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Lee M.C.W.,University of New South Wales | Mikulik Z.,University of New South Wales | Kelly D.W.,University of New South Wales | Thomson R.S.,Cooperative Research Center for Advanced Composite Structures 2Web page Ltd. | And 2 more authors.
Composite Structures | Year: 2010

Robust design is a philosophy that aims to ensure that a structure will be tolerant to unknown variations and imperfections. This is an important consideration as highly optimised critical structures are required to survive unexpected loading and operating conditions. In some ways, robust design appears to be similar to damage tolerant design but its application to aerospace structural design is neither well established nor understood. In order to demonstrate the differences between the two concepts, a stiffened composite panel has been analysed for damage tolerance and robustness properties. Damage tolerance has been studied experimentally with the panel subjected to impact damage. The effect of laminate stacking sequence on the robustness of the panel has been assessed using finite element analysis and a Robust Index applied to quantify the robustness. The differences between designs are discussed together with the possible future directions for robust design applied to aerospace composite structures. © 2009 Elsevier Ltd. All rights reserved. Source

Degenhardt R.,German Aerospace Center | Degenhardt R.,Private University of Applied Sciences, Gottingen | Kling A.,German Aerospace Center | Bethge A.,German Aerospace Center | And 5 more authors.
Composite Structures | Year: 2010

In the field of aerospace engineering but also in the fields of civil and mechanical engineering the industry demands for significantly reduced costs for development and operating. Reduction of structural weight at safe design is one avenue to achieve this objective. In many cases it results in thin-walled structures, which are prone to buckling if subjected to compression or shear. The presented paper is based on a recent European Space Agency (ESA) study, conducted at DLR Braunschweig, on Probabilistic Aspects of Buckling Knock-Down Factors and contributes to this goal by striving for an improved buckling knock-down factor (the ratio of buckling loads of imperfect and perfect structures) for unstiffened CFRP cylindrical shells. Buckling tests and buckling simulations were performed to investigate the imperfection sensitivity and to validate the applied simulation methodologies. Test results as well as deterministic and probabilistic buckling simulation results are presented and compared. Finally, improved knock-down factors are deduced and discussed. © 2009 Elsevier Ltd. Source

Arbelo M.A.,Private University of Applied Sciences, Gottingen | De Almeida S.F.M.,Brazilian Technological Institute of Aeronautics | Donadon M.V.,Brazilian Technological Institute of Aeronautics | Rett S.R.,Brazilian Technological Institute of Aeronautics | And 5 more authors.
Thin-Walled Structures | Year: 2014

Nondestructive experimental methods to calculate the buckling load of imperfection sensitive thin-walled structures are one of the most important techniques for the validation of new structures and numerical models of large scale aerospace structures. Vibration correlation technique (VCT) allows determining equivalent boundary conditions and buckling load for several types of structures without reaching the instability point. VCT is already widely used for beam structures, but the technique is still under development for thin-walled plates and shells. This paper intends to explain the capabilities and current limitations of this technique applied to two types of structures under buckling conditions: flat plates and cylindrical shells prone to buckling. Experimental results for a flat plate and a cylindrical shell are presented together with reliable finite element models for both cases. Preliminary results showed that the VCT can be used to determine the realistic boundary conditions of a given test setup, providing valuable data for the estimation of the buckling load by finite element models. Also numerical results herein presented show that VCT can be used as a nondestructive tool to estimate the buckling load of unstiffened cylindrical shells. Experimental tests are currently under development to further validate the approach proposed herein. © 2014 Elsevier Ltd. Source

Castro S.G.P.,Private University of Applied Sciences, Gottingen | Castro S.G.P.,Embraer SA | Castro S.G.P.,Clausthal University of Technology | Mittelstedt C.,HIGH-TECH | And 5 more authors.
Composite Structures | Year: 2014

Semi-analytical models for the linear buckling analysis of unstiffened laminated composite cylinders and cones with flexible boundary conditions are presented. The Classical Laminated Plate Theory and the First-order Shear Deformation Theory are used in conjunction with the Donnell's non-linear equations to derive the buckling equations. Axial, torsion and pressure loads can be applied individually or combined in the proposed models. The stiffness matrices are integrated analytically and for the conical shells an approximation is proposed to overcome non-integrable expressions. Comparisons with the literature show that the classical base functions available for axial compression cannot capture the buckling modes for non-orthotropic laminates. For torsion loads these classical shape functions do not catch the buckling modes even when applying the assumption of pure orthotropy, and it is shown how the proposed models correlate well with experimental data from the literature and finite element results. The use of elastic constraints at the boundaries allows the simulation of different boundary conditions in a versatile way and it is shown how those constants can be adjusted in order to change from one type of boundary condition to another. © 2014 Elsevier Ltd. Source

Arbelo M.A.,Private University of Applied Sciences, Gottingen | Degenhardt R.,Private University of Applied Sciences, Gottingen | Castro S.G.P.,Private University of Applied Sciences, Gottingen | Zimmermann R.,German Aerospace Center
Composite Structures | Year: 2014

Currently, imperfection sensitive shell structures prone to buckling are designed according to the NASA SP-8007 guideline, from 1968, using its conservative lower bound curve. In this guideline the structural behavior of composite materials is not appropriately considered, since the imperfection sensitivity and the buckling load of shells made of such materials depend on the lay-up design. In this context a numerical investigation about the different methodologies to characterize the behavior of imperfection sensitive composite structures subjected to compressive loads up to buckling is presented in this paper. A comparative study is addressed between a new methodology, called "Single Perturbation Load Approach", adopted by the European project DESICOS, and some classical approaches such as non-linear analyses considering geometric and thickness imperfection obtained from real measurements. An extension of the Single Perturbation Load Approach called "Multiple Perturbation Load Approach" is also introduced in this paper to investigate if one perturbation load is enough to create the worst geometrical imperfection case.The aim of this work is to validate these numerical methodologies with experimental results and point out their limitation, advantage and disadvantage, to calculate less conservative knock-down factors than the obtained with the NASA SP-8007 guideline for unstiffened composite cylinders. © 2013 Elsevier Ltd. Source

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