Microstructure related mechanical response and fatigue crack growth behavior of polymer electrolyte membrane under in-situ loading

Polymer electrolyte membrane (PEM) is a key component in fuel cells, however, its mechanical degradation behavior driven by fatigue is not yet well understood. Herein, combined with digital image correlation and microscopic observation, the multiscale mechanical response and crack growth behavior of PEM associated with microstructure were investigated using multiple in-situ tests including uniaxial tensile, stress relaxation and crack growth with different stress ratios. Results show that PEM clearly presents the rate dependence and anisotropy. Combined with the area statistics of hydrophobic main chains before and after tension, the plastic deformation mechanism associated with molecular chain rotation and unwinding was explained, and a modified multilayer viscoelastic-plastic constitutive model in consideration of the effects of plane stress, anisotropy and true stress was developed. Furthermore, based on the analysis of strain field at the near crack-tip, the size of cyclic plastic zone tends to increase with the increasing of crack length and stress ratio, but the effect of crack length on crack growth rate is more significant due to the larger stress concentration effect. Finally, the failure mechanism associated with ligament, tearing plane and resilient fatigue striation was elucidated.