34 research outputs found

    Studies of Impefection Sensitive Conical Composite Structures

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    The stability of shell structures has been an object of studies for more than a century. Thin walled cylindrical and conical structures are widely used in aerospace, offshore, marine, civil and other industries. Nowadays, with the growing application of composite materials a deep understanding of the influence of their properties and the laminate stacking sequence on the mechanical behaviour of shell structures is increasingly more important. As it is already known, one of the most significant sources of discrepancy between theoretical predictions and experimental results for the buckling load is the presence of geometric imperfections. Currently, imperfection sensitive shell structures are generally designed, at the preliminary design phase, according to the guideline NASA SP-8007 for cylinders and NASA SP-8019 for truncated cones using the conservative lower bound curve, which does not consider composite material characteristics. Hühne developed the Single Perturbation Load Approach (SPLA), a robust design method that stimulates a single buckle, which is assumed as a “worst-case” geometrical imperfection [1]. There have been carried out considerably more numerical, analytical and experimental studies on cylindrical shells than on conical shells. Currently typical composite launcher structures are investigated by 12 partners in the European project DESICOS [4]. The aim of this paper is to study the SPLA on a conical shell structure and compare it with the NASA design approach

    On the development of shell buckling knockdown factors for imperfection sensitive conical shells under pure bending

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    Thin-walled conical shells are used as adapters between cylindrical shells of different diameters in launch-vehicle systems or as tailbooms in helicopters. A major loading scenario for conical shells is pure bending. The buckling moment of these shells is very sensitive to imperfections (geometry, loading conditions) which results in a critical disagreement between theoretical and experimental results for conical shells under pure bending. The design of these stability critical shells is based on classical buckling loads obtained by a linear analysis which are corrected by a single knockdown factor (0.41 - NASA SP-8019) for all cone geometries. This practice is well established among designers and hasn't changed for the past 50 years because the buckling behavior is till today not very well understood. Within this paper a reduced stiffness analysis for conical shells under pure bending is performed. Data of previous experimental testing campaigns are used to validate the new design criteria for different conical shell geometry configurations. The results show that the application of the new design recommendation for conical shell structures results in increased knockdown factors for the buckling moment which in turn may lead to a significant weight reduction potential. All ABAQUS-Python scripts and the results generated for this article are deposited in the Elsevier repository

    Experimental and numerical investigation of CFRP cylinders with circular cutouts under axial compression

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    Thin-walled cylindrical structures are widely used in aerospace, offshore, civil and other engineering fields. Parts of space launcher transport systems are one example for the application of such shells. Buckling of thin-walled structures is a very important phenomenon to be considered during their design phase. This is true not just because such structures are often imperfection sensitive (geometry, boundary conditions, load introduction, thickness, etc.) but also due to operational requirements set on these thin-walled structures which often lead to the need for introducing cutouts to accommodate access panels, doors and windows. These cutouts constitute an additional factor that influences the overall stability and needs to be understood in order to enable a safe operation and an effective design of these structures. The study deals with buckling experiments on two axially compressed, unstiffened CFRP cylindrical shells with circular unreinforced cutouts, performed by DLR. Moreover, a FE model is described that is validated with the experimental results. The objective of the study is to investigate the effect of the size of the cutouts on the buckling characteristics of the tested shells

    Towards robust knockdown factors for the design of conical shells under axial compression

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    Thin-walled conical shells are used as adapters between cylindrical shells of different diameters in launch-vehicle systems. Conical shells carry heavy payloads and are consequently subjected to axial compression. The buckling load of these shells is very sensitive to imperfections (geometry, loading conditions) which results in a critical disagreement between theoretical and experimental results for axially loaded conical shells. The design of these stability critical shells is based on classical buckling loads obtained by a linear analysis which are corrected by a single knockdown factor (0.33 - NASA SP-8019) for all cone geometries. This practice is well established among designers and hasn't changed for the past 50 years because the buckling behavior is till today not very well understood. Within this paper an analytical and numerical lower-bound procedure for conical shells under axial compression is proposed. Data of previous experimental testing campaigns are used to validate the new design criteria for different conical shell geometry configurations. The whole design concept is demonstrated by means of the Interstage 1/2 of the Vega launcher and it is concluded that a revision of the current design recommendation for conical shell structures may results in a significant weight reduction potential

    ROBUST DESIGN CRITERION FOR AXIALLY LOADED CYLINDRICAL SHELLS – SIMULATION AND VALIDATION

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    A currently used guideline for cylinder structures under axial compression is the NASA SP-8007 which is based on empirical data from the 1960s. This guideline provides knock-down factors (KDF) for the lower bound of the buckling load which depend on the cylinder radius-to-thickness ratio but neglect the influence of the cylinder length L. Experimental results indicated an influence of the cylinder length on the buckling load but a clear dependency could not be established because of the insufficient amount of available data. A comprehensive numerical investigation was performed in order to study the influence of length effect on the lower bound of the buckling load. The numerical analysis is based on the single boundary perturbation approach (SBPA) for cylindrical shells. The results verify that there is a significant influence of the cylinder length L on the lower bound of the buckling load. Semi-analytic knock-down factors for the stability failure of axially loaded cylindrical shells were determined which can be used for a simple and fast approximation of the lower bound of the buckling load. The corresponding SBPA thresholds were validated with a number of high fidelity buckling experiments and deliver much higher KDFs than currently used empirical guidelines
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