Boundary Layer Analysis
of the Ridge Singularity in a Thin Plate

Alexander E. Lobkovsky
The James Franck Institute
The University of Chicago
5640 South Ellis Avenue Chicago, Illinois 60637

Large deformations of thin elastic plates and shells present a formidable problem in continuum mechanics which is generally intractable except by numerical methods. Conventional approaches break down in the limit of small plate thickness due to appearance of discontinuities in the solution which require boundary layer treatment. We examine a simple case of a plate bent by forces exerted along its boundary so as to create a sharp crease in the limit of infinitely small thickness. We find a separable boundary layer solution of the von Karman plate equations which is valid along the ridge line. We confirm a scaling argument \cite{fullerine} that the ridge possesses a characteristic radius of curvature $R$ given by the thickness of the sheet $h$ and the length of the ridge $X$ {\it viz.} $R \sim h^{1/3} X^{2/3}$. The elastic energy of the ridge scales as $E \sim \kappa (X/h)^{1/3}$ where $\kappa$ is the bending modulus of the sheet. We determine the dependence of these quantities on the dihedral angle of the ridge $\pi - 2\alpha$. For all angles $R \sim \alpha^{-4/3}$ and $E \sim \alpha^{7/3}$. The framework developed in this paper is suitable for determination of other properties of ridges such as their interaction or behavior under various types of loading. We expect these results to have broad importance in describing forced crumpling of thin sheets.