On the identification of the coefficient of moisture expansion of polyamide-6: Accounting differential swelling strains and plasticization

Polyamide based composites are increasingly used in automobile industry. During their lifetime, they are subjected to aggressive environments including moisture which acts as a plasticizer of the polymer network, affecting its mechanical behaviour. Besides, the heterogeneous in-depth moisture content induces a differential hygroscopic swelling that may eventually cause damage. In practice, hygroscopic expansion depends on both the moisture content and coefficient of moisture expansion. Often, the polymer database do not provide detailed information on the dependence of the coefficient of moisture expansion on the moisture content. This work describes a multiscale and multiphysics approach enabling to identify the local evolution of the materials parameter of the hygro-mechanical behaviour law of a neat PA6 polymer, occurring during the transient stage of humid aging tests, from the numerical simulation of the dimensional change experienced by macroscopic samples. This study highlights the nonlinear evolution of the measured hygroscopic strain at the beginning of the moisture diffusion, as well as the need to account for the water induced plasticization effects (i.e. the fall down of the Young's modulus of the polymer material), for building an appropriate, effective modelling of the material behaviour.