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Mechanics of Composite Materials (v.38, #4)

Adhesion of Thermoplastic Matrices with Different Molecular Weights to Fibers by N. V. Korneeva; Yu. A. Gorbatkina; V. G. Ivanova-Mumjieva; V. I. Nedel'kin; L. M. Bolotina (pp. 238-290).
The dependence of the interfacial strength in polymer-fiber systems on the molecular weight (MW) of the matrix is investigated. Adhesive joints of poly(phenylene-sulfide sulfones) and polysulfones with a steel wire 150 μm in diameter and glass fibers 200-300 μm in diameter are examined. The MW of both polymers was controlled during the synthesis process. The shear adhesive strength τ was determined by the pull-out technique. For all the systems investigated, it is shown that the adhesive strength in relation to the duration t f of their isothermal formation is described by curves with a maximum, whose values depend on the formation temperature T f. For each polymer with a given MW, the greatest possible of these values, called “optimum” and denoted by τopt, are determined. It is found that τopt increases with molecular weight. It is speculated that the effect observed stems from the different packing of polymer chains with distinct lengths in the near-surface layers.

Keywords: molecular weight; fiber-polymer joints; polymer chains; shear adhesive strength; pull-out technique; substrate; adhesive

Creep of Poly(Vinyl Chloride)/ Chlorinated Polyethylene Blends by R. D. Maksimov; T. Ivanova; J. Zicans (pp. 291-298).
The results of experimental investigations of the creep behavior of blends of poly(vinyl chloride) (PVC) with chlorinated polyethylene (CPE) are presented. Eight types of specimens with the PVC/CPE weight ratios of 100/0, 90/10, 80/20, 60/40, 40/60, 20/80, 10/90, and 0/100 are examined. The creep tests were continued for 1000 h. It is discussed how the blend composition affects the elastic and inelastic behavior of the material. The elastic compliance of the blend can be determined from the properties of its components by using the Kerner and Budiansky equations for heterogeneous systems with a phase structure of statistic-dispersion type. The creep compliance (the total current compliance minus the elastic compliance) obeys the power law of creep with coefficients depending on the blend composition.

Keywords: poly(vinyl chloride); chlorinated polyethylene; polymer blend; creep

Effect of Water Absoption, Elevated Temperatures and Fatigue on the Mechanical Properties of Carbon-Fiber-Reinforced Epoxy Composites for Flexible Risers by J. O. Jansons; K. Glejbøl; J. Rytter; A. N. Aniskevich; A. K. Arnautov; V. L. Kulakov (pp. 299-310).
The influence of water absorption, under different temperatures and thermal aging in an oven, on the elastic and strength characteristics of carbon-fiber-reinforced epoxy composites is investigated by comparative tests in three-point bending. The tension-tension fatigue behavior of the composites is also studied.

Keywords: flexible riser; three-point bending; water absorption; fatigue

Nonlinear Deformational Properties of Dispersely Strengthened Materials by L. P. Khoroshun; E. N. Shikula (pp. 311-320).
A method and an algorithm for determining the effective deformational properties of dispersely strengthened materials with a physically nonlinear matrix and quasi-spheroidal linearly elastic inclusions are elaborated based on the stochastic differential equations of the physically nonlinear theory of elasticity. Their transformation to integral equations and the application of the method of conditional moments reduce the problem to a system of nonlinear algebraic equations, whose solution is constructed by the iteration method. The deformation diagrams as functions of the volume content of inclusions are investigated.

Keywords: composite; stochastic equation; probability; effective properties; disperse particles; macrostresses; macrostrains; conditional moment; fluctuations; averaging; physical nonlinearity

Mixed Antiplane Problem of Electroelasticity for a Piezoceramic Half-Space with a Tunnel Hole by D. Bardzokas; M. L. Fil'shtinskii (pp. 321-328).
Harmonic vibrations of a piezoceramic half-space weakened by a tunnel hole with a system of active surface electrodes are investigated. Based on integral representations for solutions to two types of conditions on the boundary of the half-space, the boundary-value problem of electroelasticity is reduced to a system of singular integrodifferential equations of the second kind. The results of parametric investigations characterizing the behavior of the electroelastic field components on the boundary and in the region of a piecewise-homogeneous half-space are presented.

Keywords: electroelasticity; piezoceramics; half-space; hole; electrode; singular integrodifferential equation

Problem-Oriented Functionals in the Theory of Nonlinearly Elastic Composite Shells by A. N. Guz'; V. A. Maksymyuk; I. S. Chernyshenko (pp. 329-334).
Based on the Kirchhoff-Love or Timoshenko hypotheses and with regard for a possible membrane or shear degeneration, mixed linearized functionals for four variants of shell theory are presented. The convergence of numerical methods is improved by choosing small strain components as additional variable functions. New classes of problems for thin and nonthin shells are solved. The stress-strain state of shells is studied using different variants of this theory.

