2446. Kinking of Integrally Stiffened Skins

Paper

Abstract

Good panel optimization programs for integral panels with ”plain” or blade stiffeners were developed a long time ago. The ones developed under my direction saved weight on airplanes and passed tests. Buckling of the stiffeners is one of the causes of panel kinking, a sudden loss of column stability. the allowables were based on buckling analysis that correlated tests run by Frank Collins at Lockheed Georgia back in 65. Wing covers are bent in both directions by transverse pressures and eccentricity. Buckling of the stiffeners is sensitive to added compression of the free edge. The old correlation was based on am extreme idealization. The panel is considered to be long with the stresses constant along the length. Every cross section has the minimum compression stress on the free edge. This would seem to be quite conservative. It is not. Analysis for large deflections is used to investigate why the buckling stress is so low. Column buckling and local buckling are not independent. Both wave forms are generated by many iterations to satisfy the principles of equilibrium and compatibility. The computer program is presented on diskette including both source code and symbols. The old tests were short beam columns that reached a maximum load in the elastic post buckled range. They did not kink. Both Frank and I were inside the screens with our noses in the test area. Four specimens had added compression on the free edge. The errors in the correlation of these test range from +6% to -2%. pretty good considering that the local buckling was simplified so that torsional equations could be used. Why plate elements with one edge free behave differently as part of a cruciform, as part of a Z-stringer, and as stiffeners is explained. Increasing the plate width (height) of stiffeners can decrease the strength.