Modeling the effect of temperature and degree of crystallinity on the mechanical response of Polyamide 6

In the current work, an extensive experimental study is performed, to investigate the influence of both, the applied thermal conditions over a wide range of temperatures and the manufacturing process induced degree of crystallinity on the mechanical response of semi-crystalline polymers. To this end, large-strain tensile experiments with different loading procedures (i.e. monotonic, cyclic, and relaxation tests) are conducted on Polyamide 6 for different loading rates. The experimental data base provides new insights into the complex dependencies of the effective material properties on the aforementioned factors and serves as the foundation for the development of a continuum mechanical constitutive framework. The phenomenological, isothermal model is developed in a reasonably general, thermodynamically consistent manner, to predict the strain rate, temperature and degree of crystallinity dependent large-deformation response of semi-crystalline polymers. A coupled nonlinear visco-elastic, elasto-plastic theory, incorporating nonlinear isotropic and kinematic hardening, is proposed to capture the complex material behavior (e.g. strain recovery and hysteresis loop after cyclic loading-unloading and nonlinear stress relaxation). A staggered characterization method is proposed, to identify a set of material parameters from the experimental data. Finally, validation studies demonstrate the great capabilities of the novel constitutive framework, to accurately predict the significant influence of the temperature and degree of crystallinity on the material response.