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The use of integrally stiffened panels (ISP's) in wings of small and medium-sized aircraft is frequent in aviation. These thin-walled structures are often subjected to compressive loads during their life-time and, although the finite element method has become an important tool for engineers to obtain the buckling load of structural members, some results based in charts published by NACA are still used in the preliminary stages of the wing design in many aircraft companies. However, these charts used for calculation of the critical compressive stress consider only idealized stiffened panels and neglect geometric details as the fillet radius used in the current design of ISP's. The objectives of this paper are twofold: (i) to show that the charts published by NACA provide good results for the critical buckling stresses for several geometries of ISP's, when compared to finite element results with proper boundary conditions, providing fillet radii are also neglected in the finite element models and (ii) to show that the values of the critical buckling stresses for local instability of ISP's may be significantly increased when one considers the effect of the fillet radius, meaning that this parameter should also be considered in the optimal design of such structures. Several numerical results obtained with finite element simulations based on different geometrical parameters of ISP's are presented in this study. [ABSTRACT FROM AUTHOR]

Abstract: In the present work, a set of finite element analyses (FEA) was carried out, using Abaqus to reproduce the mechanical behaviour of integrally stiffened panels when subject to longitudinal compression. Since most fabrication processes, such as welding, introduce distortions and affect the material properties, the sensitivity to these defects was assessed. Different shapes and magnitudes of the initial geometrical imperfections were tested and a high sensitivity was observed to both factors on the ultimate load. The existence of a heat affected material showed no influence on the ultimate strength of the tested panels. [Copyright &y& Elsevier]

Reinforced structures for aircraft fuselages are conventionally composed by base (skin) aluminium plates and reinforcement elements (stringers), joined by riveting operations. During the last decade more effective approaches for reinforced fuselage and wings, such as the Integrally Stiffened Panels (ISP), have appeared. These homogeneous reinforced structures are obtained in an integral form by extrusion, allowing for lower manufacturing costs. During service conditions, these structures can be subjected to extreme compressive loading conditions and, due to their slenderness and low weight, ISP design must account for a reliable determination of buckling loads. However, complexities of the cross-sectional geometrical shapes, together with the occurrence of elastoplastic non-linear effects prior or after buckling, completely impair the use of analytical tools, being the analysis by the Finite Element Method (FEM) imperative in a reliable design process. In the present work, the structural performance of ISP structures is assessed, accounting for buckling in the elastoplastic range, by means of numerical simulation with the Finite Element Method. Also, the buckling load-carrying capacity of multiple sets of reinforced structures, composed by a finite number of ISP and joined by friction stir welding (FSW) operations, is also studied. In doing so, it is possible to numerically infer about the influence of the presence of FSW zones in the overall stiffness and mechanical behaviour of ISP structures with complex cross-section geometries. [ABSTRACT FROM AUTHOR]

Aircraft fuselage panels are conventionally differential reinforced structures, with aluminum skin and stringers being conventionally joined by riveting operations. As a consequence, these structures can present a number of distinct components, making assembly operations time consuming and expensive. Recently, developments in aluminum extrusion have opened up possibilities for new design procedures for aircraft applications. Particularly, complex-section panels known as Integrally Stiffened Panels (ISP), which are reinforced structures integrally obtained from extrusion, are a good example. In structural terms, ISP may be considered as thin-walled structures in which the stability for compressive loads is strongly dependent on the buckling strength of the cross section itself. Besides the difficulties in the project for buckling deformation modes within the elasto-plastic range, in aeronautical design it is mandatory to find the best compromise between weight and structural performance. In this sense, the present work presents an optimization algorithm based on a Hybrid Differential Evolution and Particle Swarm Optimization (HDEPSO) formulations, applied in finding the optimal cross section shape and dimensions for general ISP structures, under compressive buckling loads and accounting for elasto-plastic behaviors. The results and design guidelines are obtained by a set of parameterized nonlinear finite element simulations in ABAQUS, accounting for buckling carrying load levels as the principal design criteria. [ABSTRACT FROM AUTHOR]

In this work, parametric finite element models are used to generate new design charts that provide the local buckling coefficient (k s l) of integrally web-stiffened panels (ISPs) subjected to uniaxial compressive loads, including the fillet radius effect. The numerical results from linear buckling analyses of hundreds of different panel geometries are presented in terms of four non-dimensional parameters: t w / t s , b w / b s , t s / b s and R / b w f , whereby the latter represents the effect of the fillet radius on k s l. An extensive survey based on ISPs geometries used in different types of aircraft wings was conducted to define the ranges of these four ratios. Within each range, discrete values of the four ratios were chosen and randomly combined to enable the construction of the finite element models used to obtain the new k s l curves. The proposed finite element modeling was validated by comparing the results with numerical and experimental results from the literature, showing a good agreement. Compared to non-filleted ISPs results, it is shown that the local buckling stresses of filleted ISPs may be significantly increased, allowing designers to optimize different ISPs geometries. • Local buckling coefficients of integrally stiffened panels with filleted junctions are presented in tabular form. • Results are presented in non-dimensional quantities, allowing the use for different panel geometries. • Results can be easily implemented in numerical programs to provide local buckling stresses for different geometries. • Finite element results were compared with other numerical and experimental results showing a good agreement. • Geometries considered in this work are based on wing upper panels found in commercial aircraft of large and medium sizes. [ABSTRACT FROM AUTHOR]