Wiggly crack paths in the tearing of thin films

Oscillating crack paths are produced in the sort of thin floppy films used for packaging when a starter hole or crack is propagated in the plane of the film by a blunt object (for example the tip of a Biro or a rod) pushing along the tear. The effect is most marked when the path of the tear is locally unsupported, that is can be deflected out of the plane of the sheet. Tearing occurs in mode III, the waves of material appearing as flaps displaced in-and-out of the film. In some cases, as a flap is produced, the previous flap gets trapped between the rod and the edge of the waveform, and this can produce permanent creases across the flap. The problem is analysed in a preliminary way and compared with experiment. A key part of the solution is to find the variable angular velocity of the contact points between rod and film. Practical films are anisotropic as regards elastic moduli, yield strength and fracture toughness, and such variations with direction in the film have a profound effect on the detailed shapes of the oscillating paths.