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Airplane crash modelling assessment of the Riera model

J.M. RAMBACH, F. TARALLO, S. LAVARENNE,
18th International Conference on Structural Mechanics in Reactor Technology (SMIRT 18), Beijing, China, August 7-12, 2005,
Rapport DSR N° 74, août 2005

Summary

The Riera approach is the most common method of defining a loading curve of a missile impacting a structure. After a brief reminder of the Riera assumptions (soft crash for crushed parts, instantaneous deceleration of uncrushed part due to local crushing strength of crushing part), some physical insights are developed. According to this model, the loading curve F(x, t), where the section S(x) at distance x (x=x (t)) from missile nose is under crushing, is given by the sum of a term Re(x), corresponding to the mechanical crushing strength of the section S(x), and of a term m(x)*v(t)², corresponding to the inertial force exerted at section S(x) by mass elements of density m(x), whose velocity decreases from v to zero. Some analytical considerations are proposed regarding the missile motion when impacting a rigid target and its energy dissipation. Results from models of missile and target structures performed with the fast dynamic Finite Element code RADIOSS are used to assess the robustness, accuracy and limits of Riera model. Significant discrepancies between these approaches are found for commercial aircraft. Models by fast dynamic explicit code display loading curves characterised by higher and sharper peaks, whereas the velocity decrease of the uncrushed part is delayed. Riera model assumptions are therefore questioned and analysed. Those discrepancies are mainly due to higher distribution peaks of crushing strength and mass density and due to some hard inclusions in aircraft structure, like the engines and the junction of wing to fuselage. Soft impact is no longer relevant for such parts, whose action is then characterised by high peaks of short duration. When rounding the loading curve and spreading the impact area, the shear rupture is underestimated. Regarding concrete damaging, the Riera approach appears too optimistic. In order to keep the simplicity of the Riera approach and to improve its accuracy, the authors propose to evaluate the loading curve by the Riera approach during fuselage and wing crushing and by SDOF elastoplastic models for engines during their impact. To evaluate damage to the structure correctly, the loading force is to be distributed either locally (turbine shaft of engine) or along lines (periphery of fuselage, line between engines).

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