Environmental fluctuations in operating fuel cells impose significant biaxial stresses in the constrained proton exchange membranes (PEM). The PEM's ability to withstand cyclic environment-induced stresses plays an important role in membrane integrity and consequently, fuel cell durability. In this thesis, pressure loaded blister tests are used to study the mechanical durability of Gore-Select® series 57 over a range of times, temperatures, and loading histories. Ramped pressure tests are used with a linear viscoelastic analog to Hencky's classical solution for a pressurized circular membrane to estimate biaxial burst strength values. Biaxial strength master curves are constructed using traditional time-temperature superposition principle techniques and the associated temperature shift factors show good agreement when compared with shifts obtained from other modes of testing on the material. Investigating a more rigorous blister stress analysis becomes nontrivial due to the substantial deflections and thinning of the membrane. To further improve the analysis, the digital image correlation (DIC) technique is used to measure full-field displacements under ramped and constant pressure loading. The measured displacements are then used to validate the constitutive model and methods of the finite element analysis (FEA). With confidence in the FEA, stress histories of constant pressure tests are used to develop linear damage accumulation and residual strength based lifetime prediction models. Robust models, validated by successfully predicting fatigue failures, suggest the ability to predict failures under any given stress history whether mechanically or environmentally induced - a critical step in the effort to predict fuel cell failures caused by membrane mechanical failure. / Master of Science

The strength and durability of proton exchange membranes for use in fuel cells has received much attention recently due to the increased push for sustainable alternatives to the internal combustion engine. To be viable, these alternatives must have comparable lifetimes and power outputs to the internal combustion engines they replace. Chemical degradation was once viewed as the most common culprit of early fuel cell failure, but as membranes and catalysts improved, mechanical failure became an important factor. As a result, fundamental research on the mechanically-induced failure mechanisms of fuel cell membranes, coupled with development and processing of less expensive membranes, has become an important topic. The use of the blister test geometry, along with digital image correlation of the deformed shape, creates a self-contained analysis tool useful for measuring the biaxial strength of membranes. In this work, blister tests are used to measure biaxial stress and strain for fuel cell membranes subjected to ramped pressure loading to form stress-strain curves that indicate the onset of yielding under biaxial stress conditions. Stress-life curves are developed experimentally for Gore-Selec? series 57 members using data collected under constant pressure conditions. These results are used to predict blister failure under ramped and fatigue loadings. A newly implemented hydrocarbon membrane system is evaluated with constant-pressure-to-leak blister testing. Improved strength following an isothermal hold at 100°C (pretreatment) is shown to occur. Ramped pressure testing indicates that the material after the pretreatment is stiffer and has a higher yield stress than the material before treatment. Morphological and constitutive characterization indicated differences in the materials that are consistent with the improved performance. / Master of Science