Water uptake and mechanical characteristics of wood fiber-polypropylene composites

The influence of mixing process (in a two-roll mill, high-speed mixer, or twin-screw extruder) on the strength properties of polypropylene/wood fiber composites was studied. The best results were obtained for composites compounded in a twin-screw extruder. The water uptake and the influence of moisture on the flexural strength (σ_fl) and modulus (E_fl) were studied by immersion of the composites in water at 20, 50, and 90°C. Most strongly the moisture affected the value of E_fl, but the degree of water uptake and the change in σ_fl and E_fl also depended on temperature and the presence of a modificator—maleated polypropylene (MAH). MAH improved the strength properties of the composites both in dry and wet states and also decreased the extent of water uptake and swelling in cyclic (soaking/drying) tests. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 1, pp. 101–114, January–February, 2006.     

Effect of the structure of a polymer matrix and its area of contact with the surface of the filler on the rigidity of a polyethylene composite

The distribution of the orientation of the macromolecules over the wall cross section of a filled polyethylene tube is analyzed. A direct proportionality is established between the degree of orientation (estimated from the shrinkage) and the elastic modulus. The effect of various fillers on the melting and crystallization of low-density polyethylene is considered. For a specified filler concentration the elastic modulus of the composites depends very considerably on the relative area of contact between the polymer matrix and the surface of the filler.Institute of the Mechanics of Polymers, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 3, pp. 387–391, May–June, 1974.     

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.     

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     

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.     

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.     

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.     

Magnetic impurity effect on superconductivity in systems with comparable Fermi and Debye energies

We obtain the basic system of equations for a superconductivity theory of a system with a chaotically distributed paramagnetic substitution impurity in which the Migdal theorem is violated (the condition ω_D ≪E _F cannot be assumed). The electron-phonon and impurity diagrams and also the additional diagrams corresponding to intersections of the electron-phonon and electron-impurity lines are taken into account. In the weak electron-phonon coupling limit, we obtain an equation for the superconducting transition temperature T_C that differs from the corresponding equation for the usual superconductors by renormalizations of T_C0 and of the impurity scattering parameter, ρ. These quantities depend essentially on the Migdal parameter ω_D/E _F and on the transferred momentum q_c. We show that the decrease of T_C with the increase of the impurity concentration is slowed, as compared with usual superconductors, to an extent determined by m and q_c. We also evaluate the isotopic coefficient α, whose behavior as a function of the impurity concentration depends on m and q_c. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 119, No. 3, pp 455–474, June, 1999     

Stress-strain state in the matrix of stochastically reinforced composites

Conclusion We proposed a method of studying the concentration of stresses and strains in the matrix of composites with a stochastic structure in a three-dimensional formulation. The method is based on the use of tensor operators assigned at the inclusion-matrix interface and results from the theory of effective moduli of stochastically reinforced composites.Advantages of the proposed approach include its relative simplicity and clarity, as well as the fact that it can be used to analyze the three-dimensional stress and strain concentration in the matrix of composites with components having very different properties. However, for high values of the volume concentration of reinforcement c₁ > 0.6, it is necessary to use the results of exact solutions obtained, for example, within the framework of deterministic models. The correction that is introduced here is connected with the average of the stresses and strains over an inclusion. The character of their distribution over the interface remains the same as before.The numerical results obtained here show the significant effect of the relative dimensions of the inclusions on the effective elastic properties and the stress concentration in the matrix. Comparison of theoretical values of the longitudinal elastic modulus with experimental results can serve as grounds for validating the proposed variant of choosing the tensor L₀ in the determination of the corrected characteristics and stress-strain state of the matrix.Translated from Mekhanika Kompozitnykh Materialov, No. 3, pp. 396–402, May–June, 1989     

Plane problems of stability of composite materials with a finite-size filler

The plane problem of three-dimensional stability is solved for a transversely compressed composite material reinforced with ribbons taking into account the inhomogeneous initial state. An approximate solution of the problems is based on the net method. The effect of the ribbon form factor, the ratio between the elastic moduli of the matrix and filler, and Poisson ratio of the filler on the critical deformation of the material is investigated. Translated from Mekhanika Kompozitnykh Materialov, Vol. 36, No. 1, pp. 77–86, January–February, 2000.     

