Linus Friedrich

Imperfection Sensitivity of Circular Cylindrical Shells of Varying Length Subjected to Axial Compression

Within the first half of the last century, a significant deviation between theoretical buckling loads of thin-walled shell structures and the buckling load determined experimentally was identified. The reason for this discrepancy is explained by the presence of imperfections having a degrading effect on the load carrying capacity of shell structures. To investigate the influence of imperfections on the real buckling load, different imperfection modelling approaches are developed. Within this paper the imperfection sensitivity of unstiffened circular cylindrical shell structures is investigated with the Single Perturbation Load Approach as well as the axis-symmetric imperfection modelling. Thereby, the influence of geometrical relations of the shell structures on the imperfection sensitivity is investigated. The achieved results are compared to empirical findings reported in literature.

Sizing strategy for stringer and orthogrid stiffened shells under axial compression

ABSTRACT Thin-walled stiffened shell structures are used in primary structures of space launcher vehicles. These structures are prone to buckling and thus fail due to a loss of structural stability. Generally, it has to be distinguished between two major modes of instability for stiffened shell structures: a global buckling of the entire structure and a local buckling of skin fields, longitudinal and circumferential stiffeners. Due to the large number of variables when designing stiffened shell structures, their preliminary design is a demanding task. To allow an efficient preliminary design, sizing strategies can be developed. For this purpose, analytical methods, which allow to assess the local and global instability of stiffened shell structures, are employed. In this article, sizing strategies based on efficient analytical methods are introduced and applied to identify suitable designs of stringer stiffened and orthogrid stiffened shell structures. To study the imperfection sensitivity of stringer and orthogrid stiffened shell structures, numerical computations are performed using the single perturbation load approach and mode shape imperfections. Finally, weight strength curves are derived for stringer and orthogrid stiffened shell structures.

Efficient and Robust Shell Design of Space Launcher Vehicle Structures

Space launcher vehicles consist of thin-walled shell structures which are prone to buckling and often are sensitive towards geometrical imperfections. Even small deviations of the shell from the perfect structure which still are within manufacturing tolerances, result in a tremendous decrease of load carrying capacity. To account for geometrical imperfections in an early design phase, empirical knock-down factors or theoretical approaches can be applied. In this paper, it is shown that the design of imperfection sensitive shell structures with unknown geometric imperfections may not lead to robust designs for the existing empirical and theoretical design methods. In contrast to unstiffened structures and grid stiffened shell structures, which are imperfection sensitive, it is known that the influence of imperfections during an early design phase of ring frame stringer stiffened shells is negligible when the post-buckling regime of the skin fields is exploited. Frame stringer stiffened structures can be designed in a robust manner, using efficient analysis methods, as imperfection tolerant structures; but, existing methods to size ring frame stiffeners of space launcher vehicles shell structures do not mandatorily lead to reliable and light designs. In this contribution a novel method for the efficient design of ring frame stringer stiffened shells is presented. The suggested approach is based on the explicit description of the mechanical behavior of the ring frame stiffeners at the onset of panel instability. Together with existing sizing methods for stringer stiffened shell panels the suggested approach allows for robust designs of ring frame stringer stiffened shells. The application of the novel method to size ring frames reveals that the minimum stiffness requirements are satisfied likewise with regard to existing methods; whereby, the lightweight potential is not mandatorily exploited using existing methods.