Abstract
Modern aircraft wings are thin-walled structures composed of ribs, spars and stiffened panels, where the top skin is subject to compressive forces in flight that can cause buckling instability. If these panels are machined from a single billet of metal then the initial buckling performance can be significantly improved by increasing the fillet radius along the line junction between the stiffener webs and skin. Typically thin-walled structures are usually modelled with two dimensional elements. To model the stiffened panel with fillets three dimensional elements are required. For the stiffened panel selected for the analysis the paper shows that the three dimensional model shows a substantial increase in skin initiated buckling if the fillet is taken account of. A 5mm radius leads to an increase of 34% increase in local buckling load performance for a skin portion of breath to thickness ratio of 100. The associated overall buckling load increases by 1.8%. The mass penalty for a 5mm radius is 5.1%. To avoid local and overall buckling interaction an accurate measure of both buckling loads is very important and may have impact for designers. The three dimensional models with no fillets show very good agreement with the two dimensional models.
Highlights
Aircraft structural systems are thin-walled structures with wing structure composed of ribs, spars and stiffened panels
The top skin is, under aerodynamic loading, subject to axial compressive forces that can cause buckling instability. These stiffened panels can have a considerable postbuckling reserve of strength, enabling them to remain in stable equilibrium under loads in excess of their critical buckling load, provided the initial buckling mode is a local one [1]
The combination of these two effects is such that simple supports can be conservatively assumed at the line junction between stiffener webs and inter-stiffener portions of skin
Summary
Aircraft structural systems are thin-walled structures with wing structure composed of ribs, spars and stiffened panels. Skin crenulations or stiffener pad-ups [10] These panels are continuous systems from integrally machined panels and the containment features significantly influence the fatigue crack growth performance of the stiffened panel. An optimum stiffened panel typically results in the critical buckling stress occurring with a skin mode and a higher buckling stress for the overall mode. When local buckling occurs the overall buckling stress will reduce further and so it is important that there is a significant difference in the initial and overall buckling loads for the panel to ensure postbuckling stability [11] This paper only examines the initial buckling performance but the work presented does have implications for the optimisation of stiffened panels with a postbuckled reserve of strength.
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