Coupled Evolution of Local Stress and Strain-Induced Crystallization Near a Circular Defect in Stretched Natural Rubber

We investigate the coupled evolution of heterogeneous strain, crystallinity, and stress fields in natural rubber (NR) sheets containing a circular hole during uniaxial stretching, with a focus on how strain-induced crystallization (SIC) influences local stress concentration at structural discontinuities. By high-resolution digital image correlation, an empirical strain-crystallinity relationship and a hyperelasticity analysis firmly grounded in stress–strain data obtained under diverse deformation modes, we quantitatvely map the spatial distributions of strain, SIC, and stress in the extreme vicinity (≈0.2 mm) of the defect. Local strain concentration at the lateral hole edges triggers pronounced SIC, resulting in strong stiffening and a stress concentration factor (Kt*) as high as 4.5─significantly exceeding classical elastic predictions. Upon further stretching, Kt* plateaus, eventually as SIC develops in the far-field region, promoting a more homogeneous stress distribution. The SIC-induced stiffening also generates a highly anisotropic stress field near the hole edges due to preferential reinforcement along the crystalline orientation. The localized SIC causes characteristic hole-shape evolution, where the hole becomes increasingly elongated along the stretching axis compared with the fully amorphous state. These findings elucidate the fundamental interplay between SIC and local stress concentration in elastomers with structural discontinuities and provide mechanistic insights for designing defect-tolerant, high-toughness rubber materials.