A. Aniskevich

Smart polymeric coatings for damage visualization in substrate materials

For damage indication needs in polymer composites, three smart acrylic coatings based on microencapsulated crystal violet lactone leuco dye with embedded developer silica gel and methyl 4-hydroxybenzoate were elaborated. Damage visualization ability upon duralumin substrates was tested. Indentation tests were conducted in the interval of 0.0005–0.55 J of mechanical work. Digital image analysis was used for evaluation of the visual response in the form of “bruise” spots. The measure of response intensity was relative excess of blue color above achromatic gray in the scanned images of the “bruise” spots. This technique allowed detecting influence of the coating formulations onto the substrate damage visualization.

Development of a composite with an inherent function of visualization of a mechanical action

A concept of a composite possessing a biomimetic function of visual response in the form of a “bruise” to an external mechanical action has been developed. The concept is implemented by creating a model layered epoxy composite. Methods for creating a sensitive layer, containing microcapsules with a leuco dye and particles of a developer, and a composite including this layer are described. The threshold of a visual response, i.e., the sensitivity of the composite to a local indentation load is controlled by means of a protective epoxy coating of various thickness. Series of mechanical tests with a gradually increasing load and subsequent digital image analyses were carried out. The data obtained were used to determine the required thickness of the epoxy coating in relation to the desirable threshold of visual response to an indentation load. The Hertz theory was used to analyze the resulting stresses in the coating in the elastic approximation. The experimental results allowed us to determine the normal threshold pressure causing the action of a sensitive layer embedded into the epoxy composite.

Analysis of reversible and irreversible strains in the creep of a nonlinear viscoelastic polymer

The results of an experimental investigation and analytical description of creep of a nonlinear viscoelasticplastic material (a Upilex-S polyimide film) in tension are presented. The elastic modulus, yield stress, and ultimate strength were found from quasi-static tests with a constant deformation rate. The total strain of the material was assumed to be the sum of elastic, viscoelastic, and irreversible strains. The reversible elastic and viscoelastic and the irreversible deformation of the material were investigated in experiments on creep with the subsequent recovery of deformations (creep recovery). The nonlinearity of its viscoelastic behavior was taken into account using the principle of stress-time superposition. The results obtained allow one to describe the creep behavior of a material at various stages of loading and unloading.

Effective electrical conductivity of carbon nanotube–epoxy nanocomposites

The electrical conductivity of carbon nanotube–epoxy composites is investigated analytically and experimentally. The theoretical predictions of the effective electrical conductivity of carbon nanotube–epoxy composites were performed by the analytical approach based on a micromechanical model of composites. The parametric analysis carried out revealed an influence of geometrical and electrical parameters of the micromechanical model on the effective electrical conductivity of carbon nanotube–epoxy nanocomposite. The nanocomposites made from the DGEBA-based and RTM6 epoxy resins filled with different weight content of Baytubes C150P and N7000 multi-walled carbon nanotubes were prepared. The experimental values of the electrical conductivity of the nanocomposites were compared with those calculated by means of the analytical model.

Multifunctional properties of nanocomposites made by 1D and 2D graphene based fillers

High aspect ratio graphene based fillers with different dimensionalities showed the ability to greatly modify rheological, mechanical, thermal and electrical properties of polymers at very low content. In this work, the effect of filler dimensionality on the multifunctional properties of an epoxy matrix reinforced by both carbon nanotubes (1D) and graphite nanoplatelets (2D) have been investigated across the percolation region.

Simplified Calculation of the Electrical Conductivity of Composites with Carbon Nanotubes

The electrical conductivity of two groups of polymer nanocomposites filled with the same NC7000 carbon nanotubes (CNTs) beyond the percolation threshold is described with the help of simple formulas. Different manufacturing process of the nanocomposites led to different CNT network structures, and, as a consequence, their electrical conductivity, at the same CNT volume, differed by two orders of magnitude. The relation between the electrical conductivity and the volume content of CNTs of the first group of composites (with a higher electrical conductivity) is described assuming that the CNT network structure is close to a statistically homogeneous one. The formula for this case, derived on the basis of a self-consistent model, includes only two parameters: the effective longitudinal electrical conductivity of CNT and the percolation threshold (the critical value of CNT volume content). These parameters were determined from two experimental points of electrical conductivity as a function of the volume fraction of CNTs. The second group of nanocomposites had a pronounced agglomerative structure, which was confirmed by microscopy data. To describe the low electrical conductivity of this group of nanocomposites, a formula based on known models of micromechanics is proposed. Two parameters of this formula were determined from experimental data of the first group, but the other two — of the second group of nanocomposites. A comparison of calculation and experimental relations confirmed the practical expediency of using the approach described.

Simulation of mechanical behaviour of polychloroprene/versatic acid vinyl ester/methyl methacrylate/2-ethylhexyl acrylate copolymer blend

Finite element modelling and homogenization approaches are very common for prediction of mechanical performance of composite materials. Polymer blends also display a heterogeneous microstructure and the use of these methods to assess elastomeric blends is poorly investigated. The main purpose of present investigation is to analyze the possibility of finite element modelling by homogenization approach for elastomeric polychloroprene (PCP)/versatic acid vinyl ester/methyl methacrylate/2-ethylhexyl acrylate (VeoVa-11/MMA/2-EHA) copolymer blend. To examine the properties of specifically obtained materials, two types (random and periodical) of finite element models of the blend microstructure were devised. The Mooney–Rivlin function was chosen for the description of both PCP and VeoVa-11/MMA/EHA copolymer. The modelling showed that there was not any difference between the periodical and random finite element models. The finite element modelling results exhibited a good agreement with the experimentally obtained values for prediction of the mechanical behaviour of the blend under large deformations. Therefore, the condition under high strain was studied to predict the effect of VeoVa-11/MMA/EHA copolymer content on the stress value of the blend. It was found that the strength of the blend with a small amount of VeoVa-11/MMA/EHA (5–10%) was lower in comparison with that of PCP strength. However, the higher amount of copolymer (15–20%) had the strengthening effect on the blend.

The system of carbon fibre-reinforced plastics micro-tubes for self-healing of glass fibre-reinforced plastics laminates

A system of pultruded carbon fibre-reinforced plastics micro-tubes is used for self-healing simulation in laminated polymer composite. The system consists of a package of micro-tubes, placed in the symmetry plane of the GFR/epoxy laminate stack. Healing agent is a mixture of the epoxy resin and hardener. The healing agent releases and penetrates into the cracks after the composite is damaged by the quasi-static indentation. The specimens are healed at 30℃ for 24 h. Rectangular specimens notched under ASTM D2733 have been subjected to tensile test to determine interlaminar shear strength. Shear strength of specimens has been compared in three states (virgin, damaged and healed) for various ways of healing. After the most effective self-healing, the interlaminar shear strength has been recovered to 70 ± 15% of those for virgin specimens that almost twice exceeds the residual strength of the damaged specimens.

Numerical modeling of heat transfer in an orthotropic I-beam

The finite-element method is used for modeling the heat transfer in an I-beam at different ratios of heat conductivities of its orthotropic material. Various schemes are proposed for numerical calculations, which allowed us to describe the process of heat transfer with different accuracy. The calculation error was estimated, by using simplified schemes, as a function of the ratio of heat conductivities. It is shown that simplified schemes can successfully be employed for modeling the transient heat transfer process with an accuracy sufficient for engineering applications.