Using a Knife Slitting Test to Characterize the Fracture Resistance of Proton Exchange Membranes

Through-the-thickness flaws or “pinholes” in proton exchange membranes (PEM) can lead to gas crossover, reducing fuel cell efficiency, accelerating degradation, and raising safety issues. The multi-physics process that causes these flaws is not fully understood, but stress state, environmental exposure, and cyclic operation may all be contributing factors. Fracture mechanics has proven to be useful in characterizing degradation of many materials, including polymers subjected to environmental challenges. Although unclear if pinhole formation can be successfully characterized and predicted from a fracture perspective, this study continues our prior work to characterize PEMs in such a manner. Because of the lack of constraint, thin films often exhibit very high fracture energies and large plastic zones, features that are not consistent with observations of PEM failures. In an effort to obtain the fracture energy with very little dissipation, knife-slitting tests were conducted to reduce the crack tip plasticity. With modifications made to the systems used by Wang and Gent (1994) and by Dillard et al (2005), a slitter that maintains a constant tearing angle during the slitting process was developed. While fracture energies on the order of 10⁴J/m² were measured with double edge notched test samples, and on the order of 10³J/m² were measured with trouser tear samples, the knife slit test resulted in fracture energies as low as several hundred J/m². An environmental chamber was used to enclose the slitting process so experiments at elevated temperature and moisture levels could be conducted. The relevance of these fracture energies to observed PEM failures in operating fuel cells is not fully understood. Nonetheless, the ability to obtain fracture energies approaching the intrinsic fracture energy of these ductile membranes is believed to be useful in studying what appear to be more brittle fracture modes that have been observed in PEMs.