3D-printing technology opens broad possibilities to manufacture structural shapes which could not be always possible by other methods. In the field of lightweight shells it allows to investigate structures with higher buckling loads than conventional shells. The buckling behavior of 3D-printed shells is studied in this paper where the shape of the cylindrical shells is modified by adding corrugation in the axial or circumferential directions. The shells are characterized by the amplitude of the corrugation and the number of the sinusoidal waves. Their elastic mechanical behavior is analyzed up to the buckling load. The numerical analysis shows that the modified surface can significantly improve the buckling load and reduces the sensitivity towards geometric imperfections. Prototypes of the shells were manufactured and tested to validate the numerical model. Regardless the experimental scatter, the average buckling load of the optimized corrugated shell twice exceeds the buckling load of the reference circular shell. At the same time stiffness and mass of the shell remain the same.@en

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%.

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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.

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The ability to steer carbon fibre tapes, varying the tow angle, can widen the designs possibilities of cylindrical shells that are one of the main components of aerospace structures. This research presents experimental and numerical investigation of two carbon fibre reinforced plastic cylindrical shells – a cylinder with conventional layup made of unidirectional prepreg and a variable-stiffness cylinder manufactured by applying fibre placement technology. The shells were tested in compression until buckling and later subjected to a vibration analysis. Load-shortening curves and buckling shapes were acquired during the compression tests, while the natural frequencies and the mode shapes were measured during the vibration tests. Both tests provide a useful data set of the mechanical response of the cylinders which can be applied for further validation of models. The acquired experimental results were compared to a simple, approximated numerical model of the variable-stiffness cylinder showing good correlation with the test results.@en

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.

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An experimental study of unstiffened graphite-epoxy cylindrical shells with cutouts subjected to bending load is presented. Two cylinders were tested: a classical laminated cylinder with constant-stiffness and a fiber-steered cylinder with variable stiffness in circumferential direction. Both cylinders were tested until collapse in displacement control. Displacements and strains were measured by displacement sensors, strain gauges, and two digital image correlation systems. Analysis of the test data indicates that the highest stressed area was the edges of the cutouts where local displacements created large compression strains. The experimental data were compared with progressive damage analysis results obtained with a finite element analysis that included measured geometrical imperfections. The research contributes to the knowledge of variable-stiffness composite structures and their applications in aircraft fuselages. @en

The ability to steer the carbon fibre tape, varying the tow angle can open new designs of cylindrical shells – the main structural component of the space launcher vehicles. This research presents experimental and numerical investigation of two carbon-epoxy cylindrical shells – a cylinder with conventional layup made of unidirectional prepreg and a variable-stiffness cylinder manufactured by applying advanced fibre placement technology. The shells were tested in compression until buckling, measuring load-shortening and capturing the buckling shape by digital image correlation systems. For the purpose of modelling the variable-stiffness cylinder, a simplified stiffness approximation approach was applied. The obtained load-shortening curves and buckling shapes demonstrated good correlation with non-linear numerical models. The results of the investigation contributes to the understanding the phenomenon of buckling of variable-stiffness cylindrical shells, and the influence of initial geometric imperfections and thickness variations. @en

A series of experimental tests have been carried out on three types of novel sandwich panels mainly designed for application in lightweight mobile housing. Two types of the panels are manufactured entirely from wood-based materials while the third one presents a combination of plywood for surfaces and corrugated thermoplastic composite as a core part. All sandwich panels are designed to allow rapid one-shot manufacturing. Mechanical performance has been evaluated in four-point bending comparing the data to the reference plywood board. Additionally, finite element simulations were performed to evaluate global behavior, stress distribution and provide the basis for a reliable design tool. Obtained results show sufficient mechanical characteristics suitable for floor and wall units. Compared to a solid plywood board, sandwich alternative can reach up to 42% higher specific stiffness, at the same time maintaining sufficient strength characteristics.@en