SIGNIFICANT PROPERTIES OF ALUMINIUM LIGHT ALLOYS IN THE DESIGN OF ENERGY ABSORBING AIRCRAFT STRUCTURES

The paper summarises the fundamental aspects of the design of structure,,; and structural parts in aluminium light alloys in cases when energy absorption performances are required. 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 failure and file strain rate sensitivity of the plastic response. The design conditions of crashworthy helicopter structures are referred to potentially survivable crash scenarios where file absorption of the impact energy by means of absorbers located in the landing gears, in the subfloor and in the seats call significantly reduce the occupant injuries (Figure 1). The deceleration experienced by the occupants is actually influenced by file response of the whole structure of the helicopter, as it call be outlined by the experimental evidence in all helicopter crash test (Figure 2). 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 (Figures 3,4). The Working mechanism of light alloy absorbers is exemplified considering a light alloy crushing tube (Figure 5) and the role that call be played by numerical analyses in the design and verification of the absorbers integrated in the helicopter structure is outlined (Figure 6). Indeed, many helicopter structural parts contribute to transmit the loads thus allowing file absorbers to properly work and for thus parts, strength requirements have to be considered far more important than energy absorbing issues. To identify the A role played by the different parts and the consequent requirements, all hybrid multi-body/finite elements modelling technique is presented With all application to an helicopter subfloor (Figure 7). 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 oil analytical formulations for the prediction of the absorber performances that are correlated with a data base of experimental results (Figures 8,9). Experiments performed with absorbers made of different light alloy are also discussed (Figures 10,11). Attention is then focused oil the issues relevant to the bird impacts oil aircraft structures, after having pointing out that this occurrence turns out to be, basing oil the current civil aviation regulation, the dimensioning condition for many parts of fixed and rotary Wing aircraft structures. The force levels that call be exerted by a bird strike are evaluated (Figure 12) and file main design philosophies developed to design bird proof leading edges are presented (Figure 13). It is evidenced that, also in this case, the problem call 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 all 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 all adequate bird proof structure, These concepts are illustrated considering the bird impact oil a hybrid light alloy/carbon composite vertical stabilizer (Figure 14). A numercal 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 (Figure 15). The completely different experimental outcomes obtained in two impact conditions are presented, indicating that the adopted simplified material characterisation call indeed evaluate the adequacy of the structural impact strength levels (Figures 76, 17). 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 file material behaviour beyond the elastic range are introduced in the models.