Influence of the stress level and hygrothermal conditions on the creep/recovery behaviour of high-grade flax and hemp fibre reinforced GreenPoxy matrix composites

This paper investigates the influence of the stress level and hygrothermal conditions on the creep/recovery behaviour of three high-grade composites made of GreenPoxy and flax and hemp fibres. The results show that the levels of instantaneous, time-delayed and residual strains increase with the applied load and the severity of the environment. The time-delayed strains of the materials are higher in the creep phase than in the recovery phase. A stiffening effect is also observed during the recovery phase. The post-creep viscoelastic properties are then identified using an anisotropic viscoelastic law from the recovery function. The relaxation time function is independent of the stress level and the environmental conditions. The viscous parameter varies with the stress level and increases substantially with severe environmental conditions. The dependence of the creep/recovery behaviour on the stress level is due to the dependence of the stiffening phenomenon and irreversible mechanisms rather than to the viscoelastic behaviour.