ABSTRACT:The bending properties of composite materials are often characterized with simply supported beams under concentrated loads. The results from such tests are commonly based on homogeneous beam equations. For laminated materials, however, these formulas must be modified to account for the stacking sequence of the individual plies. The horizontal shear test with a short-beam specimen in three-point bending appears suitable as a general method of evaluation for the shear properties in fiber-reinforced composites because of its simplicity. In the experimental part of this work, the shear strength of unidirectional-glass-fiberreinforced epoxy resin composites was determined in different fiber directions with the short-beam three-point-bending test. Also, the elastic constants and flexural properties of the same materials were determined from bending experiments carried out on specimens in the 0, 15, 30, 45, 60, 75, and 90°fi ber directions with high span-thickness ratios.

A simplified theoretical approach for the prediction of the longitudinal elastic modulus and Poisson's ratio in fiber-reinforced composites is developed in this paper. The method considers that the main parameter affecting the elastic behaviour of com posite materials is the existence of the mesophase layer, between fiber and matrix, which possesses different physico-chemical properties than those of the constituent phases. The simplest and most convenient laws of variation are a linear, a parabolic, a hyperbolic and a logarithmic variation of E, and v, for the mesophase material, versus the polar radius from the fiber-surface. In this paper, therefore, these laws are con sidered for evaluating the overall moduli of the composite. Each one of these laws is applied to the representative volume element of the fiber composite and compares favorably with the unfolding model, introduced by one of the authors (PST), as well as with respective data existing in literature.

A strength of materials and elasticity approach was used in this study to determine the stiffness of fiber-reinforced composites, by taking into account the concept of boundary interphase. Theoretical expressions for longitudinal transverse and shear moduli, as well as for longitudinal Poisson's ratio, were derived by use of this model. Results derived from these expressions were compared with others, to observe the discrepancies and evaluate their validity

The elastic constants of particulate composites are evaluated employing a theoretical cube-within-cube formation. Two new models of four and five components, respectively, formed by geometrical combination of three-component models existing in the literature, are used as Representative Volume Elements. Using the governing stress and strain equations of the proposed models, two new equations providing the static elastic and shear moduli of particulate composites are formulated. In order to obtain the dynamic elastic and shear moduli, the correspondence principle was applied successively to components connected in series and/or in parallel. The results estimated by the proposed models were compared with values evaluated from existing formulae in the literature, as well as with values obtained by tensile, dynamic, and ultrasonic experiments in epoxy/iron particulate composites. They were found to be close to values obtained by static and dynamic measurements and enough lower compared with values obtained from ultrasonic experiments. The latter is attributed to the high frequency of ultrasonics. Since measurements from ultrasonic's and from dynamic experiments depend on the frequency, the modulus of elasticity estimated by ultrasonic's is compared with that (storage modulus) estimated by dynamic experiments.

Abstract:The thermal expansion coefficient of particle-reinforced polymers was evaluated using a theoretical model which takes into account the adhesion efficiency between the inclusions and the matrix -an important factor affecting the thermomechanical properties of a composite. To measure the adhesion efficiency a boundary interphase, i.e. a layer between the matrix and the fillers having a structure and properties different from those of the constituent phases, was considered. This layer is assumed to have varying properties.To obtain information concerning the properties and extent of the interphase, an experimental study of the thermal behaviour of aluminium-epoxy composites was undertaken. Differential Scanning Calorimetry (DSC) measurements were performed to evaluate heat capacity with respect to temperature. In addition, the effects of different factors, such as heating rate and filler concentration on the glass transition temperature of the composite, were examined. The sudden changes in heat capacity values in the glass transition region were used to estimate the extent of the boundary interphase according to an existing theory.Finally, the values of the thermal expansion coefficient, predicted by this model, were compared with theoretical results obtained by other authors and with experimental results.