  Large deformations of soft materials are customarily associated with strong
constitutive and geometrical nonlinearities that originate new modes of
fracture. Some isotropic materials can develop strong fracture anisotropy,
which manifests as modifications of the crack path. Sideways cracking occurs
when the crack deviates to propagate in the loading direction, rather than
perpendicular to it. This fracture mode results from higher resistance to
propagation perpendicular to the principal stretch direction. It has been
demonstrated that such fracture anisotropy is related to the microstructural
stretch of the polymer chains, also known as strain crystallization mechanisms.
However, the precise variation of the fracture behavior with the degree of
crosslinking is not fully understood. Leveraging experiments and computational
simulations, here we show that the tendency of a crack to propagate sideways in
the two component Elastosil P7670 increases with the degree of crosslinking. We
explore the mixing ratio for the synthesis of the elastomer that establishes
the transition from forward to sideways fracturing. To assist the
investigations, we construct a novel phase-field model for fracture where the
critical energy release rate is directly related to the crosslinking degree.
Our results demonstrate that fracture anisotropy can be programmed during the
synthesis of the polymer. Then, we propose a roadmap with composite soft
structures with low- and high-crosslinked phases that allow for control over
fracture, arresting and/or directing the fracture. By extending our
computational framework as a virtual testbed, we capture the fracture
performance of the composite samples and enable predictions based on more
intricate composite unit cells. Overall, our work offers promising avenues for
enhancing the fracture toughness of soft polymers.
