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This paper experimentally analyses the delamination behaviour of a composite material under mode III fracture as a function of the loading mode applied during the fatigue tests was carried out in two different ways. First, the test specimens were loaded cyclically in a single direction of delamination until reaching a certain fracture energy and subsequently returning to the starting point. Second, cyclic loading was applied both in one direction of delamination and in the opposite direction until reaching the same fracture energies, though in two opposite directions. The experimental results revealed clear differences in the fatigue behaviour of the material when the way of applying the load varied. In both cases, a statistical analysis of the results was carried out in order to evaluate the existing differences more suitably. Furthermore, a fractographic analysis was conducted for both types of loading in order to distinguish possible differences at the microstructural level.

The present paper studies the fatigue delamination behaviour of an epoxy/carbon composite material under mode III loading using a longitudinal half fixed beam (LHFB) device initially designed for mode III static tests of composite materials formed by the stacking of plies. For this purpose, a series of tests was carried out at different levels of loading representative of the fatigue behaviour of the material, from the crack onset phase through the delamination phase to final fracture. The experimental results were treated statistically, obtaining the values of the fatigue limit for probabilities of fracture of 5% and 50%. Finally, a fractographic analysis of the fracture surfaces was performed which allowed us to identify the same characteristic patterns of static mode III fracture, namely broken fibres, cusps and saw-teeth, in addition to a new morphology consisting of the formation of agglomerations of resin produced by the friction between the lips of the specimen in the fracture zone that point to dynamic mode III fracture. These agglomerations eventually crack and become detached from the fibres, leaving these free of resin.

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