Nonlinear tensile behaviour of elementary hemp fibres. Part I: Investigation of the possible origins using repeated progressive loading with in situ microscopic observations

The aim of this study is to achieve a better understanding of the nonlinear tensile behaviour of the elementary hemp fibre. This is of great importance in view of the need to develop an efficient predictive tool for the design of natural fibre reinforced composites. This first paper investigates the possible mechanisms responsible for the nonlinear behaviour, using repeated progressive tensile loading with in situ polarised light microscopy.The persistence of residual strain has been confirmed during testing when the tensile load was released. Only a certain fraction of this residual strain is reversible, and the reversibility is time-dependent. Beyond the yield level, the fibre’s rigidity is not deteriorated, but significantly increased as a function of the number of loading cycles and the level of strain. A new scenario involving a stick–slip mechanism, extension and re-orientation of the microfibrils and shear strain-induced crystallisation of the amorphous cellulose is proposed.