Cyclic Buckling Tests of Pre-damaged Cfrp Stringer-stiffened Panels

Experimental results obtained from cyclic buckling and postbuckling tests of pre-damaged stiffened CFRP panels are presented in this paper. This work was conducted within the COCOMAT project funded by the EU with the objective of contributing to the reduction of structural weight at safe design. COCOMAT was targeted at establishing a new design scenario for composite stiffened panels which are understood as part of an aircraft fuselage. This design scenario aimed at exploiting considerable reserves in the load carrying capacity in fiber composite fuselage structures by accurate simulation of collapse. The project results cover an experimental database, improved slow and fast computational tools, as well as design guidelines. A reliable simulation of the collapse load requires taking degradation into account. For the validation of the tools, a sound database of experiments is needed which gives information about the progress of damage during the loading process. In this context, the present paper focuses on the investigation of pre-damaged stringer-stiffened panels under cyclic axial compression. A set of four panels of the same design was split into two variants which differ only in the position of an artificial Teflon disbond beneath a stringer. One panel of each variant was tested statically until collapse in one step as reference, while the other panel was tested cyclically with different amplitudes. Before the test, all test structures were assessed by ultrasonic inspection and the geometric imperfections were measured. During the test, advanced measurement systems such as the ARAMIS system for the measurement of the buckling pattern and thermography for monitoring the skin-stringer separation were utilized in addition to strain gauge measurements and the record of the load shortening data. The test structures, their preparations for testing, the buckling test facility, and the measurement systems used are described. The test results as to the influence of the cyclic loading on the damage progression in the skin-stringer connection are presented and discussed.