Disappearance of stretch-induced wrinkles of thin sheets: a study of orthotropic films

A recent paper [Healey et al., J. Nonlin. Sci., 2013, 23, 777-805.] predicted the disappearance of the stretch-induced wrinkled pattern of thin, clamped, elastic sheets by numerical simulation of the Föppl-von Kármán equations extended to the finite in-plane strain regime. It has also been revealed that for some aspect ratios of the rectangular domain wrinkles do not occur at all regardless of the applied extension. To verify these predictions we carried out experiments on thin (20 µm thick adhesive covered) elastomer sheets with different aspect ratios under displacement controlled pull tests. We found that the wrinkled shapes are strongly influenced by the emerging orthotropy during the extension. To carry out quantitative comparisons we abandoned the assumption about material isotropy and derived the governing equations for an orthotropic medium. In this way we found good agreement between numerical simulations and experimental data. Analysis of the negativity of the second Piola-Kirchhoff stress tensor showed that the critical stretch for a bifurcation point at which the wrinkles disappear must be finite for any aspect ratio. On the other hand, there is no such a bound for the aspect ratio as a bifurcation parameter. Physically this manifests as complicated wrinkled patterns with more than one highly wrinkled zones on the surface in case of elongated rectangles. These arrangements have been found both numerically and experimentally. These findings also support the new, finite strain model, since the Föppl-von Kármán equations based on infinitesimal strains do not exhibit such a behavior