Contents

Keywords: cellular plastic, load-carrying element, structural element, orientation distribution function, rigidity tensor, orientational averaging

A model is presented for calculating the linear elastic constants of high-porosity cellular plastics by orientationally averaging the rigidity tensor of a structural element consisting of an air sheath and a load-carrying element in the form of a straight strut with a piecewise constant cross section. The load-carrying element can resist the axial and shear loads and bending moments applied to its ends. The nonuniform orientational distribution of the elements is also taken into account. The calculation results obtained are compared with some literature data.

Keywords: MoS₂, polyimide, carbon nanotubes

The aim of this work was to investigate the effect of surface modification of carbon nanotubes (CNTs) on the mechanical properties of polyimide (PI) composites with and without MoS₂. A three-phase system of CNT/PI/MoS₂ laminates were fabricated with an extrusion-grade PI to produce CNT-reinforced laminates with MoS₂ volume fractions of 1-5%. The tensile and impact properties of CNT/PI composites and CNT/PI/MoS₂ laminates were also measured and compared. Results showed that the introduction of CNTs as reinforcing additives improved the tensile properties of the composites, but worsened their impact properties. Furthermore, the addition of MoS₂ increased the impact strength of the CNT/PI composites greatly. The optimum contents of CNT and MoS₂ have been found.

Keywords: fullerene С₆₀, epoxy resins, Young's modulus, impact strength, nanocomposites

The effect of С₆₀ 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 С₆₀ increases by about 100-200%. A molecular mechanism of the toughening effect of С₆₀ on epoxy resins is suggested.

Keywords: composites, single-walled and multiwalled carbon nanotubes, processing technologies, epoxy

An epoxy resin (Epon 828) was filled with single- and multiwalled carbon nanotubes (SWCNTs and MWCNTs) in two steps by using the high shear mixing and ultrasonication techniques. The melt flow of the composites was characterized in a plate/plate rheometer. The thermomechanical properties of the composites were determined in dynamic mechanical analysis tests performed at various frequencies and temperatures. It was found that the incorporation of SWCNTs or MWCNTs increased the viscosity and stiffness of epoxy above its glass-transition temperature. The time-temperature superposition principle was employed to estimate the storage modulus of the composites as a function of frequency (f = 10^–33-10³ Hz) in the form of master curves.

Keywords: anisotropy, equation of state, shock wave, composite, generalized decomposition of stress tensor

Using an analytical relation between the Hugoniot states (anisotropic and isotropic) and other thermodynamic states (anisotropic and isotropic) at high pressures, the effect of fiber orientation on the structure of shock waves in carbon fiber-epoxy composites of various symmetry is investigated. A correct nonlinear model of propagation of shock waves in anisotropic materials is proposed, which employs the conception of total generalized pressure and the pressure corresponding to the thermodynamic response, i.e., to the equation of state. The equation generalizes the nonlinear Hugoniot equation to anisotropic materials and is reduced to the classical variant in the case of isotropy. Invoking the relations of nonlinear anisotropic solids and the generalized decomposition of stress tensor, the double structure of shock waves, consisting of nonlinear anisotropic and isotropic elastic parts, is examined. The numerical calculations of Hugoniot levels of stress agree well with experimental data for a carbon fiber-epoxy composite selected.

Keywords:  particulate composites, polymer-matrix composites, mechanical response, finite-element analysis

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.

Keywords: composite piezoelectric wedge, contact surface, transcen­dental equation, singularity of electroelastic field

With the help of a local solution, the behavior of an electroelastic field in the vicinity of border of the contact surface of a composite piezoelectric wedge is investigated. Numerical results showing the relation between the degree of singularity of stresses and the electroelastic properties of joined materials and joint geometry are presented. In the plane of openings of homogeneous wedges, the limit curves separating the region of low stresses from the region of concentrated stresses at the border of contact surface are constructed. Some special cases are considered.

Keywords: low-velocity impact, composite sandwich panels, drop-weight impact test, experiment

The objective of this research is to compare the numerical and experimental results with a closed-form solution for the dynamic responses of composite sandwich panels subjected to a low-velocity impact. An equivalent two-degree-of-freedom system is used to predict the response at different boundary conditions. The experimental results were obtained from drop-weight impact tests. Numerical calculations were performed using the LS-DYNA software to verify the analytical results. The numerical simulation showed a good agreement with experimental data. Meanwhile, with increasing impact velocity the analytical model showed some disagreement with experiment. This can be explained by the static analysis and minimization of the potential energy used in the model.

Keywords: POM/CF, nano-SiO₂, fracture surface

Polyoxymethylene/carbon fiber(POM/CF)composites containing nano-SiO₂ were prepared, and their mechanical properties were investigated. At a content of 1-5 vol.%, the nano-SiO₂ exerted an obvious reinforcing effect on POM, leading to an increase in the elastic modulus and stiffness of the composites.

Keywords: polylactic acid, nanoclay, polymer rheology, die swell

The purpose of this research was to investigate the rheological behavior and extrudate swell of polylactic acid (PLA) filled with a nanoclay. The effects of the amount of nanoclay and surface treatment were studied by a capillary rheometer. The dispersion of the nanoclay was inspected by using the scanning electron microscopy. Generally, PLA/nanoclay composites exhibited a pseudoplastic rheological behavior, as the shear stress and die swell increased with increasing shear rate. However, the shear viscosity decreased. The die swell also increased with increasing shear stress. The elongation viscosity decreased with increasing elongation rate. The incorporation of nanoclay decreased the shear stress and shear viscosity to a lesser degree than the elongation viscosity.