This paper deals with the study of the influence of fiber/matrix interface on the fatigue behavior of unidirectional carbon/epoxy composites. These materials are based on a T300/DGEBA/DDM system and the sizing nature, fiber oxidation, and degree of cure are varied independently. The interfacial properties were evaluated by the microdebonding test. The results show the good adhesion developed in our systems. The epoxy specific sizing increases the interfacial strength and the lack of oxidation treatment slightly decreases it.
In three-point bending, the failures of the composites all appear on the compressive side, due to undesirable fiber kinking phenomena. To overcome these difficulties, which are linked to the compressive effects of the loading nose, a compression bending test was adapted to perform monotonic as well as fatigue tests. Under this configuration, progressive failure initiates on the tensile side and slowly propagates through the thickness of the sample. The Wöhler's curves can be plotted with moderate scattering. They are very flat, in comparison to the GFRP curves, and thus show the excellent fatigue behavior of these materials.
The effect of the interface on both the monotonic and cyclic behaviors is clearly pointed out. The lack of fiber oxidation increases the monotonic properties without changing the fatigue resistance significantly. On the other hand, a slight matrix under-cure leads to a better intrinsic fatigue resistance but to smaller properties-to-failure. The analysis of both monotonic and cyclic properties reveals the existence of different optima depending on the interfacial properties. The “best” interface does not exist; it is a function of both the fiber/matrix system properties and the mechanical performance studied.