Biaxiality-Controlled Crystallization Dictates Multiaxial Fracture in Elastomers

Fracture in elastomers is commonly described by a critical energy release rate determined under uniaxial loading, yet its applicability to multiaxial deformation and microstructurally evolving materials remains unclear. Here, we quantify crack-growth onset in elastomer sheets under controlled biaxial stretching by combining energetic analysis with high-resolution measurements of near-tip deformation fields. For predominantly elastic elastomers, crack initiation occurs at a material-specific critical energy release rate that is insensitive to deformation biaxiality. In contrast, strain-induced crystallizing elastomers such as natural rubber exhibit a breakdown of this universality driven by a biaxiality-controlled transition in near-tip fields. Below a critical deformation state, crystallization remains spatially confined, with minimal impact on crack resistance. Above this threshold, crystallization expands across a larger region around the crack tip, significantly elevating the crack-opening displacement and the critical energy release rate. Consequently, fracture resistance is governed by the size and development of the crystallized zone rather than an intrinsic constant. These results delineate when elastomer fracture follows universal energetic laws versus deformation-activated structural transformations. This framework provides practical guidance for designing robust elastomers for complex loading in tires, soft robots, and biomedical devices.