Polymer electrolyte membrane (PEM) fuel cell, which relies on an ionomer-based membrane electrolyte to enable the transport of protons and the separation of electrochemical redox reactions, is currently widely pursued for transportation applications. High cost and poor durability remain to be significant barriers for commercial market penetration of PEM technology. PEM performance degradation is an extremely complicated process that involves the numerous mechanisms across several disciplines. Mechanical failure of membranes in the form of pinholes and fractures/tears is commonly observed in PEM stacks. Rapid performance loss due to gas cross-over and sometimes catastrophic failure shortly follow the mechanical breach of membranes. Improving the mechanical endurance of the electrolyte membrane is critical for extending the life of PEM stacks. An approach based on experiments and numerical modeling is being developed to help understand and elucidate the mechanical failure modes and mechanisms. Experiments have revealed that membrane strength can degrade rapidly under certain PEM operation conditions. The molecular nature of such degradation process is also being investigated. Membrane failure occurs when its “strength” drops below the mechanical stress/strain induced by RH cycling. A mechanistic approach is being developed to predict the remaining strength and stack life by tracking the evolution of these two variables.