The commercial viability of polymer electrolyte membrane fuel cells (PEMFCs) for transport applications relies on the mitigation of the degradation induced by highly-transient mobility operation. Current computational strategies largely bifurcate into high-fidelity 3D-CFD models with decoupled degradation or one-dimensional mechanistic models, thereby failing to capture the critical spatial-temporal feedback loops driving the stack failure [1]. This work addresses this gap by presenting a fully coupled, transient 3D-CFD framework that integrates mechanistic degradation kinetics within a Eulerian-Eulerian multiphase mixture solver [2]. The proposed framework couples a mechanistic six-step carbon corrosion model in the catalyst layer and a radical-induced ionomer depolymerization model [3,4]. Unlike conventional decoupled approaches, this solver dynamically updates macroscopic properties of the electrolyte, such as membrane thickness, equivalent weight and ionic conductivity, based on instantaneous and local electrochemical states. The model is applied to a sampled real-world urban driving cycle, characterized by dynamic load variations typical of a light-weight hybrid vehicle. Simulation results demonstrate the framework's capability to spatially resolve degradation heterogeneities, identifying critical spots where the material stress is maximized. Furthermore, the bidirectional coupling reveals the causes for a synergistic acceleration of failure: localized membrane thinning enhances gas crossover, which in turn fuels further radical generation via the Fenton reaction. This spatially resolved analysis provides a robust platform for optimizing durability strategies against realistic fatigue stressors.