Tracking the Evolution of Heterogeneous Crystallization Driven by Complex Deformation Scenarios in Natural Rubber

Natural rubber (NR) has regained attention due to its sustainability and exceptional mechanical properties driven by strain-induced crystallization (SIC)─a unique self-reinforcing mechanism. Despite extensive research, the SIC behavior under complex deformation conditions, frequently encountered in NR products, remains insufficiently understood. This study investigates the evolution of nonuniform SIC in unfilled NR under heterogeneous deformation using a specially designed geometry where diverse local deformation modes are achieved within a single tensile test. By integrating digital image correlation and high-speed infrared thermography, we map the spatial distributions of strain and the associated crystallinity evolution across the specimen. The findings reveal that local SIC initiates at nearly the same critical longitudinal strain, regardless of local strain biaxiality characterized by the lateral-to-longitudinal true strain ratio (μ₁₂ = −ε₂/ε₁). However, the subsequent evolution of SIC is strongly influenced by local deformation characteristics. At a constant μ₁₂, local crystallinity (χ) increases with ε₁. Conversely, at a constant ε₁, χ increases with μ₁₂, indicating that uniaxial stretching promotes higher crystallization than other deformation modes. An empirical relation describing the strain–crystallinity relationship, using ε₁ and μ₁₂ as variables, enables comprehensive tracking of crystallinity evolution under nonuniform deformation. These insights deepen the understanding of SIC mechanisms, and guide the design of high-performance, sustainable rubber materials.