Under typical PEM fuel cell operating conditions, part of membrane electrode assembly is subjected to humidity cycling due to variation of inlet gas RH and/or flow rate. Cyclic membrane hydration/dehydration would cause cyclic swelling/shrinking of the unconstrained membrane. In a constrained membrane, it causes cyclic stress resulting in mechanical failure in the area adjacent to the gas inlet. A mathematical modeling framework for prediction of the lifetime of a PEM FC membrane subjected to hydration cycling is developed in this paper. The model predicts membrane lifetime as a function of RH cycling amplitude and membrane mechanical properties. The modeling framework consists of three model components: a fuel cell RH distribution model, a 2 hydration/dehydration induced stress model that predicts stress distribution in the membrane, and a damage accrual model that predicts membrane life-time. Short descriptions of the model components along with overall framework are presented in the paper. The model was used for lifetime prediction of a GORE-SELECT membrane. I: Introduction:Mechanical degradation of PEM membrane can limit stack lifetime. Operation of a fuel cell under realistic load cycles results in both chemical and mechanical degradation of the polymer electrolyte membrane. Degradation of the membrane causes opening up of pinholes or crazing of polymer [1,2,7] increasing gas-crossover and subsequently resulting in catastrophic failures of the fuel cell stack.Understanding and modeling the mechanical degradation mechanism and kinetics enables prediction of membrane lifetime as a function of PEM operational conditions and optimization of membrane structure through the choice of reinforcement [4], the membrane processing methods and the operating conditions. The physics based model of membrane mechanical degradation could guide the synthesis of new membranes with tuned mechanical properties and enhanced life in a fuel cell.Hydration cycling is a primary cause of the mechanical degradation of a geometrically constrained polymer, which exhibits dimensional changes with varied water content. A simple way to impose mechanical damage to a geometrically constrained PEM membrane is to subject it to humidity cycling. At high RH, the membrane absorbs water, and at low RH, the membrane desorbs water. Such RH cycling would result in swelling 3 and shrinking of an unconstrained polymer. RH cycling of a constrained polymer causes cyclic stress. In PEM, such geometrical constraints are imposed by bipolar plate ribs through the gas diffusion layers (GDLs), catalyst layers adjacent to the membrane and at the seals. Moreover, we hypothesize that internal stress in the membrane can be caused by the difference in gas RH at the anode and cathode membrane surfaces that routinely occurs at fuel cell operational conditions. Cyclic stress in the membrane causes irreversible elongation of the membrane [7] and subsequent formation of crazes and cracks that causes gas crossover through the membrane and stack failure.The goal of...

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