A typical sequence of replicas of the fatigue damage development in glass-fibre/PP is found in Fig. 5, and of lower magnification, in Fig. 6. The fatigue damage in the glass-fibre/PP is characterized by large distributed debonds originating from flaws or fibre breaks. The former grow in an accelerating-decelerating manner, and the debonds become more pronounced with thicker and darker appearance with increasing number of cycles. This is due to attrition of the crack surface asperities as the local loading is mainly in mode II. The mismatch in elastic properties of the fibre and the surrounding matrix results in shear loading. When the effective frictional constraint has become low enough, the interfacial crack will propagate before being arrested by a microstructural obstacle. There is a variability in interfacial properties on the micrometer size scale which will further enhance the erratic debond growth pattern (see e.g. Zhang et al.[17]). The debond mechanism is not critical per se, since it does not directly result in ultimate failure of longitudinal composites, but as the debonds grow, the overload in the adjacent fibres redistributes. When the redistributed overload exceeds the local strength of a weak fibre segment, it will fail and serve as an initiation point for the growth of new debonds