Abstract
<jats:p>Fracture in elastomers is commonly described by a critical energy release rate determined under uniaxial loading, yet its applicability to multiaxial deformation and to materials with evolving microstructure remains unclear. Here we quantify crack-growth onset in elastomer sheets subjected to controlled biaxial stretching by combining energetic analysis with high-resolution measurements of near-tip deformation fields. For predominantly elastic elastomers that do not undergo deformation-induced structural evolution, we confirm that crack initiation occurs when the energy release rate reaches a material-specific critical value that is insensitive to deformation biaxiality. In contrast, strain-induced crystallizing elastomers, such as natural rubber, exhibit a breakdown of this universality. This fracture response is governed by a biaxiality-controlled transition in near-tip fields: below a critical deformation state, crystallization remains spatially confined and contributes minimally to crack resistance, whereas above this threshold, crystallization is activated in an extended region surrounding the crack tip. This localized structural reinforcement substantially elevates crack-opening displacement and the critical energy release rate. Consequently, fracture resistance is no longer an intrinsic material constant but is instead controlled by the size and development of the crystalized zone. These results delineate the conditions under which fracture in elastomers follows universal energetic laws and when it becomes dominated by deformation-activated structural transformations. This framework provides practical guidance for designing robust elastomers for applications involving complex loading, including tires, soft robotic components, and biomedical devices.</jats:p>