Shear moduli of structural composites from torsion tests

A combined experimental/analytical approach for effective evaluation of in-plane and out-of-plane shear moduli of structural composite laminates from torsion is presented. The test samples are produced by pultrusion and consist of E-glass fiber systems and vinylester resin. Three types of rectangular samples are used: unidirectional samples cut from plates; angle-ply and angle/cross ply samples cut from wide-flange beams. The shear moduli are obtained from the experimental torsional stiffnesses (T/phi) and data reduction techniques based on torsion solutions by Lekhnitskii and Whitney. The classical approach of using paired samples of different widths but same material orientation can grossly underestimate out-of-plane shear moduli. Thus, to overcome this problem, samples with material orientations normal to each other are used, and to obtain samples of larger dimensions than the available thicknesses of the material, the pultruded laminates are bonded flat-wise, from which transverse strips of different widths are cut. Consistent results are obtained by the two data reduction techniques, and the experimental values are compared to predictions by micro/macro-mechanics models. In relation to more elaborate methods, the proposed torsional approach call be easily implemented in the laboratory, and by testing a representative volume of the material with this method, it is expected to obtain reliable shear moduli data for structural composites.