Estimation of the macroscopic stress-strain curve of a particulate composite with a crosslinked polymer matrix

The main focus of the present paper is the estimation of the macroscopic stress–strain behavior of a particulate composite. A composite with a cross-linked polymer matrix in a rubbery state filled with an alumina-based mineral filler is investigated by means of the finite-element method. The hyperelastic material behavior of the matrix is described by the Mooney–Rivlin material model. Numerical models on the basis of unit cells are developed. The existence of a discontinuity (breaking) in the matrix at higher loading levels is taken into account to obtain a more accurate estimate for the stress–strain behavior of the particulate composite investigated. The numerical results obtained are compared with an experimental stress–strain curve, and a good agreement is found to exist. The paper can contribute to a better understanding of the behavior and failure of particulate composites with a polymer matrix.     

Effect of anisometry of a platelike nanofiller on the elastic constants of a transversely isotropic composite

Analysis results for the elastic properties of a composite with a small amount of coplanarly arranged platelike filler particles are presented. The geometrical form of the particles is described by an oblate ellipsoid of revolution. The calculations are performed by formulas obtained by using the Eshelby approach for media with a low concentration of inclusions. The effect of anisometry of the ellipsoidal particles and of the ratio between the elastic moduli of the filler and matrix on the effective elastic constants of the composite is discussed. Calculation results are compared with experimental data for the elastic moduli of a nanocomposite containing completely exfoliated particles of an unmodified montmorillonite. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 4, pp. 493–504, July–August, 2008.     

Operational properties of a heat-insulation composite with expanded polystyrene crumbs

The operational properties (strength, heat conductivity, hygroscopic sorption, vapor permeability, and frost resistance) of a heat-insulation composite of density 150–350 kg/m³ are investigated. The matrix consists of an aerated concrete substance and a disperse filler of expanded polystyrene crumbs obtained by crushing the household polystyrene package waste. The macro-and microstructure of the composite are considered. It is found that the operational properties of the material directly depend on its density, determined by the amount of the cementing substance in the matrix. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 4, pp. 591–604, July–August, 2008.     

Numerical modeling of deformation and fracture of a multiphase polymer concrete

A numerical method for predicting the deformational and strength characteristics of a calcite-quartzitic polymer concrete from the known properties of its components is developed based on the finite-element method. Components of the material are assumed elastic and isotropic, and the filler particles are modeled by round inclusions perfectly bonded to the polymer matrix. The size distribution of the inclusions correspond to that of actual fillers. The destruction process of the components is simulated by sequentially excluding the particles in which the maximum principal stress has achieved the ultimate value for this component. A comparison of calculated and experimental characteristics of the polymer concrete showed errors of 2–4% for the elastic modulus and about 10% for the ultimate strength if the finite-element cell included not less than 20–30 average-size particles and 2–5 large ones. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 42, No. 6, pp. 817–824, November–December, 2006     

Modelling a new composite from a recycled GFRP

The mechanical reprocessing of different types of plastic scrap in high-energy disintegrator mills in direct and selective milling systems is considered. The grindabilty and separation of components of composites-an acrylic plastic and glass fibers-are analyzed. The relation between the density of the filler material and the fractions of a polymethylmethacrylate powder is modelled on the basis of experimental data. Artificial neural networks and a hybrid genetic algorithm are used to design a new composite from a recycled glass-fiber-reinforced plastic. Russian translation in Mekhanika Kompozitnykh Materialov, Vol. 44, No. 6, pp. 881–894, November–December, 2008.     

Interfacial strength of joints between fibers and dispersedly filled epoxy matrices

The concentration dependences of adhesive strength are investigated for fiber/dispersedly filled epoxy matrix systems. The measurements were carried out using an improved model of adhesiometer under normal conditions at a constant rate of loading. It is shown that the adhesive strength as a function of filler concentration has a maximum, which is more or less pronounced. The location of the maximum depends on the nature of filler and particle geometry. The increase in the adhesive strength at the maximum reaches 20–30% in comparison with that for the unfilled epoxy matrix. Since the interfacial strength between steel wire and all the mineral powders investigated is zero, the growth in the adhesive strength upon introduction of a finely divided filler in polymer binders is rather un expected. The possible reasons for the phenomenon observed are discussed. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 43, No. 1, pp. 3–14, January–February, 2007.     

Formation of a transition layer on the fillers of polymer composites

Based on a plane model of composites, the effect of a transition layer on the elastic modulus E_c of the composites is analyzed in the case where, under the action of a load, the transition layer is formed both on the side of matrix and filler. In evaluating E_c, it is assumed that the elastic modulus in the layer grows linearly from the elastic modulus of matrix to that of filler, but pores in the filler are impermeable to matrix macromolecules. Analytic relation ships are found which allow one to determine the volume fractions of the transition layer on the side of matrix and filler if the experimental elastic modulus of the composite is known. These relationships are used to find the magnitude of the layer in epoxy composites with various fillers and to evaluate its effect on the compressive elastic modulus of the composites. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 42, No. 5, pp. 693–700, September–October, 2006.     

