Mechanical fracture of Nafion® membrane limits the life of PEM FC stacks. This is likely a result of gradual strength degradation and mechanical stress/strain transients induced by the cycling relative humidity (RH). Mechanical properties of Nafion® membrane strongly depend on water content. The objectives of the authors’ work are (1) to understand the fundamental mechanical behavior of an ionomer membrane, i.e., Nafion®, as a function of RH and (2) to develop physically meaningful models to perform stress/strain analysis of membrane electrode assemblies under RH and temperature variations. To characterize the mechanical response of an ionomer as a function of temperature and relative humidity, an environment chamber capable of generating temperatures from 25 to 100 degrees Centigrade and relative humidities from 5 to 85 percent was designed and built. An electromechanical membrane test (load) frame was mounted as an integral part of the system. An optical strain measurement device was used to record axial extension and lateral contraction of the membrane specimens without contact. Extensive mechanical tests on a commercial ionomer membrane were conducted under carefully controlled hydration and temperature. Fully nonlinear, fully anisotropic elasto-plastic constitutive representation of this ionomer material was obtained as function of temperature and RH. Water content significantly affects the elastic modulus of the membranes. Experimental data show that the elastic modulus of the membrane continuously increases up to about twice the original value during dry out. Such has been taken into account in order to accurately model the stress/strain history of the membrane during dry-out. The collected experiment data were represented in material constitutive models for use in a finite element code, ABAQUS. A 3-layer membrane electrode assembly (MEA) structure has been modeled to observe stress/strain distribution during RH and T cycling. Non-uniform electrode/membrane interfaces have been modeled as well as uniform sections to see the effects of geometric irregularities on the extreme values of stress and strain.

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