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Investigation of lower bound methods to predict the buckling loads of cylindrical sandwich shells under axial compression
2013 (English)Independent thesis Advanced level (degree of Master (Two Years)), 20 credits / 30 HE creditsStudent thesis
Abstract [en]

Cylindrical thin-walled structures are typically used in space launcher applications. Such structures show high sensitivity to small initial geometrical imperfections, which favours the triggering of global buckling at a much lower load level than the analytical predictions. The traditional solution to this problem, still currently applied by industry, is based on the KDF methodology proposed in the NASA design guidelines, whose publication dates back to the 1960s. However, several authors have agreed these KDFs seem to be overly conservative for the material and structural solutions available nowadays.Research projects like the presented herein have been conceived for the investigation of improved KDFs, which would result in more comprehensive and robust design guidelines. This is necessary to make the best out of the new materials and solutions currently available to designers, such as CFRP materials and honeycomb sandwich structures.Therefore, it is essential to understand the influence of different geometrical imperfections on the buckling response of such shells. This investigation makes use of the Finite Element Method to study the nonlinear unstable buckling of shells subjected to five different imperfection patterns, which are implemented as initial nodal displacements in the numerical model.These patterns consist of traditional imperfections - eigen-mode affine and axisymmetric imperfections - and more recent ones -a dimple defined by sine and cosine functions, a single perturbation load and a stress-free single perturbation load imperfection-, that gained importance after the development of the Single Perturbation Load Approach (SPLA) developed by Hühne et al. The SPLA considers a perturbation load applied in radial direction as a worst imperfection which can directly lead to a design load N_1 which accounts for the imperfection sensitivity of the shell.This project is intended to assess the effect of these imperfections on both a small-scale cylinder that will be tested in DESICOS and the full-scale Ariane 5 Inter Stage Structure hardware. These structures are sandwich cylinders, made of CFRP facesheet and a softer core.The buckling response of the shells is assessed and lower-bound buckling loads yielded by the selected imperfection patterns are produced. Appropriate analysis parameters that deliver accurate nonlinear buckling predictions for monolithic and sandwich shells are defined and the errors introduced by the simple discretization of the shells are quantified. Experimental results from the literature are successfully reproduced using such parameters.The shells show moderate imperfection sensitivity to single buckle imperfections. In the case of the sub-scale model, lower-bound from the SPLA is more difficult to obtain. Therefore, special take should be taken when applying the SPLA on sandwich shells. It yielded KDF values around 0.88 and 0.82 for the small and full-scale cylinders respectively.On the contrary, traditional imperfections yield much smaller lower-bounds, namely around 0.6 and 0.5 for the small and full-scale respectively. These values are even below the ones provided by the NASA SP-8007 approach, so there would be no benefit in using such lower-bounds, whereas the SPLA seems to yield promising results for future design methods, but its effects and applicability have to be fully understood.Lastly, the differences regarding the imperfection sensitivity of sub-scale and full-scale shells are found to be minimal, despite the unalike geometry and materials.

Place, publisher, year, edition, pages
2013. , 122 p.
Keyword [en]
Technology
Keyword [sv]
Teknik
Identifiers
URN: urn:nbn:se:ltu:diva-57088Local ID: dca45ea2-1819-4f1c-9e3a-2ebba4dea8b0OAI: oai:DiVA.org:ltu-57088DiVA: diva2:1030475
External cooperation
Subject / course
Student thesis, at least 30 credits
Educational program
Space Engineering, master's level
Examiners
Note
Validerat; 20140504 (global_studentproject_submitter)Available from: 2016-10-04 Created: 2016-10-04Bibliographically approved

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