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 an d hoop filament-wound tubular specimens produced from carbon-fiber-rei nforced epoxy resin in order to clarify the effects of fiber/matrix bo nd quality. The latter was set by using carbon fibers with different s urface treatments. The mechanical response of the composites was impro ved by enhanced fiber/matrix adhesion; this resulted in a change in th e failure mode from adhesive-type interfacial failure towards a cohesi ve-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 i n transverse losipescu, direct laminate shear and transverse tensile t esting failure initiation occurs in local mode I (crack opening type), whereas crack propagation and thus final failure take place in mode I I (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 modif ications. For the combined tension and torsion tests on tubes it was f ound primarily that interfacial variations are clearer the higher the transverse tensile load component is. Results on microcomposites indic ated that the microdroplet pull-off test is the most sensitive techniq ue in resolving interfacial effects.