Modelling of impact forces and pressures in Lagrangian bird strike analyses

The paper aims at evaluating and improving the accuracy of bird impact numerical analyses performed with finite element explicit codes, focusing on the modelling of the spatial and temporal pressure distributions exerted on the target by the impacting body. A Lagrangian approach is adopted, interfacing the ESI/Pam-Crash solver code with an automatic trial-and-error procedure for the elimination of the excessively distorted elements. The theoretical formulation relevant to the impact of a cylindrical soft body against a rigid target is reviewed and this idealised case is adopted to validate the presented approach with increasingly refined finite element schemes. A sensitivity study is then carried out, adopting differently shaped bird models and varying the material hydrodynamic and deviatoric responses. A set of models is selected comparing the results with the experimental average values and the scattering reported in literature for the most significant loading parameters in impacts on rigid targets. The model shape and the calibration parameters of the bird material used in these models are subsequently adopted in the analyses of impacts on a deformable polycarbonate plate. The numerical results obtained with increasingly refined bird models are presented and discussed. A range of modelling parameters is finally suggested to perform reliable numerical analyses on aircraft structures and a criterion is proposed to select the models for a reasonably conservative approach to the design of a bird proof structure.