Yu. M. Tarnopol'skii

Effect of fiber curvature on the modulus of elasticity for unidirectional glass-reinforced plastics in tension

The effect of curvature of the fibers on the modulus of elasticity of unidirectional glass-reinforced plastics (GRP) in tension has been investigated on the basis of the theory of layered reinforced media with random initial irregularities [1,2]. It is shown experimentally that relatively minor distortion of the fibers during manufacture may result in important changes in the modulus of elasticity. A model is proposed for determining the effect of fiber curvature on Youngs modulus; experiments on specimens with a given regular fiber curvature indicate good agreement with the theory. The effect of prestressing the fibers on the modulus of elasticity has been studied using commercial AG-4S material and the optimum prestress has been established.

Shear effects during bending of oriented glass-reinforced plastics

An oriented glass-reinforced plastic may be treated as a regular multilayer material. Given large number of layers we can replace the discrete model with a continuous medium whose elastic constants are determined by the properties of the components. Using this approach and assuming that the reinforcing and the resin obey Hookes law, we examine the problem of the bending of oriented glass-reinforced plastics and by means of two examples demonstrate the importance of shear effects in the bending of beams. The experimental results are in good agreement with theory.

Programmed winding of glass-reinforced plastics

It is proposed to employ a variable winding tension designed to give a required initial stress distribution. The program is constructed on the basis of solutions describing the loss of tension associated with winding and the softening of the resin during heat treatment. The problems are solved in the elastic formulation. Programs are obtained for three cases: constant tension in a ring sill on the mandrel, compensation of the stresses that develop after removal from the mandrel, and compensation of the thermoelastic stresses that develop during cooling.

Structural characteristics of materials reinforced with high-modulus fibers

Research on the mechanics of boron and carbon-reinforced plastics is briefly reviewed. The design and testing characteristics of these materials associated with the high degree of anisotropy of their elastic properties, as compared with those of glass-reinforced plastics, are discussed. Problems relating to testing at an angle to the direction of the reinforcement, the effect of misorientation and distortion of the fibers, and the consequences of the low shear strength are considered. Experimental confirmation has been obtained by testing unidirectional (1 : 0), orthogonally reinforced (1 : 1 and 2 : 1), and tridirectional (1 : 1 : 1 in the 0°, +60°, and −60° directions) boron and carbon-reinforced plastics.

Load transmission mechanism in oriented glass-reinforced plastics

Using Outwaters model, the authors give a theoretical description of the load transmission mechanism in oriented glass-reinforced plastics both for the assumption that Hookes law holds true for both components and for the real strain diagram of the resin. Experimental results confirm the presence of two load transmission mechanisms in glass-reinforced plastics. A method of estimating the monolithicity of reinforced plastics is proposed.

Modern trends in the development of fibrous composites

The two main trends in the development of high-modulus composites are considered. Improved methods of calculation sensitive to the effects associated with the weak shear and transverse characteristics are reviewed. It is shown that the disadvantages of composites with a traditional arrangement of the reinforcement can be overcome. The properties of boron- and carbon-reinforced plastics with a traditional reinforcement structure are described and compared with those of three-dimensionally structured materials with two- or three-strand reinforcing. Whiskerized fiber reinforcement is also considered. A program of further research on high-modulus composites is outlined.

Some negative characteristics of fiber-reinforced materials

The effect of low resistance to shear and tension-compression at right angles to the layers of reinforcement on the state of stress and strain of elements composed of fiber-reinforced materials is analyzed. It is shown that to take these characteristics into account the calculations must be refined. In certain cases this leads to the appearance of qualitatively new effects. Typical problems in which the negative characteristics of the material must be allowed for are examined.

Variation of prestress in filament-wound glass-reinforced plastics

The authors examine the process of winding prestressed parts from oriented glass-reinforced plastics. It is shown that the essential anisotropy of the properties of these materials in responsible for variation of the specified prestress. The law of distribution of the tensile forces is investigated in the linear-elastic approximation for the case of a ring wound onto a rigid mandrel. A method of calculation is proposed that permits the change in prestress to be estimated and gives the critical number of turns beyond which the pressure on the mandrel ceases to increase.

Filament-wound rings tested by the application of concentrated loads

The stiffness and strength of rings and ring segments of fiber-reinforced materials have been investigated theoretically and experimentally in short-time tests with concentrated loads applied in the plane of the specimen. It is shown that the low shear resistance and the geometric nonlinearity must be taken into account in testing thick- and thin-walled specimens, respectively. The equations and graphs presented make it possible to allow for these effects in analyzing the test results or in design calculations. Experiments on unidirectional glass-reinforced plastics indicate good agreement with the proposed theoretical equations.

Flexure of beams with low shear strength on an elastic foundation

The deflection of a beam on an elastic foundation is determined with allowance for shears and without postulating a law of shear-stress distribution. The accuracy of approximating this law with a quadratic parabola is estimated. The effect of shears on deflection is investigated in relation to beams of different length made of materials of the oriented glass-reinforced plastic type with low shear strength.