9 – Stability of composite stringer-stiffened panels

Design and analysis of stiffened composite panels including postbuckling and collapse: The aircraft industry demands for reduced development and operating costs, by 20% and 50% in the short term and long term, respectively. Contributions to this aim were provided by the project POSICOSS (5th FP) and COCOMAT (6th FP), both supported by the European Commission. As an important contribution to cost reduction, a decrease in structural weight can be reached by exploiting considerable reserves in primary fiber composite fuselage structures through an accurate and reliable simulation of postbuckling up to collapse. The POSICOSS team has developed fast procedures for the postbuckling analysis of stiffened fiber composite panels, has created comprehensive experimental databases, and has derived suitable design guidelines. COCOMAT built up on the POSICOSS results and considers in addition the simulation of collapse by taking degradation into account. The results comprise an extended experimental database, degradation models, and improved certification and design tools, as well as extended design guidelines. One major task of POSICOSS and COCOMAT is the development of improved analysis tools that are validated by experiments performed within the framework of the projects. Because the new tools must comprise a wide range of various aspects, a considerable number of different structures had to be tested. These structures were designed under different objectives (e.g., large postbuckling region). For the design process the consortiums applied state-of-the-art simulation tools and brought in their own design experience. This section deals with the design process as performed within both projects and with the applied analysis procedures. It is focused on the DLR experience in the design and analysis of stringer-stiffened carbon-fiber-reinforced plastic panels gained within the scope of these two projects. A structural robustness–based design strategy for thin-walled stiffened composite structures: The purpose of this chapter section is to present a robustness-based design strategy for thin-walled composite structures under compressive loading, which combines strength requirements in terms of the limit and ultimate load with robustness requirements evaluated from the structural energy until collapse. To assess the structural energy, the area under the load-shortening curve between several characteristic points such as local buckling, global buckling, onset of degradation, and collapse load is calculated. In this context, a geometrically nonlinear finite element analysis is carried out, in which the ply properties are selectively degraded by progressive failure. The methodology is demonstrated with a stiffened composite structure under compressive loading. Moreover, the deterministic and probabilistic results are compared. In this context, the uncertainties and energy-based structural robustness measures are employed to investigate the robustness and reliability of thin-walled composite structures in the postbuckling regime. In the design of composite structures, this innovative strategy might lead to a more robust design when compared to an approach that only accounts for the ultimate load. An analysis tool for the design and certification of postbuckling composite aerospace structures: In aerospace, carbon-fiber-reinforced polymer (CFRP) composites and postbuckling skin-stiffened structures are key technologies that have been used to improve structural efficiency. However, the application of composite postbuckling structures in aircraft has been limited because of concerns related to both the durability of composite structures and the accuracy of design tools. In this section, a finite element analysis tool for design and certification of aerospace structures is presented, which predicts collapse by taking into account the critical damage mechanisms. The tool incorporates a global–local analysis technique for predicting interlaminar damage initiation, and degradation models to capture the growth of a preexisting interlaminar damage region, such as a delamination or skin–stiffener debond, and in-plane ply damage mechanisms such as fiber fracture and matrix cracking. The analysis tool has been applied to single-stiffener and multistiffener fuselage-representative composite panels, in both intact and predamaged configurations. This has been performed in a design context, in which panel configurations are selected based on their suitability for experimental testing, and in an analysis context for comparison with experimental results as the representative of aircraft certification studies. In all cases, the tool was capable of accurately capturing the key damage mechanisms contributing to final structural collapse and was suitable for the design of next-generation composite aerospace structures.