Micromechanism Based Material Models for Natural Fiber Composites

In tensile tests the PP/flax fiber composites clearly shows a nonlinear behavior in loading and hystheresis loops in unloading. In creep tests performed at different load levels the response was nonlinear viscoelastic, and after recovery, viscoplastic strains were detected. No degradation in stiffness could be seen and thus nonlinear viscoelasticity and viscoplasticity were assumed to be the main cause for the observed behavior. The viscoplastic response was described by a power law with respect to time and to stress level in the creep test. Schapery’s model for nonlinear viscoelasticity and a power law for viscoelastic compliance with respect to time have been used to characterize the material. The developed model has accuracy sufficient for practical applications. However, at high stresses the attempts to describe the viscoelastic compliance by a power law with a stress independent exponent were unsatisfactory and therefore stress dependence of this exponent was included in the data analysis. The accuracy is perfect but the thermodynamic consistency of this procedure has to be proven.