Non-linear model applied on composites exhibiting inelastic behavior: development and validation

Sammanfattning: The polymeric composite materials are in high demand by industries where light and strong materials are required. Although manmade fiber (e.g. glass, carbon, aramid fibers) are most often used to reinforce polymers, natural fibers due to their environmental friendliness and sustainability have been also considered. Natural fiber composites have shown to have great potential as a substitute for conventional glass fiber materials. However, bio-based composites exhibit highly non-linear behavior, besides they are very sensitive to elevated moisture and temperature. Therefore, careful design and optimization of composite properties defined by constituents, composition and internal structure is needed to meet requirements of real-life applications. This can be done by using accurate models that can take into account factors responsible for inelastic behavior of these materials. The initial part of this thesis is dealing with development of phenomenological approach to predict inelastic behavior of composites in tension. Viscoelasticity and viscoplasticity was analyzed in short term creep tests and modulus degradation in stiffness degradation tests. Schapery’s model for viscoelasticity and Zapa’s model for viscoplasticity was used to characterize nonlinearity. This method was then validated on short, randomly oriented fiber composites with different cellulosic fibers (flax, viscose) and bio-polymers (PLA, Lignin). The elastic modulus, tensile stress-strain curves and failure were analyzed at different humidity and temperature levels. Results showed high sensitivity to moisture and temperature and highly non-linear behavior of these materials. Modeling showed good agreement between experimental data and simulations. Since there is need for simulations of strain controlled tests, this model was rewritten in inverted incremental form. Simulations of stress-strain curves showed, that predictions are more accurate, when characterization of viscoelastic and viscoplastic parameters was done at stresses close to failure. However, due to creep rapture it was not always possible to characterize material at high stresses and in this case viscoelastic functions have to be extrapolated. The stress-strain curves can be then used to further adjust extrapolation of model parameters. The model developed in the first part of the thesis proved to be capable of predicting behavior of short fiber composites with good accuracy. However, in order to carry out simulations input parameters have to be experimentally obtained and it has to be done for every composite that is studied. The second part of this thesis is dedicated to development of constitutive model which uses parameters of constituents to predict behavior of material with any composition. This model then is applied on semi-structural natural fiber composites consisting of bio-based resins reinforced with continuous cellulosic fibers. Mechanical properties of different bio-based thermoset resins and regenerated cellulose fibers have been analyzed. Results showed comparable properties of bio-based and synthetic epoxy resins, even at elevated humidity levels, but high scattering of properties from sample to sample. They also showed that bio-based resin exhibit limited non-linearity whereas regenerated cellulose fiber is highly non-linear. In order to avoid large scatter typical for bio-based materials and improve accuracy of the model, methodology for parameter identification for viscoplastic model with use of only one sample has been suggested. The objective here is to simulate strain controlled tests and the most convenient way to do it is with Schapery’s strain formulation model. The parameters for such model can be obtained from relaxation tests, where viscoelastic strain is kept constant but due to presence of viscoplastic strain component such experiments are difficult to perform. Instead, constituents exhibiting viscoplastic behavior have been characterized in creep and viscoelastic parameters for Schapery’s strain formulation are obtained from simulations of relaxation tests with inverted incremental model. Then these parameters are used to simulate behavior of composite subjected to iso-strain conditions.