Crack-Parallel Stress Effects in Soft Material Fracture: Insights From Gap Tests and the Microplane Modeling Perspective

Abstract Fracture in soft materials such as elastomers and biological tissues is traditionally characterized by a constant fracture energy, independent of nonsingular stresses. Recent advances in stiff quasi-brittle and ductile solids have revealed, however, that the nonsingular crack-parallel stresses strongly influence the fracture behavior and cause the fracture energy to be a variable. Here, this concept is extended to soft materials undergoing large strains. Gap tests on synthetic rubber demonstrate that crack-parallel pre-compression increases both the peak load and the fracture energy significantly. This can be explained only by the presence of a finite-width damage zone at the fracture front. Complementary cutting tests further confirm increased energy dissipation under pre-compression. To gain insights into these findings, a microplane-based constitutive framework is proposed. The framework captures orientation-dependent inelasticity, frictional dilatancy, fiber stiffening, softening damage, and the vertex effect, while ensuring thermodynamic consistency at large strain. The results establish that fracture in soft materials cannot be fully described by a line crack with a singular crack-tip field and a constant fracture energy. Rather, a damage zone of finite width must exist at fracture front. Because of the damage zone, an incremental inelastic finite element modeling is required, which brings in further complications in finite-strain modeling compared to the hyperelastic case. Experience with damage under impact and with friction shows that the microplane deformations are best characterized by the stretch and angle change of two initially normal vectors calculable from the Green–Lagrangian strain.