Cumulative Damage Modeling of Fuel Cell Membranes

Abstract

The strongly varying humid environment under which fuel cell membranes operate introduces detrimental mechanical stresses that inflict damage to the membrane1. The damage incurred by the membrane during each humidity cycle when accumulated over time may lead the membrane to fatigue failure. The residual fatigue life of the membrane is determined here by using a cumulative-damage model2. A time-temperature-humidity dependent constitutive model and a multi-physics fuel cell finite element model are first developed to characterize the material properties and estimate the coupled mechano-hygral-thermal stress field in the membrane. Using this multi-level modeling approach, and with the aid of experimentally obtained fatigue data, fail-safe regions (see Figure) are identified for the case of accelerated stress test loading. For general loads simulating fuel cell operation, the distribution of stress-reversals is calculated for the load history using rain-flow counting algorithm. The regions of membrane that are more susceptible to fatigue damage are identified. It is also found that high amplitude stress cycles with low occurrence percentage are more detrimental than high occurrence low-amplitude cycles.This work was supported by Natural Sciences and Engineering Research Council of Canada, Simon Fraser University Community Trust Endowment Fund, Canada Research Chairs, Mitacs through the Mitacs Accelerate program, and Ballard Power Systems. 1 Borup, R., et al., Scientific aspects of polymer electrolyte fuel cell durability and degradation. Chemical Reviews, 2007. 107(10): p. 3904-3951. 2 Fatemi, A. and L. Yang, Cumulative fatigue damage and life prediction theories: a survey of the state of the art for homogeneous materials. International Journal of Fatigue, 1998. 20(1): p. 9-34. Figure 1