Modal nudging and elastic tailoring for blade-stiffened wing structures

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Aerospace requirements put an emphasis on developing lightweight structures to help reduce fuel consumption and related costs. Typically, designers in aerospace engineering use thin-walled structures which are periodically stiffened with ribs, frames and longerons, i.e. semi-monocoque structures, as an efficient solution. However, slender and thin-walled structures often exhibit undesirable elastic nonlinearities and instabilities that need to be remedied at the cost of mass efficiency. However, it has been demonstrated that incorporating well-behaved elastic nonlinearities offers the means to recuperate the baseline's efficiency or even improve upon it. Modal nudging is a recently introduced tailoring technique, whereby mode shapes from the post buckling regime are seeded as initial perturbations to the geometry of the perfect structure. The small alterations to the geometry of a structure can be used to connect stable pre-buckling responses to stable post-bucking ones. This characteristic increases the load carrying capability of the structure by removing critical bifurcations and stabilising the post-buckling response removing any of the undesirable instabilities which may typically be encountered post-bucking. As an additional benefit, in stabilising the post-buckling response, modal nudging also ameliorates imperfection sensitivity. Ultimately, the stabilisation of post-buckling responses, the increase in load-carrying capacity, and the reduction in imperfection sensitivity, are conducive to further lightweighting of aerospace, semi-monocoque structures. Preliminary work has shown that modal nudging via geometric alterations can successfully be used to increase the load carrying capacity and the compliance of blade-stiffened wing structures. With a judicious selection of the post-buckling modes seeded onto the original geometry, the nonlinear load-displacement trajectory of a structure can be closely controlled and optimised for compliance, load-carrying capacity, or additional functionality. The drawback of the geometric approach is that small perturbations to the initial geometry are difficult and costly to manufacture. Moreover, certain applications do not permit geometric changes. That is the case, for instance, in aerodynamic structures where any geometric alteration would disrupt flow and performance. A more suitable approach to nudging could then be controlling the nonlinear behaviour by elastic tailoring. This can, for example, be achieved by localised shifting of the neutral axis, by laminate design, or by smoothly varying the material properties using composite tow-steering. This project will investigate the efficacy of elastic tailoring through composite materials to replace geometric imperfection seeding for the modal nudging technique. The first objective is to demonstrate, by design and analysis of numerical prototypes, that semi-monocoque structures can be nudged through stiffness tailoring. The second objective of the project is to verify the numerical findings in experimental tests, by designing, building, and testing a prototype blade-stiffened aircraft panel. The challenge is to design and build a physical prototype that exhibits the desired structural behaviour, and which is robust to manufacturing imperfections. To achieve this objective, an understanding of the effect of manufacturing imperfections on the mechanical behaviour of the nonlinear structure is necessary. Accurate experimentation on the nudged prototype structures will enable the validation of the numerical analyses and will enable practical applications of well-behaved nonlinear structural responses.