Non-linear behaviour of PLA based flax composites

The mechanical behaviour of polylactic acid/flax fibre composite in tension was investigated by analysing elastic properties and loading curves. The observed non-linearity was attributed to microdamage, viscoelastic and viscoplastic response, suggesting Schapery's type of model for viscoelasticilty and Zapas' model for viscoplasticity. It was found that 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. Viscoplastic and viscoelastic strain development was analysed in creep and strain recovery tests at several high stress levels. The identified and validated material model is non-linear viscoelastic and viscoplastic with slight non-linearity even in the elastic strain term. It appears that there is no region of linear viscoelasticity for this material. Non-linear elasticity, viscoelasticity and viscoplasticity are equally responsible for the observed non-linearity in the tensile tests.