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Vol. 47/2011, No.3 July

Keywords: carbon nanotubes, polymer, nanocomposite, interphase layer, elastic constants, anisotropy, orientational averaging

Variant of a stepwise analysis of the elastic properties of a carbon-nanotube-reinforced composite with account of the effect of interphase layers between the nanotubes and the polymer matrix is reported. The preliminary calculation of the elastic constants of a structural element incorporating a nanotube and an interphase layer and the subsequent calculation of independent elastic constants of a composite with such transversely isotropic structural elements oriented in one direction are both performed by using the Mori–Tanaka theory of an equivalent medium. The calculations are carried out for a wide range of ratios between the elastic moduli of the interphase layer and matrix. The elastic constants of a composite with randomly oriented nanotubes are obtained by using the method of orientational averaging.

Keywords: fracture mechanics, polyolefin pipe, polymer weld, stress intensity factor, graded structure

A three-dimensional model of a pressured polymer pipe with a weld is created in order to estimate the stress intensity factor for a crack located inside the weld. A comparison with values obtained for an edge-cracked tension specimen, usually employed for an experimental determination of weld properties, is performed. The difference between the stress intensity factor in a homogeneous pipe and the results taking into account the changes in material properties inside the weld is presented. A conservative and easily applicable relationship for calculating the stress intensity factor in a pipe weld is proposed.

Keywords: plate, finite-element method, transverse shear strains

A four-node rectangular finite element is elaborated for a plate in calculated with account of transverse shear strains. On varionational realization of the finite-element method, the stiffness matrix and the vector of equivalent nodal forces are obtained for the case of a surface load. The basic nodal kinematic parameters, adopted also include the angles of transverse shear.

Keywords: potted anchor, composite rod, interfacial debonding, pullout test

An analytical model for the estimation of load-carrying capacity of cylindrical potted anchors with round rods made of high-strength composites is developed taking into account a partial debonding at the rod-pottant interface under tension loading. A detailed parametric analysis of the influence of debonding propagation on the load-carrying capacity of the potted anchor is carried out.

Keywords: strain localization, interfacial debonding, shear failure, DIC, steel-fiber-reinforced concrete

The full strain field near the interface of steel fiber/concrete in a half-mold specimen was measured using a combined method of pull-out test and digital image correlation (DIC). The strain localization mechanism of the interfacial failure is discussed. The strain distributions near the interface at a straight fiber under different loads show that the interfacial shear failure has a distinct characteristic of intervals in time and space directly related to the strain localization, which makes the interfacial failure initiate, develop, and transfer successively. In particular, the local strain distributions around pores near the straight fiber interface demonstrate that the strain changes its sign at the irregular parts of the pore where the initial debonding took place and the deformation path is affected by the pore.

Keywords:  polymer-matrix composites, environmental degradation, gravimetric analysis

The degradation of mechanical properties and the moisture absorption of a carbon/epoxy composite is studied through immersion of specimens in two common types of glycol-based (corn-based and synthetic) deicing fluids at two temperatures. Test results indicate that the effects of the fluids are similar at room temperature, while at elevated temperatures, the specimens immersed in the corn-based deicing fluid show a greater drop in the tensile strength. Moreover, gravimetric analysis results show a mass gain in all specimens, with a significantly lower gain from the corn-based deicing fluid than from the synthetic one at the two temperatures selected in this study.

Keywords: impact damage, impact energy, loss of strength, criterion of an equivalent hole, criterion of concentration

Theoretical and experimental investigations into the loss of the static tensile strength of a KMKU-2M.120.E01 composite after low-speed impacts of different energy have been carried out. All calculations are performed by the method of finite elements. The drop in the strength as a function of impact energy is estimated according to two criteria: the criterion of an equivalent hole (the upper limit of loss of strength) and the criterion of concentration (the lower limit of loss of strength). The results obtained agree closely with experimental data and therefore can be recognized as reliable.

Keywords: nanocomposites, polyimide, inorganic nanoparticles, mechanical properties, oxygen permeability

Polyimide (PI) films based on poly(pyromellitic dianhydride-co-4,4′-oxydianiline) (PI-PM) were filled with nanoparticles of organically modified montmorillonite (MMT), vapor-grown carbon nanofibers (VGCF), and silicate nanotubes (SNT) of different concentration.Rheology measurements and structural investigations showed a relatively good dispersion of the nanoparticles in the PI matrix to a degree of filling that depended on the type and morphology of the nanoparticles used. Mechanical (tensile modulus, strength, and deformation at break) and barrier (oxygen permeability) properties of the PI-PM nanocomposite films were investigated. The tensile modulus of the polyimide nanocomposites filled with tubular SNT and VGCF nanoparticles increased with volume concentration of the nanoparticles without a catastrophic decrease in the elongation at break. In addition, the MMT particles chemically modified with 4,4′-bis-(4′′-aminophenoxy)diphenylsulfone improved the barrier properties of the PI-PM films significantly more than VGCF or SNT. The relative poor oxygen barrier and mechanical properties of the PI-PM/VGCF nanocomposite films is ascribed to the relative poor adhesion between the VGCF and the polyimide matrix, which was confirmed by scanning electron microscopy of the fracture surface of these films.

Keywords: nanocomposites, nanofiber, covering material, local curvature, geometrical nonlinearity, stress distribution

Within the framework of a piecewise homogeneous body model, with the use of the three-dimensional geometrically nonlinear exact equations of the theory of elasticity, the method developed for determining the stress distribution in nanocomposites with unidirectional locally curved covered nanofibers is used to investigate the normal stresses acting along nanofibers. The investigation is carried out for an infinite elastic body containing a single locally curved covered nanofiber in the case where there exists a bond covering cylinder of constant thickness between the nanofiber and the matrix material. It is assumed that the body is loaded at infinity by uniformly distributed normal forces in the fiber direction. Upon formulation and mathematical solution of the boundary value problem, the boundary form perturbation method is used. Numerical results for the stress distribution in the body and the influence of geometrical nonlinearity on this distribution are presented and interpreted.

Keywords: Taguchi method, fiber orientation, shear layer

A fast and effective methodology integrating the finite-element and Taguchi methods is presented to determine the optimal design conditions of the injection molding process for short-fiber-reinforced polycarbonate composites. The finite-element-based flow simulation software, M-flow, was employed to simulate the molding process to obtain the fiber orientation distributions required. The Taguchi optimization technique was used to identify the optimal settings of injection molding parameters to maximize the shear layer thickness. The effects of four main parameters — the filling time, melt temperature, mold temperature, and injection speed — on the fiber orientation or the shear layer thickness were investigated and discussed. It is found that the dominant parameter is the filling time. The best levels of the four parameters to acquire the thickest shear layer are also identified.

 

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Last update: 10.08.2011