Elasticity of composites with irregularly oriented shape-anisotropic filler particles

A variant of determining the elastic characteristics of composites containing irregularly oriented shape-anisotropic filler particles of two types (short fibers and thin platelets) is considered. The effective elastic constants of the composites are calculated by using the method of orientational averaging of elastic characteristics of isolated transversely isotropic structural elements reinforced with unidirectionally oriented short fibers or coplanarly arranged thin platelets. The superposition of elastic properties of the irregularly oriented structural elements, with account of their orientational distribution in the composite material, is accepted. The calculation results are compared with experimental data for the effective elastic moduli of polymeric composites reinforced with short glass fibers and of polymeric nanocomposites containing the platelet-type particles of organically modified montmorillonite. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 42, No. 3, pp. 285–300, May–June, 2006.     

Effect of dispersed filter particles on the strength behavior of composites

An attempt is made to find a solution by which the effect of neighboring filler particles on the strength of composites could be indirectly estimated. The stresses in composites with a great amount of filler are calculated. Composites based on epoxy-resin binder are tested. A comparison of different models shows that the modified Kerner model reflects only the tendency for the compression strength to change with the degree of filling. The relations between the compression strength and the degree of filling are better described by models allowing for constraint of the plastic deformations of the matrix. However, in this case, the effect of the neighboring particles is not properly taken into account. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 575–584, September–October, 1999.     

A nanocomposite based on a styrene-acrylate copolymer and native montmorillonite clay 2. Modeling the elastic properties

A variant of stepwise determination of the elastic characteristics of quasi-isotropic polymer-silicate nanocomposites with randomly oriented platelike filler particles is considered. First, the elastic constants of transversely isotropic structural elements with coplanarly placed filler particles are obtained, and then they are averaged over all spatial directions by using the orientational averaging method. The characteristic features of the hierarchical structure of exfoliated and intercalated nanocomposites are also considered. The concentration dependences of elastic constants of a nanocomposite based on a styrene-acrylate copolymer and native montmorillonite clay are modeled, and the results obtained are compared with experimental data. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 42, No. 2, pp. 235–246, March–April, 2006.     

Creep and Long-Term Strength of Press-Powder Composites

The mechanical state of a press-powder filler for pyroautomatics system units of aerospace equipment during their long-term storage is investigated. With the use of the endochronic approach, the rheological properties of a dispersedly reinforced composite are simulated based on the conceptions of linear and nonlinear creep. Analytical expressions of constitutive relations of creep and strength for the composites are obtained. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 6, pp. 803–818, November–December, 2005.     

The effect of C<Subscript>60</Subscript> fullerene on the toughening of epoxy compositions

The effect of C₆₀ fullerene on the mechanical properties of epoxy resins has been investigated. It is found that this filler affects the tensile modulus and tensile strength of epoxy compositions only slightly, but their impact strength at a 0.01–0.12 wt.% content of C₆₀ increases by about 100–200%. A molecular mechanism of the toughening effect of C₆₀ on epoxy resins is suggested.     

Deformation of a composite plate on an elastic foundation by local loads

Bending of an elastic annular composite plate with a light filler lying on an elastic foundation is considered. The plate is subjected to local loads. To describe the kinematics of the package, asymmetric across its thickness, the hypotheses of broken normal is accepted. The reaction of foundation is described based on the Winkler model. A system of equilibrium equations is constructed, and its exact solution in displacements is found. Numerical solutions for a metal-polymer sandwich plate are presented. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 43, No. 1, pp. 109–120, January–February, 2007.     

Polytetramethylene glycol-modified polycyanurate matrices reinforced with nanoclays: synthesis and thermomechanical performance

