Plasticity-Based Constitutive Framework for Multi-Scale Modeling of Mechanical Degradation in PEM Fuel Cell Membranes

The long-term durability of polymer electrolyte membranes (PEMs) remains one of the key barriers to the widespread deployment of fuel cells. This work presents a plasticity-based constitutive framework for predicting membrane degradation under cyclic hydration and thermal loading. The model combines hydration- and temperature-dependent elasticity, swelling, and plastic strains with a pinhole growth law that is activated only when both mechanical plasticity and chemical degradation are present. A binary fluoride release rate (FRR) switch is introduced to couple chemical activity with defect growth in a computationally efficient manner. Simulation results reproduce key experimental and modeling trends from the literature. Larger hydration amplitudes Δλ accelerate void growth and plastic strain accumulation, while thermal swings increase hydrostatic stresses and material softening, thereby enhancing defect evolution. The framework thus provides a physically consistent rationale for the coupled influence of hydration variability, thermal cycling, and chemical degradation on membrane lifetime. While simplified in geometry and chemistry, the present approach captures the irreversible nature of plastic deformation and remains tractable for long-term cycling studies. It provides a robust basis for future extensions toward 2D/3D analyses and for integration into multi-physics durability assessments of PEM fuel cells.