Rolf Zimmermann

Investigation of Buckling Behavior of Composite Shell Structures with Cutouts

Thin-walled cylindrical composite shell structures can be applied in space applications, looking for lighter and cheaper launcher transport system. These structures are prone to buckling under axial compression and may exhibit sensitivity to geometrical imperfections. Today the design of such structures is based on NASA guidelines from the 1960’s using a conservative lower bound curve generated from a database of experimental results. In this guideline the structural behavior of composite materials may not be appropriately considered since the imperfection sensitivity and the buckling load of shells made of such materials depend on the lay-up design. It is clear that with the evolution of the composite materials and fabrication processes this guideline must be updated and / or new design guidelines investigated. This need becomes even more relevant when cutouts are introduced to the structure, which are commonly necessary to account for access points and to provide clearance and attachment points for hydraulic and electric systems. Therefore, it is necessary to understand how a cutout with different dimensions affects the buckling load of a thin-walled cylindrical shell structure in combination with other initial geometric imperfections. In this context, this paper present some observations regarding the buckling load behavior vs. cutout size and radius over thickness ratio, of laminated composite curved panels and cylindrical shells, that could be applied in further recommendations, to allow identifying when the buckling of the structure is dominated by the presence of the cutout or by other initial imperfections.

Thermo-mechanical design aspects for primary composite structures of large transport aircraft *

Thermo-mechanical design aspects for primary carbon fibre reinforced plastics (CFRP) components, especially wings, of future large transport aircraft are discussed. Numerical simulations of the heating of a wing under intensive sun irradiation and of cooling during taxiing and take-off result in the recommendation of an appropriate design temperature. The study of flat stringer-stiffened panels reveals the influence of imperfections and boundary conditions on the buckling load under simultaneous thermal and mechanical loading.

CFRP Fuselage Structures - Postbuckling Permitted

CFRP (carbon fibre reinforced plastics) fuselage structures are expected to be realized with future generations of aircraft. Going into the postbuckling regime with these structures requires improved, fast and reliable procedures for analysis and design of stiffened fibre composite panels. The EC project POSICOSS, which was started in the year 2000, develops such procedures. This article deals with the main objectives of the project, the theoretical and experimental work to be carried out, and with first results.

Dealing with Imperfection Sensitivity of Composite Structures Prone to Buckling

The Space industry demands for lighter and cheaper launcher transport systems. Structural weight reduction by exploitation of structural reserves in composite launcher structures contributes to this aim, however, it requires accurate, fast and experimentally validated stability analysis of real structures under realistic loading conditions. Structures in space applications can be imperfection sensitive because their maximum load is often equal or close to the first buckling load. The current design guidelines were developed only for metallic structures and are from 1968. For composites structures no appropriate guidelines exist. To fill this gap DLR developed a promising “Single Perturbation Load Approach” which exploits the worst imperfections idea efficiently. In the running EU project DESICOS (New Robust DESIgn Guideline for Imperfection Sensitive COmposite Launcher Structures) this approach will be further investigated and combined with a stochastic approach resulting in a future design approach. This chapter deals with the state-of-the-art in buckling of imperfection sensitive composite structures, recent investigations on the new design approach, and the DESICOS project. It describes the line of actions of the new design approach, and specifies the theoretical and experimental work to be carried out.

CYCLIC BUCKLING TESTS OF PRE-DAMAGED CFRP STRINGER-STIFFENED PANELS

Experimental results obtained from cyclic buckling and postbuckling tests of pre-damaged stiffened CFRP panels are presented in this paper. This work was conducted within the COCOMAT project funded by the EU with the objective of contributing to the reduction of structural weight at safe design. COCOMAT was targeted at establishing a new design scenario for composite stiffened panels which are understood as part of an aircraft fuselage. This design scenario aimed at exploiting considerable reserves in the load carrying capacity in fiber composite fuselage structures by accurate simulation of collapse. The project results cover an experimental database, improved slow and fast computational tools, as well as design guidelines. A reliable simulation of the collapse load requires taking degradation into account. For the validation of the tools, a sound database of experiments is needed which gives information about the progress of damage during the loading process. In this context, the present paper focuses on the investigation of pre-damaged stringer-stiffened panels under cyclic axial compression. A set of four panels of the same design was split into two variants which differ only in the position of an artificial Teflon disbond beneath a stringer. One panel of each variant was tested statically until collapse in one step as reference, while the other panel was tested cyclically with different amplitudes. Before the test, all test structures were assessed by ultrasonic inspection and the geometric imperfections were measured. During the test, advanced measurement systems such as the ARAMIS system for the measurement of the buckling pattern and thermography for monitoring the skin-stringer separation were utilized in addition to strain gauge measurements and the record of the load shortening data. The test structures, their preparations for testing, the buckling test facility, and the measurement systems used are described. The test results as to the influence of the cyclic loading on the damage progression in the skin-stringer connection are presented and discussed.

A robustness-based design strategy for composite structures

Purpose – This paper aims to present a new 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. Design/methodology/approach – In order 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. Findings – The advantage of the proposed methodology is observed by analyzing unstiffened composite plates under compressive loading, wherein the lightest plate that satisfies both strength and robustness requirements can be attained. Practical implications – As a practical implication, this methodology gives a ne...

A robustness-based design strategy for composite structures – probabilistic approach

Purpose – The purpose of this paper is to present the probabilistic approach to a new robustness-based design strategy for thin-walled composite structures in post-buckling. Design/methodology/approach – Because inherent uncertainties in geometry, material properties, ply orientation and thickness affect the structural performance and robustness, these variations are taken into account. Findings – The methodology is demonstrated for the sake of simplicity with an unstiffened composite plate under compressive loading, and the probabilistic and deterministic results are compared. In this context, the structural energy and uncertainties are employed to investigate the robustness and reliability of thin-walled composite structures in post-buckling. Practical implications – As practical implication, the methodology can be extended to stiffened shells, widely used in aerospace design with the aim to satisfy weight, strength and robustness requirements. Moreover, a new argument is strengthened to accept the coll...