Tearing path: instabilities for crack paths in thin sheets cut by blunt objects

A thin sheet is a very soft object, which therefore undergoes very large deformation when torn.
We found that a straight crack which propagates quasistatically in such a sheet when driven by a blunt object may become unstable in several configurations : oscillatory fracture when the object is displaced along a line, or spiraling when using a perforating cone.
We wish to compare these two configurations and show how these instabilities compare in involving the very soft bending mode.