Pedro M. Reis
Manchester Centre for Nonlinear Dynamics, Unversity of Manchester, UK
| There has been a recent upsurge of interest in the study of oscillatory fracture, a subject which is, as yet, far from understood. Here we present results in a novel experimental context. An archetype of the phenomena is the opening of a blank CD case by tearing its packaging film with a blunt object. In our experiments, a cutting tool is perpendicularly driven through a brittle thin polymer film, clamped at its boundaries, that progressively cuts the material as it advances. For large enough tools, the crack tip follows a highly periodic path, leaving behind a striking oscillatory pattern. A discontinuous transition from oscillatory to straight crack paths - the Unzip instability - is observed as the width of the cutting tool is incrementally decreased. The amplitude and wavelength of the oscillatory crack paths scale linearly with the size of the cutting tool, over a wide range of length scales, but are independent of both the cutting speed and the width of the sheet. An additional continuous transition occurs, from oscillatory to straight cracks, as the angle between the cutting tool and the thin sheet is decreased. We develop a geometrical model that accurately reproduces the behaviour observed experimentally, far away from threshold. The central idea of our formulation is a coupling between the Griffith criterion for crack propagation and geometrical considerations of the thin elastic sheet. |
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