Local postbuckling of hat-stringer-stiffened composite laminated plates under transverse compression

In this paper, a closed-form method for the analysis of the local postbuckling behaviour of aircraft panels that are braced by hat-stringers is presented. The stiffened panels are loaded by transverse compression which is a load case that has been treated only scarcely in the open literature, and the corresponding buckling and postbuckling behaviour that eventually leads to failure of the panel is quite different to what is observed when a panel under longitudinal compression is considered. This contribution clarifies that the ultimate load bearing capacity of a stiffened panel with closed-profile stringers under transverse compression is governed by several consecutive stability cases. Firstly a closed-form approximate analysis method for the linear buckling analysis of the skin between two stringers taking the torsional stiffness of the hat-stringers into account is derived (stability case 1). Secondly, a simple Marguerre-type postbuckling analysis method is presented that accounts for the geometrically nonlinear behaviour of the panel skin after buckling (stability case 2) and enables a closed-form analysis of its effective width. Thirdly, the linear buckling analysis is adapted to the analysis of the panel skin under a stringer (stability case 3). It will be shown that stability cases 2 and 3 cannot be treated independently, but have to be considered interactively which necessitates an iterative procedure. The accuracy of the proposed analysis method, relying on the simplifying assumption of a perfectly flat plate rather than considering a cylindrically curved panel is established by comparison with results of accompanying geometrically nonlinear finite element calculations.