-transverse winding. Equations for calculating the thermal conductivity of oriented fiberglass plastics with other types of reinforcement featuring various angles to the direction and plane of reinforcement are particular cases of the derived equation. The results of an experimental check for this equation for calculating the thermal conductivity of epoxy fiberglass plastics are presented.Translated from Mekhanika Kompozitnikh Materialov, Vol. 29, No. 3, pp. 381–386, May–June, 1993.     

Some characteristics of the use of the acoustic emission method in the study of the fragmentation of single fibers in a polymer matrix

Conclusions The energy spectra of acoustic signals during stretching of an epoxy specimen reinforced with carbon fiber and of a pure epoxy matrix differ substantially. Results of a comparison of these spectra make it possible to distinguish acoustic emission signals due to failure of the fiber during stretching of a reinforced composite and to plot curves of the average length of failing fiber fragments as a function of strain.Translated from Mekhanika Kompozitnykh Materialov, No. 6, pp. 734–740, November–December, 1993.