Soft materials can break under large deformations experiencing both geometric and material nonlinearities. Since the stress fields at their crack tips are highly nonlinear, the fracture process is not adequately described by traditional linear elastic fracture mechanics (LEFM). Several experimental studies with neo-Hookean hydrogels have demonstrated the existence of a nonlinear length scale ℓNL that controls crack propagation [1]. This length scale is materialspecific, as soft materials exhibit different levels of strain-stiffening or softening. In contrast, geometric nonlinearity is universal, an inherent characteristic of any material, and its relevance to the fracture process is still not understood. Recent numerical simulations have shown its effects on a propagating crack within a linear elastic material under mode I loading [2]. Driven by geometric nonlinearities, the crack-tip accelerates from the sub-Rayleigh to the super-shear wave speed range, exceeding the theoretical limit predicted by LEFM. Therefore, given the limitations of LEFM, we aim to elucidate the fundamental role of nonlinearities in the fracture process. Specifically, we present a numerical and theoretical study focused on the geometric nonlinear effects in a static crack under mode I loading. We reveal a geometric nonlinear length scale λNL that controls the stress state around the crack-tip. It is observed that the stress singularity α differs from the standard square root singularity predicted by LEFM. A numerical procedure is proposed to compute α and λNL as a function of the stress intensity factor. Furthermore, we find good agreement between our numerical results and theoretical predictions. Therefore, our research establishes geometric nonlinearity as a foundational requirement for the development of a nonlinear elastic fracture mechanics theory (NLEFM), providing a starting point for investigating other complex nonlinear fracture phenomena.