Design of a Combined Loads Machine for Tests on Fuselage Barrels and Curved Panels

The design of a combined loads test machine for fuselage barrels and curved reinforced panels is presented. The test facility allows to test different configurations of full scale panels, fuselage and wing box shells, with a combination of mechanical loads and internal pressure. The combined loads can be applied to perform both static and cyclic tests. The design of a test fixture for panel tests is then described. The fixture is used to carry out experimental investigations on reinforced metallic and composite curved stiffened panels with different radii of curvature. Moreover, such a fixture allows to simulate the actual state of stress on the panel as it would be part of a cylindrical reinforced shell such as a fuselage barrel. Finite element models have been used to simulate the behavior of the fixture – panel assembly. A detailed description of the external frame is presented. INTRODUCTION The development of wide body aircraft and the utilization of advanced composite materials and new manufacturing techniques has stimulated in the last decades the necessity to provide large test facilities to perform experimental investigations that would substantiate analytical and numerical analyses. On the other hands, the complexity and the high related costs of full scale tests suggest to improve the efficiency of the experimental work in order to develop accurate and reliable techniques able to reduce full scale tests to subcomponent tests. It should be also pointed out that sometimes the complexity and the size of structural components can not be reduced, as in the case of the analysis of crack propagation phenomena. In the ____________________________________________ Full Professor in Aerospace Structures PhD, Assistant Professor in Aerospace Structures PhD student in Aerospace Engineering PhD student in Aerospace Engineering Senior Aerospace Engineer Senior Aerospace Engineer Senior Aerospace Engineer Principal Engineer analysis of the crack propagation in a reinforced shell (e.g. a fuselage panel) a minimum number of stringers and frames is required in order to correctly simulate the propagation. A variety of test facilities are available in research centers for this purpose. In most cases it is not possible to apply a combined set of independent loads including internal pressurization or the allowable size of the test articles is not suitable for the experimental investigation to carry on. The COLT test machine is probably the only facility allowing such tests (full scale barrels and panel tests subjected to combined loads and pressurization), although the wide flexibility and the complex nature of the mechanical and thermal loads to be applied require a very complex architecture of the data acquisition, control and test management systems, with very high costs associated to the realization and operations of the facility. Within this context the design of a combined loads test machine for fuselage barrels and curved reinforced panels was carried out and is presented in the paper. The activities are coordinated by the Dipartimento di Progettazione Aeronautica of the University of Naples in cooperation with Alenia Aeronautica and Piaggio Aeroindustries within the Centro Regionale di Competenza Trasporti (CRdCT) project, funded by the Regione Campania. The CRdCT project is network of experimental laboratories mainly leaded by some Regione Campania’s Universities and associated research centers. The main goal of the project is to support the industries involved in the design and realization of aerial, terrestrial and maritime means of transport. The motivation for the development of such a machine is the need of a facility able to test different configurations of full scale and sub scale panels, fuselage and wing box shells, with a combination of mechanical loads and internal pressurization. DESCRIPTION OF THE TEST MACHINE The facility allows to carry out complex, combined loads tests on full scale fuselage barrels with 1-1.9 radii and a maximum length of 5 m. It has the capability to apply a 3000 kN axial load, a 5000 kNm torque 45th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials Conference 19 22 April 2004, Palm Springs, California AIAA 2004-1628 Copyright © 2004 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved. American Institute of Aeronautics and Astronautics 2 moment, a 3000 kN shear load and a 1350 mbar internal pressure. The facility is designed to perform both static and cyclic loading tests. As schematically showed in Figure 1, the barrel is accommodated between a fixed reaction platen and a loading platen. The loading platen consists of an assembly of three platens mounted on a guide bar system in order to allow the displacements and rotation required for each loading condition (Figure 2). A first vertical platen is mounted on linear, horizontal guide bars to allow the displacements mainly produced by the axial loads (z direction). A second platen is mounted on the first one, adopting two linear vertical guide bars to allow vertical displacements (y-direction). A third platen realized with circular guide bars is connected to the second one in order to allow the rotation about z axis. Figure 1 Scheme of the combined load test machine with a cylindrical shell test article Figure 2 Loading platen assembly Four horizontal 750 kN hydraulic actuators located between the reaction and the loading platens introduce the axial load. Two supplementary actuators can be used to increase the total axial load capacity. Two vertical 1500 kN hydraulic actuators, connected to the loading platen system, introduce the shear and torsion loads (Figure 3). The internal pressure load is provided by a pneumatic system. Since full scale tests on large structures are expensive, a special fixture capable to perform tests on curved reinforced panels representative of these structures was designed. The fixture can accommodate a 4 m by 5 m reinforced curved panels with 1.3 to 3.5 m radii and 0.48 to 0.50 m frame spacing. Axial, torsion and internal pressure static and cyclic loads can be applied to the panel fixture assembly. The basic idea is to reproduce on the test panel mounted on the fixture the same loading and boundary conditions as for corresponding cylindrical shell for a given loading condition. One of the major problem in the sub-structural testing is the reproduction of the actual boundary conditions. Furthermore, the panel-fixture assembly is loaded as a unique structure and consequently a portion of the load is reacted by the fixture. Finally, the fixture is to be designed to have infinite life at the cycling loads. To ensure the proper boundary conditions, the panel is hinged to the fixture and a system of transverse constraints is provided to avoid undesired stresses during the pressure loading. The axial stiffness of the fixture is about 8% the panel axial stiffness. This requirement is to be satisfied in order to reduce the test axial load. At the same time the fixture should have a sufficient strength to resist the other load conditions. A panel-fixture assembly finite element model and a full-scale model of the corresponding cylindrical shell have been realized to size the fixture and to perform the sub-scale vs full-scale comparison. Results will be presented in the following paragraphs.