Numerical-experimental approach to identify the effect of relative humidity on the material parameters of a rate-dependent damage model for polyamide 66

This work aims at identifying the behavior of polyamide 66 (PA66) under different Relative Humidity (RH) conditions using a phenomenological model that accounts for viscoelastic and viscoplastic rheology coupled to ductile damage. An experimental approach is designed considering different loading conditions, namely: monotonic at several strain rates, loading-unloading, creep-recovery, and cyclic tests. These experiments are chosen to discriminate the various active mechanisms governing the nonlinear behavior of PA66. The thermodynamic background of the phenomenological model, the evolution laws, and the accompanying RH-dependent material parameters are presented and discussed. Using the experimental findings, an optimization algorithm is adopted to identify the model parameters. The latter are investigated with regard to the relative humidity, leading hence to the development of a model that accounts for the effect of RH on all inelastic mechanisms and ductile damage. Validation through experimental data for RH = 0%, 25%, 50%, 65%, and 80% reveals that the current model captures the effect of RH and yields mechanical responses in good agreement with experimental findings, notably at higher RH levels. The current numerical-experimental framework presents a unified model for a wide range of humidity conditions and provides a better insight into material properties and rate-dependent inelastic mechanisms under humidity exposure, which typical models do not provide. In addition, the present constitutive law is easily adoptable in micromechanics schemes for the study of polymer based composite materials and structures.