V. L. Kulakov

Tests Methods for Composites. Survey of Investigations Carried out in the PMI of Latvian Academy of Sciences in 1964-2000

A review of long-standing research of the static mechanical testing of composites in tension, compression, bending, and shear carried out on flat, ring, and tubular specimens is presented. Methodical recommendations and restrictions for each test method considered, as well as the shapes and sizes of specimens and the loading schemes are given in summary tables. Particular attention is focused on the problem of determining the shear characteristics of 3D textile composites.

COMPOSITE RISERS FOR DEEP-WATER OFFSHORE TECHNOLOGY: PROBLEMS AND PROSPECTS. 1. METAL-COMPOSITE RISER

Prospects for the application of advanced composites in the offshore technology of oil production are considered. The use of composites in vertical pipelines-risers seems to be the most efficient. The operating loads are studied and the attendant problems are formulated. A comparative analysis of the characteristics of metal, composite, and metal-composite deep-water risers is presented. A technique is developed for designing multilayered risers, taking into account the action of internal and external pressures, gravity, and the axial tensile force created by tensioners, as well as the residual technological stresses due to the difference in coefficients of thermal expansion, physical-chemical shrinkage, and force winding. Numerical estimations are given for a two-layered riser with an inner metal layer of steel, titanium, or aluminum alloys and a composite layer of glass- or carbon-fiber plastics formed by circumferential winding. It is shown that the technological stresses substantially affect the characteristics of the riser.

Effect of Water Absoption, Elevated Temperatures and Fatigue on the Mechanical Properties of Carbon-Fiber-Reinforced Epoxy Composites for Flexible Risers

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.

FRP Reinforcing bars — designs and methods of manufacture (Review of Patents)

It is been suggested to devide all FRP reinforcing bars into three groups in accordance with the method used for establishing a bond between the core of the bars and concrete. Patents related to each group are cited and briefly described. At the end of the paper, ideas proposed in the patents are discussed. Problems connected with a practical implementation of FRP reinforcing bars will be considered in the next paper.

Grips for the transmission of tensile loads to a FRP strip

Designs of grips for flat strips made of fiber-reinforced polymers (FRPs) are put forward. They can be used for tests in tension, prestretching, and long-term loading. In order to prevent the rise of stress concentrations, special profiled grips with a varying surface curvature allowing the transmission of tensile loads to the strip by means of gradually increasing shear stresses (without peaks) have been elaborated. The results of a mathematical analysis of the stress state in the strip, an example of calculation, and experimental data obtained from testing a carbon-fiber-reinforced polymer strip in profiled model grips specially constructed for this purpose are presented.

Composites in offshore technology in the next century

The advisability of application of advanced composites in deep-water offshore technology is shown using risers and tendons as examples. Numerical estimations of the parameters of multilayered metal-composite risers are obtained. Two new processes of spatial braiding for creating the external jackets of compound tendons are considered. Advanced composites are the only possible choice for exploitation of deposits at depths greater than 1500 m. That is why they are the most promising structural materials for offshore technology in the next century.

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.

Fastening of a High-Strength Composite rod with a Splitted and Wedged end in a Potted Anchor 1. Experimental Investigation

The effectiveness of fastening of high-strength unidirectional CFRP/epoxy rods in potted anchors was investigated experimentally. The rods had splitted ends, in which duralumin wedges were glued. The experiments, performed for three types of contact between the composite rods and the potted material, showed that the most effective were full adhesion and adhesion–friction contacts, when the maximum load-carrying capacity of CFRP rods under tension could be reached. The full friction contact was ineffective because of the shear failure of CFRP rods inside the anchorage zone.

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.

Fastening of a High-Strength Composite Rod with a Splitted and Wedged End in a Potted Anchor 2. Finite-Element Analysis

A finite element analysis is carried out to determine the stress-strain state of anchors for round rods made of a high- modulus, high-strength unidirectional carbon-fiber reinforced plastic. The rods have splitted ends in which Duralumin wedges are glued. Three types of contact between the composite rods and a potted epoxy compound are considered: adhesion, adhesion-friction, and friction ones. The corresponding three-dimensional problems in the elastic statement are solved by the finite-element method (FEM) with account of nonlinear Coulomb friction. An analysis of stresses on the surface of the composite rod revealed the locations of high concentrations of operating stresses. The results of FEM calculations agree with experimental data.