Effect of incision curvature on measuring fracture energy of soft materials

Fracture energy is the property that characterizes how a material resists crack growth. In a standard measurement of fracture energy, an incision is typically introduced into the specimen. It is known that the measured fracture energy may depend on the incision curvature. However, the underlying mechanism of such dependence remains unclear. In this paper, we prepared polyacrylamide/Ca-alginate hydrogel specimens featuring incisions with circular tips of varying diameters. The fracture energy was subsequently measured through a pure shear test. We observed that the fracture energy is proportional to the incision diameter, with a slope comparable to the work of fracture for larger tip diameters. Conversely, for smaller tip diameters, the fracture energy remains independent of the incision diameter and aligns with the intrinsic fracture energy. This transition occurs at an incision diameter comparable to a material-specific scale known as the fractocohesive length. Notably, the fractocohesive length, rather than the inelastic zone scale, successfully explains the dependence of fracture energy measurement on incision curvature. The difference between these two length scales of the material here spans three orders of magnitude. These results will be helpful for establishing standards for measuring fracture energy of soft materials.