Design, Testing and Validation of a Composite Box Under Combined Loading up to Collapse

This paper presents part of the activities performed by Politecnico di Milano within the European research project COCOMAT, consisting of the design, testing and validation of a closed box structure made up of CFRP stringer-stiffened curved panels. The design phase is carried out using ABAQUS/Explicit with the purpose of obtaining a structure with a ratio between the collapse and the buckling load that is greater than 3 under axial compression and greater than 2 under torque. The box is manufactured by Agusta/Westland and tested at Politecnico di Milano. The first test results are used to validate the FE model, removing some restrictive hypothesis formulated during the design process. Finally, a comparison between the results under combined axial compression and torque loads obtained in the collapse test and in the FE analysis reproducing the collapse is shown. A good correlation is obtained, both in the buckling and postbuckling behavior and in the collapse modality, demonstrating the capability of the dynamic explicit analyses to capture the highly nonlinear response of the aeronautical panels. The results also show the possibility of CFRP structures working deeply in the postbuckling region if properly designed and, consequently, the possibility of a weight reduction of aerospace structures thanks to the possibility of moving up the ultimate load toward the collapse load without any loss in the structural safety