Cyclic Buckling Tests of Stiffened Composite Curved Panels under Compression and Shear

This paper presents buckling tests results of curved stringer-stiffened panels made of graphite-epoxy assembled in closed boxes. Tests were performed both statically and cyclically under combined axial compression and torque loading. The effect of different application sequences of combined loading is derived. Moreover, the influence of repeated post-buckling combined loads is investigated in terms of global behavior and of collapse modalities. Results show the reliability of these structures to safely operate in the postbuckling field, even when the buckling load is reached thousands of times during the operative life. I. Introduction omposite materials are getting more and more widespread in aerospace engineering, and in particular for structural applications graphite-epoxy composites are of great interest because of their high ratios of strength over weight and of stiffness over weight. This leads, from one hand, to progressive advances in structural optimization, but, on the other hand, raises many structural phenomena that have still to be investigated. One of these phenomena is the possibility for graphite-epoxy aerospace structures to work in the post-buckling field 1 during the operative life. Indeed, nowadays it is well known that graphite-epoxy stiffened structures can work in the post-buckling range, measuring also a collapse load equal to three or four times the buckling load 2-8 , but there are few data dealing with the effect of repeated buckling, when the buckling load is reached thousands of times during the operative life. Few studies 9-10 have been performed regarding how the cyclic buckling influences the nonlinear behavior and the collapse modalities of graphite-epoxy panels, and how deep into the post-buckling field it is possible to operate without loosing structural safety. The lack of data results in over-conservative design criteria, while weight savings could be achieved if it is demonstrated the capability of composite structures to safely operate in the post-buckling field thousands of times without loosing efficiency, in order to include the buckling load in the range of allowed loads during the operative structural life, and to move up the design ultimate load near the collapse load. It is so necessary to obtain for composite structures the same level of knowledge of metal alloy structures, for which the phenomena of postbuckling, plasticity and fatigue are well known 11-12 . To obtain this level of knowledge analytical, experimental and numerical investigations must be carried out at the same time. From the experimental point of view, accurate tests must be performed to reproduce appropriate loading conditions and to carefully record data regarding the structural responses 13-14 . The experimental data will allow to develop and validate analytical models as well as finite element analyses that can be used for the description and the forecast of the behavior of graphite-epoxy structures. The study here presented is part of the European research project COCOMAT 15 (Improved MATerial Exploitation at Safe Design of Composite Airframe Structures by Accurate Simulation of COllapse) aiming to exploit the post-buckling field, and in particular the structural field between the actual ultimate load and the collapse load of graphite-epoxy stiffened panels by means of experimental tests, and to develop validated tools able to capture the damage mechanisms leading to collapse. This paper presents experimental results on two closed structures, obtained by coupling graphite-epoxy stiffened curved panels.