About the fatigue endurance of unidirectional flax-epoxy composite laminates

Even if the knowledge on the fatigue behaviour of plant fibre composites has increased steadily in the last few years, some issues still remain open at the present time. Such is the case, for instance, of the high-cycle fatigue strength. Actually, most of the fatigue studies available in the open literature to date are limited to a maximum of 1-2 million cycles. All available stress-life plots exhibit linear trends with constant slope and does not reveal any fatigue limit for these given cycle numbers. So, this paper proposes to investigate the High-Cycle Fatigue behaviour of a flax-epoxy laminated composite. The effect of loading frequency is firstly evaluated on the Low Cycle Fatigue behaviour using a multi-instrumented analysis including infrared thermography and acoustic emission. Results show that high frequency could be a suitable method to shorten the fatigue tests and study the High Cycle Fatigue behaviour of this type of composite material. Based on this result, high-frequency (30 Hz) is used to investigate the behaviour of the flax-epoxy composite on a range of 10(6)-10(8) cycles. Results show that fatigue damage continues to evolve and the maximum stress continues to decrease as a function of increasing number of cycles, following a power-law trend. This result suggests that, if a fatigue limit does exist for unidirectional flax-epoxy composite laminates, it is so low that it cannot observed in tests up to 10(8) cycles. It is also recommended to take it into consideration when designing plant fibre composite structures.