Creep and stress relaxation of unidirectional flax fiber reinforced laminates

The growing demand for the use of biobased composites in different engineering and structural applications requires proper test methods and models to predict their long-term behavior. Assessing the creep resistance and relaxation effects in natural fiber-reinforced polymer (NFRP) laminates is of major interest, and reliable data are not yet available in the literature. To date, limited information is available on the creep resistance of composite materials based on natural fibers or biodegradable matrices. In the present work, the long-term behavior of flax fiber-based composites (FFRC), realized through Vacuum Assisted Resin Transfer Moulding (VARTM) technique is presented. The mechanical properties and the creep and relaxation behavior of unidirectional FFRCs under tensile stress at ambient temperature are investigated. The results highlight the role of viscoelastic (VE), viscoplastic, and pseudo-plastic components of the NFRPs’ mechanical response, unlike traditional composites (as glass and carbon FRP) whose response is generally approximated as linear-elastic. Moreover, starting from the experimental data, basic rheological models are implemented and utilized to replicate the stress–strain–time behavior of the FFPs. The limitations of VE rheological models in fitting the observed behavior of FFRPs are argued: A set of implementations highlights the need for more complex visco-elasto-plastic predictive models able to fit the measured stress–strain–time behavior of FFRC. Consequently, the need for robust methods to identify the large number of parameters associated to VP models arises.