Non-linear behavior of bio-based composite characterization and modeling

Abstract: The development and application of bio-based composite materials have been frequently studied. Most of the work is done on quasi-static performance of these materials. However, these composites are highly non-linear therefore there is need for investigation of their viscelastic and viscoplastic behavior. This thesis is dealing with characterization and modeling of behavior of bio-based composite. The effect of temperature and relative humidity on mechanical behavior of natural fiber reinforced bio-based matrix composites subjected to tensile loading was investigated. Composites with different natural fibers (flax, viscose) and bio-based matrices (PLA, Lignin) were studied. Elastic modulus, the nonlinear tensile stress-strain curves and failure were analyzed showing that all materials are temperature sensitive. The nonlinearity was evaluated by studying modulus degradation as well as development of viscoelastic and viscoplastic strains as a function of applied load. The time-dependent phenomena were investigated in short term creep and strain recovery tests at several high stress levels. These tests demonstrated significantly higher viscoplastic strain in lignin than PLA based composites. Both, viscoelastic and viscoplastic strains are larger at higher relative humidity. The observed nonlinearity was attributed to microdamage, viscoelastic and viscoplastic response suggesting Schapery’s type of model for viscoelasticilty and Zapas’ model for viscoplasticity. PLA and lignin based flax fiber composites have been analyzed in order to obtain parameters needed for model. It was found that in PLA based composites after loading at stress levels below the maximum possible the elastic modulus is not affected and, therefore, damage does not need to be included in the material model. The modulus reduction in lignin based composites in tension starts before the maximum in stress-strain curve is reached and it can be as large as 50%. With increasing relative humidity these effects are slightly magnified. It appears that there is no region of linear viscoelasticity for PLA based composites. Nonlinear elasticity, viscoelasticity and viscoplasticity are equally responsible for observed nonlinearity in tensile tests.