Fatigue-induced membrane fracture due to dynamic stresses is an important lifetime limiting failure mode in automotive fuel cell applications. Here, a series of ex situ experiments are first conducted to measure the rate of crack growth in Nafion NRE211 membranes for a range of stress, temperature (23-70 degrees C), and relative humidity (50-90%) conditions relevant to automotive fuel cell operation. The crack growth rate is found to be similar to 1-10 nm per load cycle and strongly depends on the stress intensity: the rate increases by an order of magnitude for a mere 10-30% increase in stress, which suggests that improved stress uniformity and avoidance of high stress points is important for durability. Moreover, the sensitivity to applied stress doubles from room conditions to fuel cell conditions, where the temperature has 2-3x stronger impact on the fracture propagation than the relative humidity. Microstructural analysis indicates that plastic deformation (60% localized thinning) at the crack tip accompanies crack growth. A semi-analytical model based on Paris law is then developed to simulate crack growth as a function of cyclic loading. The model incorporates elastic-viscoplastic mechanical behaviour of ionomer membranes and provides crack growth predictions in agreement with ex situ data up to 100% strain. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.