The effect of fibre-matrix adhesion upon crack bridging in fibre reinforced composites

Raman spectroscopy has been used to study the effect of matrix cracking on the deformation micromechanics of high-performance fibres embedded in a brittle matrix. The ability of a fibre to bridge a matrix crack is considered in terms of a balance between the interfacial strength, the fibre strength and the fibre modulus. Compact tension epoxy resin specimens were prepared with HM Twaron aramid fibres embedded normal to the direction of the notch and then loaded critically such that a matrix crack ran through the fibres. The point-to-point distribution of strain was measured in the aramid fibres bridging the faces of a static crack, thus enabling the modes of interfacial failure to be monitored. The specimen was subsequently unloaded and reloaded incremently. The behaviour of the fibres bridging the faces of the crack was observed, particularly with respect to fibre/matrix debonding and the occurrence of reverse sliding at the fibre matrix interface. The observed deformation is compared both quantitatively and qualitatively to theoretical models for partial debonding (loading) and unload/reloading. In the case of a loaded crack-bridging fibre it has been possible to determine a critical interfacial shear stress for debonding and frictional shear stress in the debonded regions. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.