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Mechanics of Composite Materials (v.40, #2)

Stress-Strain State in the Zone of Load Transfer in a Composite Specimen under Uniaxial Tension by V. L. Kulakov; Yu. M. Tarnopol'skii; A. K. Arnautov; J. Rytter (pp. 91-100).
The stress-strain state in the zone of load transfer in a uniaxially stretched specimen made of a unidirectional epoxy carbon-fiber-reinforced plastic (CFRP) is investigated. A parametric analysis of the influence of geometric and mechanical characteristics of the specimen on its stress-strain state is performed by means of finite-element modeling. The parameters allowing us to significantly reduce the dangerous concentration of transverse and tangential stresses are revealed. The mechanical tensile characteristics of a high-strength pultruded unidirectional CFRP are determined experimentally, and the size effect of its strength is estimated.

Keywords: uniaxial tension; unidirectional carbon-fiber-reinforced plastic; tensile strength; size effect; finite-element analysis

Homogenization Models for Carbon Nanotubes by A. Muc; M. Jamróz (pp. 101-106).
Two homogenization models for evaluating Young's modulus of nanocomposites reinforced with single-walled and multi-walled carbon nanotubes are presented. The first model is based on a physical description taking into account the interatomic interaction and nanotube geometry. The elementary cell, here a nanotube with a surrounding resin layer, is treated as a homogeneous body — a material continuum. The second model, similar to a phenomenological engineering one, is obtained by combining the law of mixture with the Cox mechanical model. This model describes the stress distribution along stretched short fibers surrounded by a resin matrix. The similarities between composite materials reinforced with short fibers and nanotubes are elucidated. The results obtained are compared with those for classical microcomposites to demonstrate the advantages and disadvantages of both the composite materials.

Keywords: carbon nanotubes; carbon nanocomposites; homogenization model

Accceleration of Fatigue Tests of Polymer Composite Materials by Using High-Frequency Loadings by R. Apinis (pp. 107-118).
The possibility of using high-frequency loading in fatigue tests of polymer composite materials is discussed. A review of studies on the use of high-frequency loading of organic-, carbon-, and glass-fiber-reinforced plastics is presented. The results obtained are compared with those found in conventional low-frequency loadings. A rig for fatigue tests of rigid materials at loading frequencies to 500 Hz is described, and results for an LM-L1 unidirectional glass-fiber plastic in loadings with frequencies of 17 and 400 Hz are given. These results confirm that it is possible to accelerate the fatigue testing of polymer composite materials by considerably increasing the loading frequency. The necessary condition for using this method is an intense cooling of specimens to prevent them from vibration heating.

Keywords: fatigue tests; polymer composites; acceleration of fatigue tests; high-frequency loading; fatigue damage accumulation; high-frequency fatigue-test rig

Prediction and Measurement of Residual Strains for a Composite Bonded Joint by G. A. Schoeppner; D. H. Mollenhauer; E. V. Iarve (pp. 119-134).
A quasi-isotropic composite laminate/adherend of IM6/3501-6 and a composite bonded specimen were manufactured and tested. The bonded specimen was fabricated by postbonding composite adherends together using a 177°C adhesive resin. Predictions for the residual curing strains in the composite adherends and the adhesively bonded composite specimen were performed using a thermomechanical linearly elastic analysis. The analysis was performed using a computer program based on a polynomial spline displacement approximation method [1]. The residual strains of the specimens were measured using the moiré interferometry technique. Diffraction gratings were replicated at room temperature onto the edges of polished laminated adherends and on the edge of a fully cured adhesively bonded specimen. The specimens were cut through their entire thickness in the middle of the diffraction grating area, resulting in a redistribution of the residual curing stresses, with corresponding changes in the strain field at the edges of the cut. A full-field deformation pattern was obtained in the grating area by analyzing the recorded fringe patterns. The deformation field induced by the cut in the laminated adherends and the adhesive bondline were estimated by the linear thermomechanical analysis. A good agreement between the analysis and the experimental results was obtained.

Keywords: residual stresses; residual strains; composite laminate; bonded joints; moiré interferometry; three-dimensional analysis

Research in the Limiting Efficiency of Plastic Deformation of Multilayer Tension (Compression) Bars by J. Bareisis; V. Kleiza (pp. 135-144).
A method is proposed for calculating the limiting loads of multilayer bars in plastic deformation. The regularities of changes in the efficiency factor (EF) of plastic deformation in relation to the elastic moduli of the materials of which the bars are composed, the stress which causes the material plasticity, and variations in the cross-sectional areas of layers are examined. Mathematical expressions are derived which allow one to easily calculate the EF for arbitrary values of variable parameters and to find the limiting values of EF when one of the parameters is changed. The equivalent tension diagram for a multilayer bar is obtained in relation to the strength and stiffness of materials and their number and cross-sectional areas.

