Hamm, EugenioSivak, IrynaRoman, Benoit2020-05-142020-05-142020-05-142020-04-2910.1103/PhysRevLett.124.174101https://infoscience.epfl.ch/handle/20.500.14299/168741WOS:000529312400012Cutting a brittle thin sheet with a blunt object leaves an oscillating crack that seemingly violates the principle of local symmetry for fracture. We experimentally find that at a critical value of a well chosen control parameter the straight propagation is unstable and leads to an oscillatory pattern whose amplitude and wavelength grow by increasing the control parameter. We propose a simple model that unifies this instability with a related problem, namely that of a perforated sheet, where through a similar bifurcation a series of radial cracks spontaneously spiral around each other. We argue that both patterns originate from the same instability.Physics, MultidisciplinaryPhysicsfracture pathstext::journal::journal article::research article