Keywords: orthotropic composite shells; physical nonlinearity; membrane and shear degeneration; “locking”; mixed functionals

Effect of Interruptions in Loading on the Durability and Deformability of Structural Plastics by Yu. G. Korabel'nikov (pp. 335-350).
An alternative way of estimating the durability of structural plastics under stresses σ = const ∼ 0.4-0.8σff is the failure stress) based on test data obtained in multiple rectangular loading-rest cycles is proposed. It is also suggested to employ the decrease in the instantaneous elastic modulus as a criterion for evaluating the residual service life of plastic parts and structures and elucidating the reasons for their early failure. For the first time, a possibility of considerably increasing the durability and endurance of structural plastics under short pulsed loadings with relatively long interruptions at the initial stage of stress concentration is considered. The cases of a significantly increased endurance of plastics caused by long interruptions after some fatigue loading by high-frequency tension cycles with a zero maximum stress are explained. First experimental confirmations of an increased durability and endurance of microcomposites subjected to short pulsed loadings alternating with long interruptions are obtained. The evolution of the effect of the loading-rest modes on the durability of massive specimens, microcomposites, and “dry” fibrous reinforcing fillers is demonstrated with examples of a glass-fabric laminate, a microcomposite, and a nonimpregnated glass strand.

Keywords: durability; deformability; multiple loadings with interruptions; relaxation processes; elastic compliance; elastic modulus; quasi-volumetric accumulation of damages; structural plastics

Effect of Water on the Physicomechanical Properties of Composites Containing Secondary Polyethylene and Linen Yarn Production Waste by J. A. Kajaks; S. A. Reihmane; J. E. Lejnieks (pp. 351-356).
The effect of the amount of absorbed water on the physicomechanical indices (tensile modulus, tensile strength, and ultimate strain) of composites based on secondary polyethylene (SPE) of two trademarks and linen yarn production waste (LW), both with and without a coupling agent (diphenylmetane diisocyanate - DIC), is evaluated. It is shown that the strength properties considerably depend on the time of water sorption and on the blend composition. The tensile strength decreases with increased amount of absorbed water (with increased time of exposure to distilled water) and with increased content of LW in the composites. This can be explained by the plasticizing effect of water molecules, which is confirmed by the increase in the ultimate strain of specimens after their exposure to water. The slight increase in the strength observed for the systems modified with DIC is probably caused by cross-linking of the free diisocyanate in the system under the action of moisture. The diisocyanate intensifies the interfacial interaction and retards the water-sorption process. Therefore, the resulting strength indices of these systems are higher than those of the unmodified compositions.

Keywords: secondary polyethylene; linen waste; composites; water influence; strength

Carbon-Fiber-Reinforced Plastics Based on an Imide Polymerizing Oligomer Binder with a Semi-Interpenetrating Structure by G. N. Gubanova; V. N. Artem'eva; V. E. Yudin; T. A. Maricheva; D. G. Tochil'nikov; B. M. Ginzburg; V. V. Kudryavtsev (pp. 357-364).
Semi-interpenetrating polymer networks (SIPNs) are synthesized based on a polyimide binder (imide polymerizing oligomer) with addition of polyamide acid. As acylating agents, they include derivatives of benzophenone-tetracarboxylic and diphenyloxide-tetracarboxylic acids and as aminocomponents - diaminodiphenylmethane, diaminodiphenyloxide, and metaphenylene diamine. It is shown that these systems form SIPNs of the “snake-in-the-cage” type. Uncured compositions forming melts at 300-330°C are used as binders for carbon-fiber-reinforced plastics (CFRP). The homophase structure of the SIPNs in CFRP is shown by dynamic mechanical tests. The interlaminar fracture toughness (G 1c) is measured by the method of a double cantilever beam. It is found that G 1c, as a function of the content (wt.%) of polyamid acid (PAA) in the initial composition used for obtaining CFRP, is of linear character, which is another confirmation of the homophase structure of the SIPNs. The interlaminar fracture toughness achieved for CFRP is 340 J/m2 at a 30% PAA content in the initial composition, and the glass transition temperature, which determines the thermal stability of the composites, reaches 320°C. The prospects of employing these plastics in tribotechnics are discussed.

Keywords: semi-interpenetrating polymer network; polyimides; carbon-fiber-reinforced plastics; interlaminar fracture toughness; thermal stability of carbon-fiber-reinforced plastics; friction and wear

Interaction at the Interface of Phosphorus-Containing Carbon Fibers and Diglycidil Ether of Bisphenol-A by V. I. Dubkova; O. I. Mayevskaya; M. M. Neverovich; G. N. Mathur; M. N. Saraf (pp. 365-374).
Two-component blends of phosphorus-containing carbon fibers (PCF) and diglycidyl ether of bisphenol-A (DGEBA) are investigated. It is found that PCF are better wetted by the epoxy oligomer considered than unmodified carbon fibers. It is stated that the equilibrium work of adhesion of the epoxy oligomer to PCF increases considerably. Heating the two-component blends is accompanied by conversion of epoxy groups and formation of a gel-fraction nonextractable from the fiber surface. The investigation results indicate that chemical bonds are formed at the fiber-oligomer interface, which causes grafting of the DGEBA immediately to the surface of PCF without the use of intermediate compositions usually employed in such cases. It is shown that a transition layer is formed whose morphology differs from that of the fibers and polymer in the blend volume.

Keywords: blend; carbon fibers; phosphorus-containing carbon fibers; epoxy oligomer; filler; curing agent; diglycidyl ether of bisphenol-A

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