Keywords: multilayer bar; limiting load; elasticity; plasticity; efficiency

Structure and Properties of Teflon Composites with Natural Diamond Powders by A. A. Okhlopkova; E. Yu. Shits (pp. 145-150).
The results of experimental investigations of the structure and properties of composites based on polytetrafluorethylene (PTFE) containing natural diamond powders (NDP) of different dispersity are presented. To obtain diamond-containing compositions for antifrictional applications, we used a preliminary mechanical treatment of NDP (40 μm) in a planetary mill. It was stated that the formation of the maximum ordered small-spherulite structure of PTFE after injection of NDP significantly increased the wear resistance and deformational and strength characteristics of the polymer composite materials. To produce abrasive materials, PTFE was filled with NDP having a larger graininess (from 40 to 125 μm). It was found that the injection of NDP did not cause evident morphological changes in the binder — the bonds between diamond grains and the polymer are created by physicomechanical forces. To strengthen the adhesion interaction at the interface between the binder and diamond grains and to raise the wear resistance of the material, a complex modification of the polymer with inorganic and organic fillers was carried out. It is shown that the injection of the complex filler significantly improves the tribotechnical and operational properties of the diamond-containing composite material. The general laws of the influence of NDP on the formation of the supermolecular structure of PTFE are revealed. It is shown that, by varying the degree of dispersity and the content of NDP in PTFE, and by applying different methods of their injection into the polymer matrix, it is possible to control the operational properties of the composites and to produce materials of different functional application, from antifrictional to abrasive ones.

Keywords: composite; filling; diamond powders; structure; mechanical characteristics

Transition Layer in Composites with a Plasticized Filler and Its Influence on the Strength Properties by I. Lukosiute; R. Levinskas; J. Sapragonas; A. Kviklys (pp. 151-158).
Composite structures consisting of an epoxy resin matrix filled with a porous perlite containing a finely dispersed polyvinyl chloride are investigated. Models of the composites are analyzed in which the formation of a transition layer between the filler particles and the matrix may arise as a result of interaction between the separate components. The influence of the perlite and plasticized polyvinyl chloride particles on epoxy composite characteristics, such as the bending, tensile, impact, and adhesion strengths, are studied.

Keywords: epoxy resin; plasticized polyvinyl chloride; diffusion; transition layer; interphase stresses; perlite; composite; strength characteristics

Surface Extension in Layered Structures with the Use of Metal Meshes for Heat-Transfer Enhancement by R. Chatys; T. Orzechowski (pp. 159-168).
The properties of multilayer metal-fiber coverings superimposed on plane base layers, which consist of various combinations of a copper mesh and different-length copper wires, are examined. Coverings from discrete copper microwires, two-layer coverings including a mesh and microwires, and three-layer coverings consisting of a mesh and two layers of microwire are investigated. The contacts between the fibers, the copper mesh, and the base layer are analyzed by using the metallography technique. Quantitative estimates for these contacts are presented. The side effects connected with the “hydrogen disease” of copper are discussed. The thermal properties of the porous structures are analyzed with the use of a termographic chamber. It is shown that the porous metal-fiber structures have good mechanical and heat-transfer properties.

Keywords: multilayer coverings; metal meshes; porosity; thermophysical properties

Influence of Modifiers on the Physicomechanical Properties of Sawdust-Polyethylene Composites by A. Viksne; L. Rence; R. Berzina (pp. 169-178).
Polymer-wood composites based on recycled polyethylene (RPE) are investigated. Dispersed alder sawdust was utilized as a filler. To improve the compatibility between the nonpolar matrix and the polar wood fibers as a reinforcement, two types of modifiers were used, which differed in their chemical nature and mechanical interaction with the constituents of the composites. The modifiers of the first type (paraffin and OP) improved the dispersibility of sawdust (SD), and those of the second type (Exxelor 1015 and OREVAC) contained groups of maleic anhydride, which interacted with the OH-groups of SD. The effect of the modifiers on the moisture sorption by SD, the dispersibility of the filler in the matrix, and the strength characteristics (ultimate strengths and moduli in tension and bending) of dry and moist RPE–SD composites and on their moisture sorption is estimated. The best results were obtained for the composites modified with paraffin, which is due to the more efficient employment of the strength and rigidity of well-dispersed SD fibers. In their strength characteristics, the RPE-based composites investigated are comparable to composites based on low-density polyethylene.

Keywords: polymer-wood composites; modifiers; dispersion of filler; strength characteristics; moisture sorption

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