Universal Shapes of Interacting Mode-I Cracks

We experimentally investigate the interaction between two parallel cracks propagating towards each other under uniaxial traction in quasi-2D slabs of gelatin. A single crack would propagate perpendicular to the direction of traction. However, after they pass each other, the two cracks rotate and ultimately meet, leaving behind a lens-shaped remnant. We find a universal length-to-width ratio for this remnant, independent of the pulling speed and initial crack separation; the same phenomenon is observed in a variety of elastic materials. Moreover, the overall dimensions of the lens-shaped remnant are set by the initial crack separation.