Buckling, Postbuckling and Stress Concentration Problems of Stiffened Composite-Panels

• Applicant: Professor Dr.-Ing. Christian Mittelstedt
• Subject Area: Mechanics
• Term: since 2019
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 426146527

Traditional metallic lightweight constructions are increasingly replaced by composite laminated structures. When analyzing and designing thin-walled laminated structures, the stability behavior is a predominant factor to be considered. This project is devoted to the development of according efficient and yet highly accurate analysis methods and is divided into three consecutive and inter-dependent research topics:Research topic 1:When designing the stiffeners of a stiffened thin-walled structure, it is an important criterion to pre-vent global buckling modes. Hence, the concept of the so-called minimum stiffness is employed, i.e. the determination of the elastic properties of the stiffeners in such a way that a local buckling mode is enforced. The state of the art concerning stiffened composite panels is scarce, and especially with respect to the application of shear deformation theories there are no investigations available. This project thus employs higher-order laminate theories and energy based analysis methods for the solution of the minimum stiffness problem of composite structures. Research topic 2:Stiffened panels can sustain loads beyond the critical local buckling load so that it is imperative to operate lightweight structures in the postbuckling range. For practical application purposes, closed-form analytical methods are the methods of choice due to their favorable computational efficiency. Given that the minimum stiffness requirements are fulfilled, the current research topic then focusses on the closed-form analytical treatment of the postbuckling behavior using adequate shape functions for all postbuckling degrees of freedom in conjunction with energy based analysis approaches, thus enabling the study of important postbuckling state variables such as the effective width. Research topic 3:Whenever a stiffened panel is operated in the postbuckling range, it is of utmost importance to de-termine the ultimate load carrying capacity. In the majority of cases of stiffened composite structures, the ultimate load is reached when delaminations in the vicinity of stiffener feet occur and grow in an unstable manner in the interface between panel and stiffener. The onset of delamination is triggered by the geometric discontinuity at the stiffener-panel junction and the according three-dimensional and potentially singular stress fields. Especially the interlaminar stresses play a crucial role in the onset of delaminations since these stresses act in a plane perpendicular to the fiber directions of the laminate layers. It is thus the aim to develop practically applicable analysis methods by which an efficient and reliable determination of these interlaminar stresses in the vicinity of the stiffener-panel junction is enabled. These investigations will include the development of a semi-analytical approximate method that is based on a layerwise laminate theory.

• DFG Programme: Research Grants