Automated Manufacture of Grid Stiffened Panels

Abstract

Rib or grid stiffened structures have been investigated for decades, mainly for application in space structures. Grid structures offer the possibility to develop more damage tolerant structures as the network of ribs can provide redundant load paths. In an ideal situation, an aircraft fuselage could have a load carrying grid with ribs that carry tensile, compressive and shear loads. The skin would only need to sustain the cabin pressure load. Such a configuration could lead to significant weight reduction as a different design philosophy can be used. Typically, a damage tolerant design approach allowing a maximum of 3000 µstrain is used for composite aircraft structures. Using a grid structure, a different approach could be developed for a design which could allow strains up to 6000 µstrain. Automated fibre placement has made composite grid structures an affordable option. However, they pose a major challenge in the development of tooling for cure due to their complex and sometimes irregular structure. Within the European funded Horizon 2020 ACASIAS programme, square antenna elements are developed to be integrated in a composite aircraft fuselage panel. For this reason, an orthogrid stiffener pattern was chosen. As the skin of the panel must be transparent for the antenna signals, a glass fibre skin is required. A glass fibre skin to carry shear loads and carbon fibre ribs to carry compression and tension loads is therefore being developed to be able to meet structural and electromagnetic requirements. This paper describes aspects of the ongoing development for the design, manufacture and testing of an orthogrid stiffened structure in an efficient way. Simple alternating cutting of tapes at the crossing appear to be a suitable solution with a tensile stiffness reduction of 10 % compared to the situation without cuts. These and other details and elements are tested to validate the final design. The use of reusable silicone vacuum bags was investigated in order to replace complex metal tooling blocks. Besides saving on mould materials and machining, the use of a preshaped silicone vacuum bag is expected to reduce labour cost due to a decrease in handling of tooling blocks for positioning and cleaning. The first trial indicates that good quality laminates can be obtained using reusable vacuum bags. More testing will be carried out to validate the concept for a large 3×1.2 m panel.

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References

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