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Mechanics of Composite Materials (v.42, #3)

Elasticity of composites with irregularly oriented shape-anisotropic filler particles by A. Lagzdins; R. D. Maksimov; E. Plume (pp. 197-208).
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.

Keywords: short fibers; platelets; fibrous composite; platelet-reinforced composite; nanocomposite; elastic constants; orientational averaging

Fracture of a piezoelectric fiber/elastic matrix composite by B. L. Wang; J. C. Han (pp. 209-222).
A piezoelectric fiber/elastic matrix system subjected to axially symmetric mechanical and electric loads is considered. The fiber contains a penny-shaped crack located at its center perpendicularly to the fiber. By using the Fourier and Hankel transforms, the problem is reduced to the solution of an integral equation. Numerical solutions for the crack tip fields are obtained for various crack sizes and different fiber volume fractions.

Keywords: fracture mechanics; piezoelectric materials; composite materials

Wear resistance of bodies protected by a thin composite coating by A. V. Maksymuk; N. N. Shcherbyna (pp. 223-230).
The mechanical contact interaction of bodies with a thin composite coating is investigated with account of wear. The thermal effects are not considered. The coating is modeled by a thin plate. Between the body and the coating is an interlayer, which is modeled by a Winkler body with one modulus of subgrade reaction. Under the action of a rigid stamp on the coating, the process of abrasive wear proceeds. The contact interaction of the coating with the base is described by using the model of an intermediate layer. To determine the stress-strain state of the coating, equations of the generalized theory of plates including the shear strains and the compression of normal are utilized. For the contact wear problem formulated, the basic integral equation with a Fredholm-type kernel is derived, and its solution algorithm is proposed. Numerical results are presented.

Keywords: wear resistance; coating; composite material; contact interaction

Investigation of free vibrations of composite beams by using the finite-difference method by K. S. Numayr; M. A. Haddad; A. F. Ayoub (pp. 231-242).
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.

Keywords: beams; laminates; composite materials; free vibration; natural frequency; modal shape; shear deformation; rotary inertia

The limiting state of a rigidly fixed nonlinearly elastic multilayer rod by R. Yu. Amenzadeh; E. T. Kiyasbeyli; 1L. F. Fatullaeva (pp. 243-252).
The loss of the load-carrying capacity of a nonlinearly elastic multilayer rod is investigated. The rod, whose layers have various thickness and are made of different materials, is rigidly fixed at both its ends. Rigid contact conditions between the layers are assumed. The problem posed is solved by using the variational method of mixed type in combination with the Rayleigh-Ritz method. The initial analysis is reduced to the solution of the Cauchy problem for a nonlinear ordinary differential equation solved for the first derivative. As the initial condition, the maximum initial eccentricity of the rod is assumed. In the case of zero eccentricity, the Shanley critical force for an axially compressed rod is determined. For a three-layer rod whose outer layers have equal thickness and are made of the same material, numerically, for various degrees of nonlinearity, the effect of physicomechanical and geometric parameters on the critical load of buckling instability is determined. It is found that, by matching the heterogeneity of the rod, it is possible to raise its load-carrying capacity.

Keywords: multilayer rod; buckling; stability; packet; critical force; rigid fixing

Quality rating of a metal matrix-diamondcomposite from its thermal conductivity and electric resistance by N. V. Novikov; A. L. Maystrenko; V. I. Kushch; S. A. Ivanov (pp. 253-262).
The efficiency of hot-pressed diamond-containing composite materials (DCM) for various tool applications is greatly affected by microdefects, namely, the residual porosity of the metal matrix, damaged diamond grains, and imperfect diamond-matrix interfaces. An instrumental evaluation of these microdefects, predetermining the quality of a tool equipped with DCM, is rather difficult due to the small size, the nonstandard shape, and the strong heterogeneity of specimens. Proposed here is an alternative, nondestructive technique of DCM quality rating, which includes the measurement of electric resistance and thermal conductivity of diamond-containing composites and processing the obtained data by the methods of composite mechanics. It exploits the fact that diamond, being a dielectric, possesses an extremely high thermal conductivity, which allows estimating the residual porosity of a sintered metal matrix from the ratio of specific electric resistances, one being measured and another predicted by a theory. These data, in turn, are utilized to predict the thermal conductivity ofDCMwith an imperfect matrix. Matching with experiments, after solving the inverse problem gives the thermal resistance of diamond-matrix interface, which, within the frame work of the given model, simulates the damage of both the diamond grains and their bonds with the matrix. Thus, the numerical rating of quality is given in terms of two dimensionless parameters. The first one, 0 < K < 1, reflects the quality of the sintered metal matrix, whereas the second one, 0 < R <1, is an aggregate measure of the integrity of diamond grains and the perfection degree of composite interfaces. The quite satisfactory agreement observed between the theory and experiment confirms the efficiency of the technique and the reliability of the data obtained.