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.     

Investigation of the three-dimensional micromechanical behavior of woven-fabric composites

A three-dimensional representative volume-element model is presented to study the micromechanical behavior of woven-fabric composites. The effects of the fiber undulation zone and the fiber braid angle on the elastic modulus of the composites are taken into account in the unit cell. Based on isostrain and isostress assumptions, a standard homogenization procedure is used to calculate the effective elastic properties of woven-fabric composites, and all the final stiffness components are expressed in an explicit form. The results obtained by the model considered agree with published experimental results very well. The relationship between the geometric parameters, such as fiber width, thickness, volume fraction, etc., and the macromechanical behavior of the composites can be obtained by this model. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 2, pp. 209–220, April–May, 2006.     

Methods of analysing ropes. The extension–torsion method

Two new approaches are used for calculating the stress–strain state of a rope and its stiffnesses. The first approach relies on the theory of fibrous composites and Saint-Venant's solution for a cylinder with helical anisotropy. The second approach is based on the solution by the finite element method of the three-dimensional problem of elasticity theory for a solid inhomogeneous cylinder formed by a finite number of elastic fibres arranged in helical lines and connected by a weak filler (in the sense that its Young's modulus is several orders of magnitude less than the Young's modulus of the fibre). The behaviour of the stiffness when the modulus of elasticity of the filler tends to zero is analysed, and the results of the limiting transition are discussed. The numerical results obtained are compared with calculations by other well-known applied theories.     

Raman spectroscopy investigation of stiffness change and residual strains due to matrix cracking

The matrix cracking models developed for cross-ply composite laminates have been poorly extended in the past to more complex geometries used in practice, and they are still under development. In this paper, a new detailed analysis of the effect of matrix cracking on the behaviour of cross-ply and [0/45]_s laminates under uniaxial tension is attempted. The model used in this work is applicable both to cross-ply laminates and unbalanced systems. It gives exact closed-form expressions for all thermomechanical properties of a general symmetric laminate with cracks in arbitrary layers. The theoretical approach is backed by experimental data obtained by microscopic strain-state variation measurements within a specimen, with using the technique of laser Raman spectroscopy. Glass-fibre-reinforced epoxy systems were investigated. Embedded aramid fibres-sensors within the 0° ply and near the 0°/θ ° interface were necessary due to the poor Raman signal of glass. Using experimental Raman data, the residual strain and the stiffness reduction are determined as functions of increase in crack density. The stiffness reduction is predicted with a high accuracy, whereas the measured residual strains are larger than predicted. The good results for the reduction in the elastic modulus show that the basic assumption of the model is accurate. The difference is explained by the viscoelastic-viscoplastic behaviour of the off-axis layer in shear, which in creases the “apparent” residual strain. Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 42, No. 6, pp. 771–786, November–December, 2006.     

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.     

Rheology of filled polymers. Steady-state shear flow and periodic deformation. 1. Relaxation time spectra,viscosity

Conclusions 1. The validity of the temperature-frequency superposition in the finite relaxation region has been confirmed for a concentrated solution of PIB filled with solid dispersed particles;2. the apparent heat of activation of the viscoelastic relaxation times increases at and above a certain filler concentration (c=15%);3. in the case of fillers for which the apparent heat of activation of the viscoelastic relaxation times is independent of the filler concentration, the filler only shifts the frequency functions of the components of the complex shear modulus along the logarithm of the modulus axis and the relaxation time spectrum H() along the log H axis. In this case G_C/G_M=G_C/G_M=_OC/_OM=_rel;4. the effective filler concentration for ccrit, as determined from the concentration dependence of the initial viscosity, exceeds the nominal filler concentration by a factor of 2.4 to 2.8;5. at a filler concentration c>c_crit, the contour of the relaxation time spectrum changes on account of the formation of a secondary filler structure;6. when the filler concentration ccrit, the dependencies of the viscosity on the shear rate for the composites and the matrix can be reduced with respect to two parameters;7. possible methods for the reduction of flow curves by means of linear coordinate transformations have been systematized.Institute of Polymer Mechanics, Academy of Sciences of the Latvian SSR, Riga. Translated from Mekhanika Polimerov, No. 6, pp. 1075–1083, November–December, 1977.     

