The paper summarises the fundamental aspects of the design of structures and structural parts in aluminium light alloys in cases when energy absorption performances are acquired. Particularly, the design of crashworthy helicopter structures and the development of leading edges, for wings or tail empennages, in bird impact conditions are considered. In both cases the structural performance turns out to be strongly influenced by the material properties beyond the elastic range, such as the ultimate strength, the overall toughness, the elongation at failuire and the strain rate sensitivity of the plastic response. The design conditions of crashworthy helicopter structures are referred to potentially survivable crash scenarios where the absorption of the impact energy by means of absorbers located in the landing gears, in the subfloor and in the seats can significantly reduce the occupant injuries. The deceleration experienced by the occupants is actually influenced by the response of the whole structure of the helicopter, as it can be outlined by the experimental evidence in an helicopter crash test. As far as the subfloor and the landing gears are concerned, some design solutions developed to integrate energy absorbing elements in their structural lay-outs are presented. The working mechanism of light alloy absorbers is exemplified considering a light alloy crushing tube and the role that can be played by numerical analyses in the design and verification of the absorbers integrated in the helicopter structure is outlined. Indeed, many helicopter structural parts contribute to transmit the loads thus allowing the absorbers to properly work and, for thus parts, strength requirements have to he considered far more important than energy absorbing issues. To identify the role played by the different parts and the consequent requirements, an hybrid multi-body finite elements modelling technique is presented with an application to an helicopter subfloor. A general evaluation of the roles played by the strength level, the elongation at failure and the strain rate sensitivity in the design of light alloy absorbers is then carried out, basing on analytical formulations for the prediction of the absorber performances that are correlated with a data base of experimental results. Experiments performed with absorbers made of different light alloy are also discussed. Attention is then focused on the issues relevant to the bird impacts on aircraft structures, after having pointing out that this occurrence turns out to be, basing on the current civil aviation regulation, the dimensioning condition for many parts of fixed and rotary wing aircraft structures. The force levels that can be exerted by a bird strike are evaluated and the main design philosophies developed to design bird proofleading edges are presented. It is evidenced that, also in this case, the problem can not be reduced to design a structure and select a material for maximum energy absorbing capabilities. In fact, the risk of bird pocketing due to an excessive structural deformation suggests to take into considerations design solutions where the bird material is actually deflected. In such cases, the strength levels of the chosen light alloy may determine the development of an adequate bird proof structure. These concepts are illustrated considering the bird impact on a hybrid light alloy/carbon composite vertical stabilizer. A numerical model of the impact test is also presented discussing the key role performed, in the performed numerical analysis, by a damage law introduced to approximately model the tearing of the structural barrier. The completely different experimental outcomes obtained in two impact conditions are presented, indicating that the adopted simplified material characterisation can indeed evaluate the adequacy of the structural impact strength levels. Globally, all the presented numerical cases highlight the potential role that can be played, in exploiting the full range of properties offered by aluminium alloys, by the execution of numerical analyses, when reliable and complete descriptions of the material behaviour beyond the elastic range are introduced in the models.


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    Titel :

    Proprieta significative delle leghe leggere d'allumino nella progettazione ad assorbimento di energia delle strutture aeronautiche


    Weitere Titelangaben:

    Signifikante Eigenschaften von Aluminiumlegierungen beim Entwurf von energieabsorbierenden Flugzeugkonstruktionen
    Significant properties of aluminium light alloys in the design of energy absorbing aircraft structures


    Beteiligte:
    Airoldi, A. (Autor:in)

    Erschienen in:

    Metallurgia Italiana ; 100 , 3 ; 5-14


    Erscheinungsdatum :

    2008


    Format / Umfang :

    10 Seiten, 17 Bilder, 40 Quellen



    Medientyp :

    Aufsatz (Zeitschrift)


    Format :

    Print


    Sprache :

    Italienisch