Fatigue strength of carbon fibre composites up to the gigacycle regime (gigacycle-composites)

Knowledge of the fatigue strength is necessary for design purposes also at very high numbers of load cycles. While static properties and fatigue strength curves in the low, medium and high cycle fatigue regime are well investigated, only few data beyond 5×106 cycles (gigacycle fatigue regime) are available. In a first part of this paper, the fatigue strength for various laminates of a carbon fibre reinforced thermoplastic material (AS4/APC-2) was investigated experimentally up to 109 cycles. The specimen shape was optimised for tensile, compression and bending load cases to get significant fatigue data. Various candidates of fatigue damage indicators have been evaluated and monitored continuously during these fatigue tests. A general trend of the fatigue strength reduction could be observed for the majority of laminates and loading conditions beyond 5×106 load cycles. A fatigue limit (endurance limit) was not found. Significant changes of the stiffness, resonance frequency and temperature could be measured. However, a correlation between these signals and the final failure of the specimen could not be found. In a second part, a fatigue design procedure was developed, which allows predicting the fatigue life of a typical structural detail under bending loads. Various failure criteria used in the static mode have been extended to fatigue loads. In a verification test program, the fatigue strength of a waisted beam has been assessed for bending loads and compared with the predicted fatigue life.