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

传统的金属轻质结构越来越多地被复合材料层压结构所取代。在分析和设计薄壁层合结构时，稳定性是需要考虑的主要因素。本计画致力于发展相应的高效率且高精度的分析方法，并分为三个连续且相互依存的研究主题：研究主题1：在加筋薄壁结构的加劲肋设计时，防止整体屈曲模态是一个重要的准则。因此，采用了所谓的最小刚度的概念，即以强制执行局部屈曲模式的方式确定加劲肋的弹性特性。关于复合材料加筋板的技术水平是稀缺的，特别是在剪切变形理论的应用方面，没有可用的调查。因此，本项目采用高阶层合板理论和基于能量的分析方法来解决复合材料结构的最小刚度问题。研究主题2：加筋板可以承受超过临界局部屈曲载荷的载荷，因此必须在屈曲后范围内操作轻质结构。对于实际应用的目的，封闭形式的分析方法的选择，由于其良好的计算效率的方法。考虑到满足最小刚度要求，当前的研究主题则集中在后屈曲行为的封闭形式的分析处理，使用足够的形状功能的所有后屈曲自由度与基于能量的分析方法，从而使重要的后屈曲状态变量，如有效宽度的研究。研究课题3：当加筋板在后屈曲范围内工作时，确定其极限承载能力是至关重要的。在大多数加筋复合材料结构的情况下，当加筋支脚附近发生分层并在板与加筋之间的界面以不稳定的方式增长时，达到极限载荷。脱层的发生是由加筋板连接处的几何不连续性和相应的三维和潜在的奇异应力场触发的。特别是层间应力在分层的发生中起着至关重要的作用，因为这些应力作用在垂直于层压层的纤维方向的平面中。因此，它的目的是开发实际适用的分析方法，通过该方法，能够有效和可靠地确定这些层间应力在加劲板连接附近。这些调查将包括一个半解析的近似方法，是基于分层层合板理论的发展。

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.