Abstract
A process to efficiently design composite wing boxes is presented. It uses analytical and semi-empirical equations for failure modes such as material strength, plate buckling, stiffener column buckling and stiffener flange or web crippling. Laminate layups for the different components are selected in accordance with basic engineering rules and guidelines and are updated as necessary to meet the local loads. The emphasis is in allowing buckling of skins at any fraction of the ultimate load and allowing local load redistribution from buckled to non-buckled panels to save weight. The design process is automated and the design can be automatically transferred over to a commercial finite-element code for detailed design and validation. The effects on weight of number of spars, ribs, and stiffeners as well as the fraction of ultimate load at which buckling is allowed are examined and insight is gained to which of these the weight is most sensitive to. In addition, the effect of minimum gage on weight was found to be a driver.
Highlights
The preliminary design of a wing box can become very complex depending on the level of fidelity of the analysis methods used, the multiplicity of failure modes, and the load sharing between components
Stability failure corresponds to global or overall buckling, and local buckling which can be broken into skin buckling including the stiffeners, skin buckling between stiffeners, column buckling of stringers, or crippling of stringers
Within the context of a preliminary design, the approach consists of minimizing the weight, while at the same time making sure that there is no failure under the applied loads and that various design guidelines and robust design practices, as selected by the designer, are met
Summary
The preliminary design of a wing box can become very complex depending on the level of fidelity of the analysis methods used, the multiplicity of failure modes, and the load sharing between components. All this work focuses on designs with buckling, strength, natural frequency, and aeroelasticity constraints, but does not account for crippling of flanges or webs of stiffeners ribs and spars This is an important failure mode which drives a significant part of the design of compression loaded members [21] and is very rarely included in any of the optimization work in open literature [22]. The objective is to include important failure modes such as crippling and investigate design approaches such as post-buckling which are not often included in the preliminary design of entire wing boxes This approach leads to very useful conclusions about the effect of different design choices such as the number and type of stringers, number of spars and ribs, choice of layup, buckling load level, etc. It is required that a direct automated link with finite elements be possible for immediate validation of results and further design refinement
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