1,721,012 research outputs found
Development of analytical methods for fuselage design: validation by means of Finite Element analyses
No full textThe paper presents the results of a set of finite element analyses (FEAs) carried out to
support the development of an integrated design procedure that, based on semi-empirical and
analytical methods, is capable of defining generic fuselage sections of a transport aircraft. The
procedure, which is implemented in a structural optimization code, defines a structure that, compliant
with durability and damage tolerance requirements, is characterized by a post-critical behaviour of
the stiffened panels and by a design of the frames that takes the frame flexibility and the presence of
the floor beams into account.
FEAs, carried out on a reference configuration defined by the optimization code, are used to
acquire a deeper knowledge of the advantages and disadvantages of the analytical approach in the
design of complex structures subjected to realistic load cases. In particular, the influence of the actual
frame flexibility on the distribution of the skin shear flow induced by the frame is evaluated;
moreover, the effects on the stress distribution in skin and frames, caused by the presence of the
stringers, and of the stiffness concentration introduced by the floor beam are addressed.
Finite element method results demonstrate the effectiveness of the analytical model of the flexible
frame in evaluating the shear flow that a single loaded frame transfers to the skin and highlight the
effects of the presence of adjacent loaded frames. By means of geometrically non-linear FEAs, the
effects of the stringers on the stress distribution of a pressurized cylinder are evaluated, as well as the
magnitude and extension of the perturbation introduced by the floor beams
Experimental Tests and Numerical Analyses of Fibre Metal Laminates Panels Under Shear Load with Bonded Window Frame
No full textExperimental and numerical activities have been developed to investigate
the behavior of Fiber metal laminates (FML) shear loaded panels having a
bonded window frame. Both static and fatigue tests have been carried out at the
Department of Aerospace Engineering of Pisa on 10 panels manufactured by Alenia
Aeronautica, in the context of the DIALFAST (Development of Innovative and
Advanced Laminates for Future Aircraft Structures) project, co-funded by the
European Commission within the Sixth Framework Programme. In addition to
technological problems, the tested specimens represent very critical components,
involving fundamental aspects of the structural design of fuselage panels; in particular,
the primary interest to exploit the postbuckling behavior of FML components,
as commonly applied in conventional aluminum alloy fuselage structures, is associated
with the effort of making reinforced holes which do not alter significantly the
stress distribution of the main body of the panel. Detailed finite element analyses,
performed by means of the ABAQUS V 6.5 package, have been used to investigate
the panel response during testing. The numerical results have shown a very good
agreement with the experimental data, thus revealing an effective instrument to study
the behavior of a postbuckled shear panel, in presence of the neutral hole issue
Post-buckling behaviour of flat stiffened composite panels: Experiments vs. analysis
No full textThe paper is focused on the development of a validated procedure for modelling, by means of Finite Element tools, the post-buckling behaviour of stiffened composite flat panels subjected to compression loads. The experimental data for model validation were collected during a test campaign on two sets of CFRP flat stiffened panels. Tests are briefly described and selected experimental results, used for the validation phase, are presented. A detailed description of modelling strategies and analysis set up for post-buckling simulation is provided and some results of the sensitivity studies, which helped in fine-tuning the model of the test specimens, are reported as well. Finally, a through comparison of the model results with the experimental data is presented and commented. The final validation is accomplished not only by qualitatively comparing the buckled shape of the panels numerically predicted with those observed in the experiments, but also by quantitatively evaluating the differences between predicted and measured strains and out-of-plane displacements. The comparison between numerical and experimental results highlights the substantial effectiveness of the FEM approach in predicting the structural response of the panels in terms of buckling occurrence and of their post-buckling behaviour. (c) 2012 Elsevier Ltd. All rights reserved
Comportamento Post-Buckling di Pannelli Irrigiditi In Composito, Realizzati con Diverse Tecnologie, Soggetti a Compressione
No full textNel lavoro vengono presentati i risultati di prove di compressione su pannelli irrigiditi in carboresina. L’attività è stata svolta nell’ambito del programma MIUR: “COFIN 2004 – Fusoliera Tutta in Composito per Velivoli Commerciali Pressurizzati di Medie e Grandi Dimensioni”. L’attività sperimentale ha preso in considerazione due lotti di pannelli piani realizzati, da Alenia Aeronautica, utilizzando due differenti tecnologie di giunzione dei correnti al rivestimento, ovvero: “co-curing”, e “cobonding”.
