Determination of the interfacial normal strength using single fibre model composites

A technique is presented which can be used to measure the interfacial normal strength (INS) in transversely loaded composites. The technique combines the measurement of local deformations during tests in a scanning electron microscope and numerical simulations, in which the measurements are used as boundary conditions. Deformations are measured by placing a grid of markers on the surface of a single fibre specimen. The displacements of these markers are measured during loading of the specimen. Numerical simulations show that the radial and tangential interface stresses increase towards the specimen surface, causing the initiation of debonding at the surface. This effect is supported by tests under an optical microscope. The INS is defined to be the maximum radial interface stress just before the initiation of debonding. The INS values are determined for carbon fibres with surface treatment levels varying from 0% to 100% commercial treatment level. The results show an increasing INS for increasing treatment level. Based on the presented results, it can be concluded that the presented technique can be used for the measurement of the interfacial normal stress