The use of a torsion machine to measure the shear strength and modulus of unidirectional carbon fibre reinforced plastic composites

Abstract

A torsion apparatus, in which a solid rod specimen is subjected to a shear stress field only, has been used to measure the shear modulus and strength of unidirectional carbon fibre reinforced plastics. Because of the absence of tensile and compressive forces, a more accurate value of the shear strength is obtained than from a test such as the short beam shear test. The shear strength is calculated allowing for the non-linear nature of the shear stress-strain characteristic. For type 2 treated fibre the shear modulus is found to increase rapidly with fibre volume loading, in reasonable agreement with the micromechanical theory of Heaton. For type 2 untreated and type 1 treated fibre composites, a slightly less rapid increase in shear modulus is noted. Results for type 1 untreated fibre composites increase with volume loading but are below both the other results and the theoretical curve. The shear strength of composite materials made from type 2 treated fibre is greater than that of the pure resin, and has a maximum for about 50% volume of fibre. For type 1 and untreated carbon fibres the shear strength decreases with increasing volume loading. By using the concepts of fracture mechanics and assuming that the bond between type 2 treated fibre and resin is completely effective, so that failure starts in the matrix, it is possible to give a plausible explanation of the shear strength results. The shear modulus, but not the shear strength, can be measured accurately, using either square or circular cross-section specimens.