Viscoelastic behaviour of single hemp fibre under constant and cyclic humidigy environment - Experiment and modelling

Natural fibres derived from annual plants are attractive candidates to reinforce organic matrix in high performance composite applications. This use requires an accurate understanding of their mechanical properties and the development of efficient models. In the last years, many efforts were focused on the characterisation of the tensile properties of bast fibres under quasi-static loading. In contrast, the time-dependent behaviour has almost not been examined. However, considering the polymeric composition of their wall, plant fibres can exhibit to greater or lesser extent viscoelastic behaviour. This point is of great importance in view of their integration in composite materials. Effectively, contrary to glass or carbon fibres reinforced plastics, the time-dependent behaviour of natural fibres reinforced plastics could arise both from the matrix and the fibres. The aim of this study is to investigate and model the time-dependent tensile behaviour of single hemp fibres. This work proposes an experimental approach, using creep tests and Dynamic Mechanical Analysis under constant and cyclic humidity environment. A 3D anisotropic viscoelastic model based on a spectral distribution of elementary mechanisms is proposed to describe the time-delayed response of these fibres. Hemp fibres are shown to exhibit both instantaneous deformation and delayed, time-dependent deformation when tensile loaded. Under constant environment, three types of creep behaviour are observed. Our results show that an inverse Gaussian spectral model with truncation associated to a reliable parameters identification strategy is able to describe accurately the 3 types of creep behaviour. The primary and secondary creep rates are also shown to be highly influenced by the moisture level and moisture cycling. Finally, the anisotropic viscoelastic constitutive law identified using creep tests is used to simulate the quasi-static tensile behaviour of such fibres. It demonstrated that the experimentally observed nonlinear tensile behaviour can be partly attributed to viscoelastic mechanisms.