Cyclic buckling tests of pre-damaged CFRP stringer stiffened panels

Abstract

Experimental results obtained from cyclic buckling and post buckling tests of pre-damaged stiffened CFRP panels are described in this paper. This work was conducted within the COCOMAT project funded by the EU with the objective to contribute to the reduction of structural weight at safe design. COCOMAT targeted to establish a new design scenario for composite stiffened panels which are understood as part of an aircraft fuselage. This design scenario aimed at exploiting considerable reserve carrying capacities in fibre composite fuselage structures by accurate simulation of collapse. The project results cover an experimental data base, 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 give information about the progress of damage during the loading process. In this context, the present paper focuses on the investigation on pre-damaged stringer stiffened panels under cyclic axial compression. A set of four panels of the same design was split in two variants which only differ in the position of an artificial Teflon disbond beneath a stringer. One of each variant has been tested statically until collapse in one step as reference while the remaining 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 are 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.