EFFECTS OF FIBER/MATRIX ADHESION ON OFF-AXIS MECHANICAL RESPONSE IN CARBON-FIBER/EPOXY-RESIN COMPOSITES

Various transverse and shear tests were performed on flat laminates and hoop filament-wound tubular specimens produced from carbon-fiber-reinforced epoxy resin in order to clarify the effects of fiber/matrix bond quality. The latter was set by using carbon fibers with different surface treatments. The mechanical response of the composites was improved by enhanced fiber/matrix adhesion; this resulted in a change in the failure mode from adhesive-type interfacial failure towards a cohesive-like matrix failure as revealed by scanning electron microscopy. It is suggested that only torsion and transverse tensile testing of tube specimens yield reliable strength values. It is also suggested that in transverse losipescu, direct laminate shear and transverse tensile testing failure initiation occurs in local mode I (crack opening type), whereas crack propagation and thus final failure take place in mode II (shear) deformation. The short beam shear technique can be regarded as an easy-to-perform and highly sensitive measure for advanced carbon-fiber-reinforced/epoxyresin composites with various interfacial modifications. For the combined tension and torsion tests on tubes it was found primarily that interfacial variations are clearer the higher the transverse tensile load component is. Results on microcomposites indicated that the microdroplet pull-off test is the most sensitive technique in resolving interfacial effects.