Stress wave propagation in aircraft structures under bird impact

Abstract

As collisions between aircraft and birds happen many thousands of times every year, all aspects of such impact events need to be taken into account in the process of designing an aircraft. One aspect that poses a risk is caused by the vibrations induced during bird impact. These vibrations are propagated through the aircraft structure by stress waves. Severe vibrations can negatively affect aircraft components and electrical systems located near the point of impact. While a vast amount of knowledge and research on the modelling of direct bird impact damage is already available, a gap in understanding exists regarding such vibrations. The purpose of this thesis is to help fill this research gap. Therefore, this thesis tries to answer whether the shock effect caused by the stress waves generated during a bird impact on an aluminium alloy plate can be accurately predicted using state-of-the-art numerical modelling techniques. Two aspects are investigated. Firstly, it is evaluated whether the stress waves that propagate the impact loads through the structure can be identified in the model. Secondly, it is analysed how well the severity of the induced vibrations can be modelled. This is done by utilising five bird impact tests, which were performed by Airbus. In each of these tests, a bird impacts an aluminium alloy test plate, which represents a simplified aircraft structure. A dummy mass, attached to these test structures, represents an electrical system influenced by the vibratory response of the plate. The displacement of the plate is recorded using digital image correlation (DIC). Accelerometers at several locations of the test structure are used to record the vibrations. The author created a dynamic explicit finite element model of the test structures to analyse how well current modelling techniques can determine these stress waves and vibrations. The structure in this model employs mostly conventional shell elements. For the bird, a smooth particle hydrodynamic (SPH) model is used. Several types of stress waves are analysed. The main stress wave types evaluated are the in-plane longitudinal and the out-of-plane Rayleigh wave. An attempt is made to identify these waves in both the numerical model and the physical test. It is found that the DIC data of the impact test shows too much disturbance to evaluate these waves. In the numerical model, both an in-plane and an out-of-plane stress wave is seen. Their recorded velocities are seen to approach their theoretical value as the mesh is refined. The propagation velocity of the out-of-plane stress wave is found to depend on the structure of the mesh. When propagating diagonally to the mesh, a 17% lower wave propagation velocity is found compared to a wave travelling tangentially to the mesh. No research was available in which the vibrations of an aircraft structure due to bird impact are analysed. Therefore, the shock response spectrum (SRS) technique, as applied in the space industry to evaluate the severity and damaging potential of vibrations, is utilised. This is the first known research in which an SRS is applied to a bird impact problem. In this research, the SRS is used to compare vibrations from the tests and the numerical model. Such an SRS is generated from the acceleration-time signal of a vibration. It is found that the trend of the SRSs obtained from the numerical model approximates the SRSs from the bird impact test closely. However, due to the fickle pattern of the SRSs, a maximum discrepancy of 5 to 10 dB between model and test is common.