The Carbon Fibre-Epoxy Interface

Abstract

Hercules AS1 fibres were embedded in epoxy resin and epoxy copolymer resm blocks, and pulled out under carefully controlled conditions. The fibre stresses required to cause the interfaces to fail were usually very high (up to 5 GPa), and varied with embedded length (L) in different ways, which appeared to be related to the shrinkage stress of the resin. At intermediate strinkage stresses, the failure appeared to be a brittle fracture pro cess, involving a fracture surface energy of about 25 Jm2. At the highest shrinkage stresses the interface yielded at a stress of 48-57 MPa, i.e. somewhat higher than the shear strength of the matrix. At the lowest shrinkage stress a different, embedded length- independent fracture process appeared to intercede, involving a fracture surface energy of about 50 Jm-2. (The effect of shrinkage stress is tentative at present, since the stress measurements gave rather variable results.) After the interface had failed pull out was governed by friction, with shear stresses in the range 7-13 MPa. It was not possible to determine directly the pressure that the epoxy exerted on the fibre since the pull out curves during the frictional process were more or less straight. Using data from other work, in stead, coefficients of friction in the range 0.35 to 0.6 were estimated for fully cured epoxy homopolymers.