Large strain cyclic fatigue testing of unidirectional carbon fibre reinforced epoxy resin

The fatigue behaviour of a unidirectional carbon fibre reinforced epoxy resin subjected to large cyclic strains has been investigated. The development of a specific testing apparatus and procedure was found to be necessary so as to successfully test the unidirectional material in fatigue. A loop configuration has been used which allowed a gradual transfer of load from the testing machine to the specimen to be achieved without producing stress concentrations in the clamps. The results of the study reveal the existence of a fatigue mechanism for unidirectional carbon fibre reinforced epoxy resin which becomes evident under large cyclic strains, greater than 0.7%. This effect was seen to be a function of the loading level and whether the material was aged in humid conditions or not. Ageing lowered the threshold level for the onset of fatigue. The failure mechanisms involved have been revealed by scanning electron microscopy.