Unzip instabilities: Straight to oscillatory transitions in the cutting of thin polymer sheets

¹  Department of Mathematics, Massachusetts Institute of Technology - Cambridge, MA 02139, USA
²  Levich Institute, CCNY, CUNY - New York, NY 10031, USA
³  Physique et Mécanique des Milieux Hétérogènes (PMMH), UMR 7636 ESPCI/CNRS/Paris 6/Paris 7 10 rue Vauquelin, 75231 Paris CEDEX 05, France, EU

Corresponding author: preis@mit.edu

Received: 21 December 2007
Accepted: 23 April 2008

Abstract

We report an experimental investigation of the cutting of a thin brittle polymer sheet with a blunt tool. It was recently shown that the fracture path becomes oscillatory when the tool is much wider than the sheet thickness. Here we uncover two novel transitions from straight to oscillatory fracture by varying either the tilt angle of the tool or the speed of cutting, respectively. We denote these by angle and speed unzip instabilities and analyze them by quantifying both the dynamics of the crack tip and the final shapes of the fracture paths. Moreover, for the speed unzip instability, the straight crack lip obtained at low speeds exhibits out-of-plane buckling undulations (as opposed to being flat above the instability threshold) suggesting a transition from ductile to brittle fracture.