A viscoelastic–viscoplastic model with hygromechanical coupling for flax fibre reinforced polymer composites

Abstract Based on experimental investigations on the influence of water content on time-dependent behaviour of flax reinforced epoxy composites, this paper proposes to model the viscoelastic–viscoplastic behaviour with hygromechanical coupling. From a rheological point of view, the proposed model is a combination of a linear spring (elasticity), a Kelvin–Voigt model (viscoelasticity) and a viscoplastic model described by kinematic hardening in which the evolution of the internal variable is collinear with the plastic deformation. The hygromechanical coupling is obtained via the dependence on water content of all the model parameters according to a power law. The developed model shows a good correlation between experimental data (tensile, creep and relaxation) and numerical predictions of the mechanical behaviour for low water contents (below 4.90%). However, the model shows its limits for high water contents ( w c ≥ 4 . 90 % ) and high loadings. Indeed, tensile tests have revealed the emergence of a stiffening phenomenon for high loadings according to water content. This phenomenon was not taken into account in the proposed model which could explain its limitations. This assumption is confirmed by comparing the evolution of experimental yield stress with the model prediction.