1,721,236 research outputs found
La piedra en el zapato : Amnistía y la dictadura uruguaya: la acción de Amnistía Internacional en los sucesos del 20 de mayo de 1976 en Buenos Aires, Argentina por Marisa Ruiz. Montevideo : Universidad de la República, 2006
Full text linkFil: Brazuna Manes, Andrea. Universidad de Buenos Aires. Facultad de Filosofía y Letras; Argentina.Reseñado por Andrea Brazuna Mane
Analysis of a textile rope with analytical models
No full textThe stresses in the wires (fibres) forming a rope for mountaineering during sharp-edge test are analysed. A reologic approach is introduced that allows modelling a rope from single fibres since the material and cross section disposition of these are known. The limits, the difficulties, the first results and the future possibilities of this new analytical approach are presented
Crack growth evaluation on helicopter fuselage under spectrum loading
No full textDamage Tolerance method for structural integrity evaluation of helicopters fuselages requests a detailed FE analysis validated through a dedicated experimental survey. This approach permits an important maintenance cost reduction and a structural safety improvement of helicopters frames.
The construction of advanced FE models for the stress assessment can be used for the identification of the most critical area for crack nucleation and growth; the detection of these area permits a continuous monitoring of the damage with a sensors network (Comparative Vacuum Monitoring, Optical Fiber Sensors, Crack Propagation Gauges, etc). Therefore the final aim of the research is to obtain a reliable method to assess the damage accumulated in the fuselage by means of an advanced prognostic models that allow the real time definition of schedule for periodic and special inspections.
Thus an increase in the safety of the aircraft by prognostic and monitoring of the frame is the main objective; in addition, the results achieved will be useful in the cost reductions regards maintenance operations and life extension for aged aircraft.
According to this aim, a review of the state-of-the-art concerning the Structural Health Monitoring applied on helicopter fuselages will be first described in this paper. The review will be also focused on the numerical FE models of cracked structures and on residual life evaluation on helicopter fuselage. Moreover an example of FE models for the stress assessment and damage propagation an a real helicopter fuselage will be described in detail and the results compared with experimental relief with good agreement
LOW VELOCITY IMPACT BEHAVIOUR OF AL-NOMEX SANDWICH PANELS
No full textIn order to asses the impact behaviour of a sandwich structure actually used on a real helicopter frame, an experimental-numerical activity has been executed. The panel specimen tested is representative of the full technological process, and it is made with a sandwich construction, with two aluminium skins (Al 2024 T3) and a Nomex honeycomb core. The experimental set up for low velocity impact has been built in order to allow the free fall of a guided impacting mass with energy up to 100 J. According to the ductile behaviour of the skin material, the tests have been carried out increasing the energy involved in order to acquire the threshold value in which a failure happens on the skin. This is a critical situation because after the skin fail, a crack could propagate under the flight spectrum loads.
A FE model has been developed to simulate the tests with particular attention to the material characterization. In order to reproduce exactly the behaviour of the Nomex honeycomb (key characteristic in the impact prediction) a flatwise test has been executed. For how concern the aluminium, a characterization of hardening and fracture locus of the material has been inserted in numerical model. The complete material characterization allows to simulate with encouraging results the fracture of the skin at near the threshold energy
Crack propagation on helicopter panel: Experimental test and analysis
No full textAerospace structures need excellent structural efficiency and damage tolerant behaviour to avoid critical failure in pres- ence of small defects and repeated small loads typical of contingent loads. With the aim of improving the performance of the structures, new materials have been developed. Such materials, as the Al–lithium alloy, are designed with the purpose of optimizing stress/strain vs. weight. Considering the distinct advantage, but also the disadvantage, of this innovative material, it is important to verify the damage tolerant behaviour of the component and this is the aim of the present work.
With this aim, some tests have been performed on full scale panel specimens representative of a rear helicopter frame. Dedicated test equipment has been designed and built in order to apply the effective service loads on artificial damaged panels. During the tests, the propagation of the crack, started from an artificial damage, has been monitored until the pro- gressive failure of the panel reaches one or more stringers. Moreover, each specimen has been instrumented with several strain gauges to obtain a strain map during the crack propagation. The crack parameters and the strains recorded during the propagation have been compared with the results from a detailed FE model and an analytical model, with a good cor- relation. In addition, a detailed FE submodel of the bolted joint (stringer and skin in the crack propagation path zone) has been constructed to obtain the crack parameter of a particular panel specimen whose test has also been carried out in the stringer bolted joint
Analisi del comportamento damage tolerant di un pannello sandwich
No full textSommario: In questo lavoro è stato analizzato il comportamento damage tolerant di un pannello a costruzione sandwich utilizzato in fusoliere aeronautiche, attraverso un duplice approccio, sperimentale e numerico. Il pannello, soggetto ad un carico a fatica pulsante di puro taglio, è stato sottoposto al carico di prova dopo che un danneggiamento artificiale è stato creato al centro di una pelle. Il successivo avanzamento della cricca, originata dal danneggiamento, è stato seguito e monitorato attraverso una strumentazione dedicata al fine di valutarne la velocità di propagazione. Due modelli ad elementi finiti sono stati costruiti per analizzare i parametri di meccanica della frattura (SIF) durante la propagazione e confrontarli con la legge di propagazione del materiale (NASGRO). In particolare i due modelli FE seguono due approcci diversi. Nel primo approccio, si utilizza un modello 3D che riproduce con elevato dettaglio la complessità costruttiva del pannello, con successiva anali..
Crack propagation on aluminum nomex panels: experimental and numerical approach
No full textAbstract This paper analyses the damage tolerance behavior of an aluminium–Nomex sandwich panel subjected to shear load. The panel was tested inside a dedicated picture frame shear test fixture in order to apply a pulsating fully shear load. An artificial crack was created at the center of the panel on one of the skins. In order to acquire the propagation data, two crack propagation gauges (CPG) were applied at the apex of this artificial crack. Two finite element (FE) models of the panel were created in order to analyze the stress intensity factor (SIF) parameter of fracture mechanics, during propagation, using two different numerical approaches to schematize the crack propagation. The analytical approach is based on a classic FE method, with a complete 3D model and manual propagation of the crack, and a dedicated program with automatic propagation on a 2D model (with a simple numerical correction to take 3D behavior into account). Comparison of the experimental data and the analyt..
Life prediction of a wire rope subjected to axial and bending loads
No full textAbstract: During normal operation, the ends of helicopter rescue hoist ropes, to which a hook is attached, can be subject to bending stress caused by vibrations. This happens in the event of partial or total recovery of the hook into its lodging without a spring-loaded blocking system. The swinging of the rope end consequently causes bending of the rope near the terminal, with resultant fatigue stress that can lead quickly to breakage or damage of the rope. A series of alternating
bending fatigue tests using a constant load, similar to those known as BoS (bending over sheave) tests, were carried out. These simulate the effective working conditions of a rescue hoist. This paper describes modifications to test machinery, the test method and the data obtained; statistical analysis of this data enabled us to make a rope life prediction which was then experimentally confirmed by run-outs on test ropes
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