Study of Skin-Stringer Separation in Postbuckled Composite Aeronautical Panels

Abstract

Aeronautical composite stiffened structures have the capability to carry loads deep into postbuckling, yet they are typically designed to operate below the buckling load to avoid potential issues with durability and structural integrity. Large out-of-plane postbuckling deformation of the skin can result in the opening of the skin-stringer interfaces, especially in the presence of defects, such as impact damage. To ensure that skin-stringer separation does not propagate in an unstable mode that can cause a complete collapse of the structure, a deeper understanding of the interaction between the postbuckling deformation and the development of damage is required. The present study represents a first step towards a methodology to assess and improve the capabilities of stiffened composite structures subjected to postbuckling deformations.

Two regions are identified in a four-stringer panel in which skin-stringer separation can occur, namely the region of maximum deformation and the region of maximum twisting. Both regions are studied using a finite element model of a representative single-stringer specimen. For the region of maximum deformation, a seven-point bending configuration is used, in which five supports and two loading points induce buckling waves to the specimen. The region of maximum twisting is approximated using an edge crack torsion configuration, with two supports and two loading points. These two configurations are studied by changing the positions of the supports and the loading points. An optimization procedure is carried out to minimize the error between the out-of-plane deformation of the representative single-stringer specimen and the corresponding region of the four-stringer panel.

The optimal configurations are applied to a finite element model of a single-stringer specimen including cohesive elements to simulate damage initiation. The types of damage initiation that occur in these configurations are compared to the global/local analysis of the two identified regions of the four-stringer panel. Furthermore, the effect of the position of the supports and loading points on damage initiation is investigated. The edge crack torsion configuration that is found to be the best at approximating the twisting deformation of the panel also shows the most similarity in damage characteristics with respect to the four-stringer panel.