ABSTRACT of the Doctoral Thesis
Stiffened composite structures which are progressively introduced especially in primary aerospace structures have become an important structural application of composite materials. The present study aims at pointing out failure prone regions in stiffened composite structures by investigating their mechanical behavior numerically as well as experimentally. Special attention is given to the determination of interlaminar stresses, free edge effects and the failure behavior in the skin-stringer transition. In addition, an analysis concept is presented for the prediction of the overall behavior and onset of failure in arbitrarily shaped stiffened composite structures.
An overview on traditional and advanced manufacturing procedures, industrial applications and design considerations is followed by a detailed description of failure mechanisms and failure prediction in stiffened structures made of composite materials.
In order to get a detailed insight into the mechanical behavior in the skin-stringer transition a 3D-solid shell finite element approach is used. The dimensions, layup and unidirectional materials of the investigated periodically repeated stiffener section, which is subjected to an inplane tension load in the direction normal to the stiffener axis, are chosen according to aerospace applications of stiffened composite structures.
Special emphasis is focused at discussing the influence of selected design parameters on the triaxial state of stress, the onset of failure and the overall behavior of stiffened composite structures. The following design parameters are investigated: the fiber orientation angle in the off-axis plies, the transition radius/layer thickness ratio, different unidirectional layer materials as well as different core materials.
Experimental investigations are performed using tensile tests of single blade stiffened specimens in combination with acoustic emission measurements. The predictions derived from the above 3D-solid shell finite element approach are compared to the experimental data and a good agreement is obtained.
Furthermore, a global-local finite element approach is developed for the computational analysis of global as well as local effects in arbitrarily shaped stiffened composite structures. A generalized plane strain approach in combination with a specific procedure for determining the boundary conditions allow a linear local analysis although the global analysis might be geometrically nonlinear. Comparisons with results obtained from the above 3D-solid shell finite element analysis prove the applicability of the generalized plane strain approach.
Finally, an analysis concept which is based on the above global-local strategy is presented. The application of this concept to a flight hardware component demonstrates on one hand how to use this concept for real structures, on the other hand, the results of the global and local analyses give a general insight into the nonlinear global behavior and the critical failure modes of stiffened structures made of composite materials. This indicates that a global as well as a local analysis (especially of the failure behavior in the skin-stringer transition) are of major importance in todays design of stiffened composite structures.