Phase-stepping photoelasticity for the measurement of interfacial shear stress in single fibre composites

Phase-stepping photoelasticity has been used to study the fragmentation of an E-glass fibre in epoxy resin and evaluate quantitatively its interfacial adhesion. To investigate the interfacial response of an E-glass fibre with differing degrees of adhesion, it was coated by plasma-polymerisation with a crosslinked conformal film of 90% acrylic acid and 10% 1,7-octadiene. The stress distribution within the resin at the fibre–matrix interface and the adjacent matrix has been described in detail using contour maps of fringe order. From these, the interfacial shear stress profiles at fibre-ends have been calculated. With a cold-cured epoxy resin a good interface was formed between uncoupled unsized E-glass, while a poor interface was achieved with the plasma polymer coated fibre. Further, it was clearly demonstrated that chemical bond formation between the functional plasma polymer coating and the matrix was incomplete leading to the formation of an interphase epoxy resin of low yield strength. We now have a technique capable of probing the micromechanical response of interphases as a function of chemical bond formation.