VISUALIZATION OF DEBONDING OF FULLY AND PARTIALLY EMBEDDED GLASS-FIBERS IN EPOXY-RESINS

Very short glass fibres have been embedded in bars of epoxy resin and the debonding process was observed under the microscope as the polymer was stressed. In addition, fibre pull-out specimens have been similar ly watched while the fibre was pulled. The interfaces of the fully emb edded fibres failed across the fibre ends at strains of 0.2-0.3%, and circumferential failure started at about 0.6% strain. The low values f or the end failures are compatible with models involving stress concen trations at the fibre ends. The circumferential failure value is in ag reement with the results of earlier pull-out studies. In the case of t he pull-out specimen, the behaviour was complex. Fibres with short emb edded lengths debonded first across the embedded end. Failure of the c ylindrical surface was too fast for the direction of crack propagation to be determined. Longer fibres first debonded at the fibre entry poi nt and then arrested while debonding occurred across the embedded end. Final failure was again very fast. Long fibres debonded continuously, starting at the entry point. A slow fracture process appeared to be i nvolved at least initially, so that the average shear stress on the re gion still bonded increased continuously throughout the process. Fast fracture occurred only in the very last stages of the process. These o bservations are compatible with the traditional theory, but reverse bo nding is not ruled out for the shortest embedded lengths.