Design, numerical and experimental characterization of the composite spar for a regional aircraft

The work is part of the Clean Sky 2 – AIRGREEN2 (AG2) project which aims at designing, analysing, manufacturing and ground-testing a full scale composite Outer Wing Box (OWB) demonstrator for a regional aircraft. In order to achieve this objective, in the current development work a small scale (in length) spar segment has been designed, manufactured and tested in order to validate the related technologies. The OWB has been designed in order to withstand stiffness, strength and stability requirements under critical loading conditions taken from the flight envelope. In particular, a minimum weight design has been obtained with an optimization based on genetic algorithms that explored several thickness distributions and stacking sequences. A further analysis has been performed on the optimized front spar of the OWB. Focusing on the first two (root) bays, a spar segment (approx. 1100 mm long and 400 mm wide, constant section - no tapering) has been considered. The spar segment was manufactured by using the Liquid Resin Infusion (LRI) method, an Out of Autoclave method (OoA), in which Dry Non Crimp Carbon Fabrics are impregnated by epoxy resin (high temperature cure) under the application of vacuum only. The impregnated Carbon fabric material is then cured in a standard oven or self heated tool, heated internally by integrating electrical resistances and externally by heated blankets. In this work, a self heated tool option was selected. The tool was designed as an egg-crate type construction in order to minimize both mass and thermal inertia. CATIA software was used for both the tool and part design. A FEM analysis was performed by implementing progressive failure analysis using the MSC Marc software in order to predict the spar segment structural response during the experimental test. Respective material allowables have been obtained by testing standard coupons manufactured by the same base materials (dry carbon fabrics and resin) and manufacturing method (LRI). Standard hand layup procedures were followed during the layup process of the fabrics. Appropriate auxiliary materials capable to withstand the respective LRI infusion and curing process specifications were used. The spar segment has been tested up to failure in a cantilevered configuration and subjected to a tip in-plane pure shear force. The fixtures and loading system for the test have been designed in order to guarantee the correct clamped boundary conditions and to avoid any torsional effect. The scope of the test was to validate the manufacturing process, as well as the design and FEM analysis in terms of allowables and final failure of the component. A numerical/experimental comparison is presented in terms of load vs displacement response curve and load vs strain (measured at some locations on the spar).