During operation, proton-exchange-membrane fuel cells (PEMFCs) are subjected to mechanical and chemical stressors that contribute to membrane degradation, performance loss, and eventual failure. Together, synergistic effects between mechanical and chemical degradation mechanisms lead to accelerated degradation. A physics-based model is developed to understand the synergistic effects of chemical and mechanical degradation and the coupled nature of performance and durability in PEMFCs. The model includes pinhole existence and growth in the membrane, which increases crossover of reactant gases as well as subsequent formation of chemical degradation agents that impact both transport and mechanical properties of the membrane. The fuel-cell model is fully coupled with a mechanical model to determine the stresses on the membrane and subsequent growth of pinholes during transient operation. Simulation results demonstrate pinhole growth under relative-humidity cycling and the resultant increased gas-crossover fluxes and decrease in polarization performance. Furthermore, the model results highlight nonlinearities and the importance of coupling mechanical and chemical degradation models in order to explain membrane degradation under various cycles, and serves as a foundation for examining coupled durability and performance. (c) The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.