Effect of the composition and concentration of fibrous wastes from linen yarn production on the physicomechanical properties of polyolefin composites

An analysis is made of the feasibility of using wastes from the production of linen yarm (scutch, spinning and carding refuse) to modify low-density polyethylene. The effect of the composition and concentration of wastes on the processing properties (melt index), physicomechanical indices (tensile strength, elastic modulus in bending), and water resistance of composites based on low-density polyethylene is studied. It is found that the melt index decreases with an increase in the content of filler and that even a composite with a high filler content (40–50% by weight) maintains values of 0.2–0.3 g/10 min. The elastic modulus in bending increases with an increase in the content of waste, regardless of the nature of the latter. Tensile strength increases slightly and depends on the choice of filler. This result, combined with the reducation in the scatter of values of elastic modulus with an increase in filler concentration, is an indicator of the relatively high degree of heterogeneity of the systems that were studied. The water resistance of the composites decreases with an increase in filleer content. To reduce the heterogeneity of the systems, mixing should be intensified and modifiers such as stearic acid and polyisocyanates should be added.Translated from Mekhanika Kompozitnykh Materialov, Vol. 33, No. 3, pp. 408–416, May–June, 1997.     

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.     

3-D time-dependent analysis of multilayered cross-anisotropic saturated soils based on the fractional viscoelastic model

The fractional Merchant viscoelastic model is introduced to simulate the viscoelasticity of soil skeleton in this study. According to the elastic-viscoelastic correspondence principle, elastic parameters including shear modulus G_v, horizontal elastic modulus E_h and vertical elastic modulus E_v are replaced by the reciprocal of the flexibility coefficient of viscoelastic media in the Laplace transformed domain. Then, based on the precise integration solutions of multilayered cross-anisotropic elastic saturated soils, 3-D solutions of viscoelastic saturated soils are derived. The final solutions in the physical domain are obtained by the Laplace numerical inversion. The correctness of theories and programs is verified by comparing the numerical results with existing references. Sensitivity analyses are conducted to investigate the effects of viscoelastic parameters, cross-anisotropic parameters and stratification of soils on time-dependent displacement and excess pore water pressure.     

Failure of particulate reinforced polymers

In this presentation, a review is given on the main effects of mineral particulate fillers (with an aspect ratio of about unity) on the deformation and fracture of amorphous and semicrystalline thermoplastic and thermosetting polymers. Elastomeric modifiers, polymer blends, and filled elastomers are not considered here. Fillers are generally used to reduce cost as well as the thermal sensitivity of mechanical properties of the matrix material and to improve, if possible, the strength and toughness. The addition of particulate fillers influences all stages of the fabrication and use of the resulting composites. We focus on the effects of a stiff second phase on elastic moduli, matrix structure, and on deformation, creep, and failure mechanisms. As the main mechanisms, particle-matrix debonding, void formation, and matrix microshear yielding are identified. Toughness is less sensitive to the quality of adhesion since particle-matrix debonding and formation of voids can be tolerated. If well controlled, debonding contributes to deformation (formation of voids should be well distributed in space and time). Reference is also made to the surprising and positive effect of CaCO₃ particles on the toughness and impact resistance of HDPE, which increases at small interparticle distances due to interfacial effects on lamellar growth in the ligament area. Submitted to the 11th International Conference on Mechanics of Composite Materials (Riga, June 11–15, 2000). Published in Mekhanika Kompozitnykh Materialov, Vol. 36, No. 3, pp. 305–316, March–April, 2000.