Keywords: diamond-containing composites; imperfections; nondestructive testing; thermal conductivity; electric resistance; quality rating

Nanocomposites based on a styrene-acrylate copolymer and organically modified montmorillonite 1. Mechanical properties by R. D. Maksimov; S. Gaidukovs; J. Zicans; M. Kalnins; E. Plume; V. Spacek; P. Sviglerova (pp. 263-272).
The preparation of polymer nanocomposites by mixing a solution of a styrene-acrylate copolymer with a suspension of organically modified montmorillonite in dimethyl formamide is described. Seven different compositions with organomontmorillonite content from 0 to 7 wt.% were prepared and tested. Results of their X-ray diffraction analysis are presented. Data on the influence of organomontmorillonite content on the tensile stress-strain curves, elastic modulus, strength, and ultimate elongation of the nanocomposites are obtained. The concentration dependences of elastic properties of materials with differently oriented platelike nanoparticles is analyzed by using an algorithm elaborated for stepwise calculations of elastic constants with account of the features of structural hierarchy of intercalated and exfoliated nanocomposites.

Keywords: styrene-acrylate copolymer; organically modified montmorillonite; exfoliated nanocomposite; intercalated nanocomposite; mechanical properties

Strength of biodegradable polypropylene tapes filled with a modified starch by N. S. Vinidiktova; O. A. Ermolovich; V. A. Goldade; L. S. Pinchuk (pp. 273-282).
The possibility of creating composite materials with high deformation and strength characteristics based on polypropylene (PP) and a natural polysaccharide in the form of a modified starch (MS) has been studied. The modified starch is shown to interact chemically with functional groups of PP, thereby positively affecting the physicomechanical properties, structure, and water absorption properties of films and oriented flat fibers based on starch-filled PP. The strength characteristics of both oriented and unoriented composites are 1.5–2.0 times as high as those of the initial PP. The water absorption ability of the materials varies symbatically with content of MS, which points to the dominant contribution of interactions at the PP-MS interface. The introduction of MS into synthetic polymers offers a possibility of producing new ecologically safe materials with high strength characteristics.

Keywords: polypropylene; modified starch; strength; plasticizer; oriented stretching; structure; IR spectroscopy; dynamic losses; water absorption

Influence of SiO2 in SiCp on the microstructure and impact strength of Al/SiCp composites fabricated by pressureless infiltration by M. I. Pech-Canul; F. Ortega-Celaya; M. A. Pech-Canul (pp. 283-296).
The effect of SiO2 in SiCp and the following processing parameters on the microstructure and impact strength of Al/SiCp composites fabricated by pressureless infiltration was investigated: Mg content in the aluminum alloy, SiC particle size, and holding time. Preforms of SiCp in the form of rectangular bars (10 × 1 × 1 cm) were infiltrated at 1150°C in an argon→nitrogen atmosphere for 45 and 60 min by utilizing two aluminum alloys (Al-6 Mg-11 Si and Al-9 Mg-11 Si, wt.%). The results obtained show that the presence of SiO2 in SiC affects the microstructure and impact strength of the composites significantly. When Al4C3 is formed, the impact strength decreases. However, a high proportion of SiC to SiO2 limits the formation of the unwanted Al4C3 phase in the composites. Also, a higher content of Mg in the Al alloy lowers the residual porosity and, consequently, increases the composite strength. The impact strength grows with decrease in SiC particle size and increases considerably when the residual porosity is less than 1%.

Keywords: Al/SiCp composites; pressureless infiltration; impact strength

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