  Material failure is mediated by the propagation of cracks, which in realistic
3D materials typically involve multiple coexisting fracture planes. Multiple
fracture-plane interactions create poorly understood out-of-plane crack
structures, such as step defects on tensile fracture surfaces. Steps form once
a slowly moving, distorted crack front segments into disconnected overlapping
fracture planes separated by a stabilizing distance $h_{\rm max}$. Our
experiments on numerous brittle hydrogels reveal that $h_{\rm max}$ varies
linearly with both a nonlinear elastic length $\Gamma(v)/\mu$ and a dissipation
length $\xi$. Here, $\Gamma(v)$ is the measured crack velocity $v$-dependent
fracture energy and $\mu$ is the shear modulus. These intrinsic lengthscales
point the way to a fundamental understanding of multiple-crack interactions in
3D that lead to the formation of stable out-of-plane fracture structures.
