Predicting the relaxation modulus for the study of the delayed behaviour of kenaf fibres in stress relaxation

Plant fibres (PFs) are preferred reinforcements of bio-composites. Knowledge of their lifespan requires a study of their viscoelastic behaviour. In this paper, a stress relaxation analysis of kenaf fibres was performed at a constant rate of deformation at room temperature. A method for extracting the relaxation modulus in the deferred zone was proposed. This method was compared, using simulation, with the Zapas-Phillips method and experimental data via three predictive models: the stretched exponential function or KWW, the inverse power law of Nutting and the prony series. The results indicate that the relaxation modulus obtained by the method proposed is in good agreement with the experimental modulus. In addition, the estimated error is of the same order of magnitude as in the case of the Zapas-Phillips method. The parameters estimated from the KWW function (β = 0.4) and prony series model showed an important contribution in the study of the delayed response of kenaf fibres. These results can have a significant impact on the use of kenaf fibres in midterm and long-term loading applications.