The outstanding improvement in the physical properties of cyanate esters (CEs) compared with those of competitor resins, such as epoxies, has attracted appreciable attention recently. Cyanate esters undergo thermal polycyclotrimerization to give polycyanurates (PCNs). However, like most thermo setting resins, the main draw back of CEs is brittleness. To over come this disadvan tage, CEs can be toughened by the introduction of polytetramethylene glycol (PTMG), a hydroxyl-terminated polyether. How ever, PTMG has a detrimental impact on Young’s modulus. To simultaneously enhance both the ductility and the stiffness of CE, we added PTMG and an organoclay (mont morillonite, MMT) to it. A series of PCN/PTMG/MMT nanocomposites with a constant PTMG weight ratio was pre pared, and the resulting nanophase morphology, i.e., the degree of filler dispersion and distribution in the composite and the thermomechanical properties, in terms of glass-transition behaviour, Young’s modulus, tensile strength, and elongation at break, were examined using the scanning elec tron micros copy (SEM), a dynamic mechanical analysis (DMA), and stress–strain measurements, re spectively. It was found that, at a content of MMT below 2 wt.%, MMT nanoparticles were distributed uniformly in the matrix, suggesting a lower degree of agglomeration for these materials. In the glassy state, the significant increase in the storage modulus revealed a great stiffening effect of MMT due to its high Young’s modulus. The modification with PTMG led to a 233% greater elongation at break compared with that of neat PCN. The nanocomposites exhibited an invariably higher Young’s modulus than PCN/PTMG for all the volume factors of organoclay examined, with the 2 wt.% material displaying the most pronounced in crease in the modulus, in agreement with micros copy results. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 45, No. 2, pp. 255–268, March–April, 2009.     

Dynamic properties of composite laminates and panels with highly contrasting directions of anisotropy

An asymptotic analysis of the dynamic stress-strain state of a thin laminar packet of anisotropic layers is presented. The statement is nonclassical, since in layer materials the ratio of elastic moduli in the longitudinal and transverse directions can generate small parameters comparable to the relative half-thickness of the packet, as, for example, in high-strength unidirectional composites. Alternation of strong load-carrying layers and a relatively soft filler with a similar difference in the elastic moduli between the layers is also allowed. The averaged two-dimensional equations and the total stress tensor in the layers are determined. The results are classified with respect to the types of anisotropy and the indices of differences in the elastic moduli. It is shown that first-approximation models lead to kinematic relations similar to those of the theories of high-order shear strains. Institute of Problems of Mechanics, Russian Academy of Sciences, Moscow, Russia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 5, pp. 605–614, September–October, 1999.     

Comparative Studies on the Mechanical Properties of a Thermoset Polymer in Tension and Compression

Results of an experimental investigation into the mechanical properties of a polyester resin in tension and compression are reported. Features of the stress-strain curves obtained are discussed. Data on the elastic modulus, Poisson ratio, and volume strains are obtained. The results of creep behavior of the material in tension and compression are also presented. It is found that the time-dependent creep obeys a power law, but the nonlinear stress dependence can be described by using the hyperbolic sine function. The effect of load type (tension or compression) on the nonlinearity of the creep is analyzed. __________ Translated from Mekhanika Kompozitnykh Materialov, Vol. 41, No. 5, pp. 633–650, September–October, 2005.     

Contact problems of thermoelasticity for a half-space of a functionally gradient material

We propose a method of solution of problems of thermoelasticity for an inhomogeneous half-space. We assume that Poisson's ratio of a material in the half-space is constant, and Young's modulus and the coefficients of linear thermal expansion and thermal conduction vary exponentially with distance from the surface of the half-space. As an example, we consider the contact problem on sliding an inhomogeneous body along the surface of a rigid base with regard for frictional heating. I. Franko L'viv University, L'viv; Warsaw University, Warsaw. Translated from Matematychni Metody ta Fizyko-Mekhanichni Polya, Vol. 41, No. 2, pp. 45–56, April–June, 1998.     

Investigation of free vibrations of composite beams by using the finite-difference method

The free-vibration behavior of symmetrically laminated fiber-reinforced composite beams with different boundary conditions is examined. The effects of shear deformation and rotary inertia, separately and/or in combination, on the free-vibration properties of the beams are investigated. The finite-difference method is used to solve the partial differential equations describing the free-vibration motion in each case. The effect of shear deformation on the natural frequencies is considerable, especially for higher frequencies, whereas the influence of rotary inertia is less significant. The study includes comparisons with results available in the literature. In addition, the impact of such factors as the span/depth ratio, fiber orientation, stacking sequence, and material type on free vibrations of the composite beams is investigated. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 3, pp. 331–346, May–June, 2006.     

Moisture permeability of a polymer nanocomposite containing unmodified clay

Results of an investigation into the kinetics of moisture sorption by an unfilled styrene-acrylate copolymer and a nanocomposite containing unmodified montmorillonite clay are reported. The sorption tests were performed at 25°C and different levels of the relative humidity of air up to 80%. The moisture diffusivity was calculated using Fick's equation for one-dimensional diffusion. Data on the influence of montmorillonite concentration on the coefficients of diffusion, solubility, and permeability are obtained. The experimental values of the permeability coefficient are compared with those calculated by using a model taking into account the increased path of a diffusing water molecule caused by the shielding effect of plate like filler particles. Translated from Mekhanika Kompozitnykh Materialov, Vol. 44, No. 5, pp. 723–736, September–October, 2008.     

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.