In the current research the buckling response of cylindrical shells manufactured using 3D-printing technology is studied numerically and experimentally. The 3D-printing technology allows quick prototyping in order to assess the influence of the corrugated surface of the shells on the buckling loads. Finite element analyses are conducted using the commercial finite element code Abaqus, and the structural behavior is analyzed up to the post-buckling field. A few shells are manufactured using additive 3D-printing technology and tested in compression. The shells are then optimized in terms of the amplitude of the corrugation and the number of the sinusoidal waves. The average buckling load of the optimized corrugated shells results 160% higher of the buckling load of the cylindrical shells with circular section, keeping the same stiffness and mass. The introduction of the corrugated surfaces, not only significantly improves the buckling load, but also reduces the sensitivity towards initial geometrical imperfections. The knowledge acquired on these small 3D-printed shells can be useful for the design of new aerospace cylindrical shells.

Thin-walled carbon fiber reinforced plastic (CFRP) shells are increasingly used in aerospace industry. Such shells are prone to the loss of stability under compressive loads. Furthermore, the instability onset of monocoque shells exhibits a pronounced imperfection sensitivity. The vibration correlation technique (VCT) is being developed as a nondestructive test method for evaluation of the buckling load of the shells. In this study, accuracy and robustness of an existing and a modified VCT method are evaluated. With this aim, more than 20 thin-walled unstiffened CFRP shells have been produced and tested. The results obtained suggest that the vibration response under loads exceeding 0.25 of the linear buckling load needs to be characterized for a successful application of the VCT. Then the largest unconservative discrepancy of prediction by the modified VCT method amounted to ca. 22% of the critical load. Applying loads exceeding 0.9 of the buckling load reduced the average relative discrepancy to 6.4%.

Two laminated composite shells, one with a conventional straight fiber laminate denoted the classical laminated shell and the second one with a variable angle tow reinforced composite, had been excited and their natural frequencies and mode shapes had been measured and monitored as a function of the axial compression load. Then, the in-situ buckling loads of the two tested specimens were predicted using the Vibration Correlation Technique (VCT) and compared with actual experimental buckling loads and Finite Element buckling predictions, yielding matching, consistent and repeatable results. It was shown that the VCT predicts the actual in-situ buckling loads of laminated composite thin walled cylindrical shells with a high accuracy, yielding 96% and 98.6% of the experimental buckling load, for the classical and variable angle tow composite shells, respectively. These results, although based on only two specimens, join the relatively small data base published in the literature, proving the nondestructive nature of the VCT approach, making it an adequate method for application on thin-walled structures, like shells. In addition, some testing recommendations are presented, to effectively enable the successful application of the VCT for in-situ buckling prediction of the buckling sensitive structures, like composite cylindrical shells.

This paper presents the numerical analysis of a thermoplastic composite stiffened panel subjected to compression load. The panel has three stringers with a non-symmetric design, with an artificial crack at the middle stringer interface and is made from a fast crystallizing polyetherketoneketone carbon composite. The finite element model includes an approximation of the geometrical imperfections which were measured using a digital image correlation system. The finite element analyses are discussed, where the crack propagation is modelled using the virtual crack closure technique. The results show that crack propagation starts rather early after buckling and the crack growth behaviour is heavily influenced by the buckling shape, which consists of three half-waves in longitudinal direction in each bay.