Non si sono riscontrate sostanziali differenze nel comportamento pre e post-buckling delle due tipologie di pannello mentre si è evidenziata una differente modalità di collasso: nei pannelli cocured si osserva un importante fenomeno di peeling dei correnti nella regione centrale assai più contenuto o assente nella tipologia cobonded; comunque queste differenze non alterano significativamente il livello di carico di collasso dei pannelli
Solar Sail Structural Analysis via Improved Finite Element Modeling
Full text linkDespite the existence of many studies about the structural analysis of a square solar sail, the need of obtaining reliable numerical results still poses a number of practical issues to be solved. The aim of this paper is to propose a new method that improves the existing analysis techniques. In this sense, the solar sail is modelled using distributed sail-boom connections, and its structural behavior in free flight is studied, using the inertia relief method, at different incidence angles of the
incoming solar radiation. The proposed approach is able to circumvent the onset of numerical convergence problems by means of suitable strategies. A non-linear analysis is carried out starting from an initial geometrical configuration in which the whole solar sail is perturbed using a linear combination of the first global buckling modes, obtained with a static eigenvalue analysis. Key points of the procedure are the application of a correct sail pre-stress, a clever choice of the type of elements to be used in the finite element analysis and the use of a suitable mesh refinement. The performance of the new approach have been successfully tested on square solar sails with side length varying from relatively small to medium-large sizes, in the range 10m - 100m. A detailed analysis is presented for a reference 20m x 20m square solar sail, where the paper shows that the suggested procedure is able to guarantee accurate results without the need of additional stabilization technique. In particular, the vibration global mode shapes and frequencies of the solar sail are correctly described even in presence of unsymmetrical loading conditions. In other terms, the numerical analysis is completed without any convergence problem and any disturbing local modes
"Size effect" in the fatigue behavior of Friction Stir Welded plates
No full textComparative fatigue tests were carried out on Friction Stir Welded specimens of a 2195-T8 aluminum-lithium alloy that differed significantly in width. The width of the larger specimens was over thirteen times greater than that of the small specimens. Fatigue results showed a clear "size effect", i.e. fatigue life of large specimens was about 40% of the corresponding value of small specimens. The Equivalent Initial Flaw Size methodology was adopted to correlate the two sets of results. Fatigue crack initiation life was disregarded with respect to crack propagation life, and fatigue life was evaluated only as propagation of a small pre-existing defect. Following this methodology, test results of small specimens were used to evaluate the initial equivalent flaw contained in each specimen. It was assumed that this data followed a normal distribution. The equivalent initial flaw in larger specimens was evaluated by simple geometrical considerations. A very good assessment of mean fatigue life and scatter in the fatigue results of large specimens was obtained by simulating the propagation of these defects. Calculations were carried out by taking also welding residual stresses into account, but the results demonstrated that this effect was not significant
Comportamento post-buckling di pannelli piani realizzati in carboresina soggetti a compressione monoassiale
No full text
Some contraindications of hole expansion in riveted joints
No full textComparable fatigue tests were carried out on aluminium alloy 7475-T7351 double butt joints with untreated and with cold expanded holes. Surprisingly, the behaviour of the specimens having cold expanded holes was worse than that of specimens with untreated holes. This result was attributed to the surface upset which was present in cold expanded holes. Several hypotheses were formulated and experimentally verified to overcome this problem, such as deeper hole deburring, rivet diameter, joint design and grip material, steel instead of aluminium alloy.
Additional tests demonstrated that the problem was not present in sealed joint as the surface upset was hidden in the sealant thickness. Sealants have detrimental effects in the fatigue resistance of riveted joints, as they increase the load transferred by rivet bearing. The hole expansion is beneficial in this condition, while in un-sealed joints its effect must be accurately evaluated.
Other authors too highlighted possible problems due to surface upset, up to suggest to eliminate it. Only in a very few cases, a reduction of fatigue life as a consequence of hole expansion was observed
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