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.

This work assesses the potential of stinging nettle (Urd.ca dioica L.) growing on trace element contaminated soils to produoe fibres for rnaterial applications. The nettles studied in this work grew spontaneously and dominated the vegetation cover in poplar short rotation coppices planted for the phytornanagement of lands contaminated by trace elements. Two sites were studied, contaminated by Hg for the first one and a mix of As, Cd, Pb and Zn for the second one. Results show that, for the considered soils, the contaminant contents in nettle bast fibres were at low levels, comparable to those collected at unpolluted control areas. lt rnakes it possible to consider this biornass for material use. The measured rnatter yield was lower than those obtained with traditional fi bre crops cultivated in Europe on agricultural lands. However, the tensile properties of the bast fibres mechanically ex tracted without field retting or prior alkaline treatment were equal to or better than those of industrial hemp and flax, rnaking spontaneous nettles an interesting supplement to traditional European fi bre crops for rnaterial applications.