Effects of debonding and fiber strength distribution on fatigue-damage propagation in carbon fiber-reinforced epoxy

In order to design new fatigue-resistant composites, the underlying fatigue damage mechanisms must be characterized and the controlling microstructura l properties should be identified. The fatigue-damage mechanisms of a unidi rectional carbon fiber-reinforced epoxy has been studied under tension-tens ion loading. A ubiquitous form of damage was one or a few planar fiber brea ks from which debonds or shear yield zones grew in the longitudinal directi on during fatigue cycling. This leads to a change in stress profile of the neighboring fibers, and an increase in failure probability of these fibers. The breakage of fibers in the composite is controlled by the fiber strengt h distribution. The interaction between the fiber strength distribution and debond propagation leading to further fiber breakage was investigated by a numerical simulation. It was found that a wider distribution of fiber stre ngth and a higher debond rate lead to more distributed damage and a higher fracture toughness. Implications to fatigue life behavior are discussed, wi th reference to constituent microstructure. (C) 2000 John Wiley & Sons, Inc .