Vulcanized blends of natural rubber (NR) and poly(butadiene) (BR) are key components of high-performance tires due to their durability and fatigue resistance. Building on prior characterization of their morphology and thermomechanical properties, this project aims to elucidate the structural origin of cyclic stress-softening (Mullins effect), a widely observed yet poorly understood phenomenon in rubber blends. We will investigate miscible and immiscible NR/BR blends with varying crosslink densities and compositions under cyclic uniaxial loading using in situ simultaneous SAXS/WAXS. WAXS will probe strain-induced crystallization and orientation, while SAXS will reveal mesoscale organization and damage mechanisms. By correlating structure and mechanical response across blend types, the study seeks to identify mechanisms driving the Mullins effect and provide design guidelines for fatigue-resistant rubber blends, supporting the development of next-generation tire materials.