Experimental and numerical investigation of soft impact loading on aircraft materials

Abstract

Bird strike poses a great hazard to aircraft components, such as the engine and windshield, during flight. Thus, it is critical to understand the impact resistance of key aircraft components under bird strike. During this PhD study, Aluminium Alloy 2024-T3 and laminated glass interlayered with thermoplastic polyurethane (TPU), which are commonly used as the materials of the aircraft fuselage and windshield respectively, are selected as impact target materials. Both laboratory-based experiments and finite element simulations are performed using a light gas gun and ABAQUS/EXPLICIT respectively. A good agreement is achieved between the experimental work and numerical predictions for the impact response. Two bird substitute materials were selected: RTV rubber and ballistic gelatine, whose dynamic pressure profiles are similar to that of a real bird during a high speed impact. Mechanical properties of both materials were investigated by conducting compression tests at quasi-static (0.25, 2.5 and 25 min-1) and intermediate rate (2760 min-1 to 22500 min-1). As a result, the relationship between true stress and strain is obtained and constitutive equations are established using the hyperelastic models, i.e. the Ogden, Mooney-Rivlin and Neo-Hookean. The 3D Digital Image Correlation technique was employed in the gas gun test of the AA 2024-T3 and TPU interlayered laminated glass. Thus, the impact compliance of both targets from rubber and gelatine impacts are attained and found to be roughly equal to the same initial projectile momentum. An FE simulation was used to model the experimental process and was validated against the DIC results. Moreover, the Hugoniot pressure plays a predominant role in laminated glass fracture during the impact, with the rubber projectile leading to a larger damage than the gelatine projectile given the same momentum. This is because the shock wave speed in the rubber is larger than that in the gelatine projectile. A validated FE simulation is therefore presented, which can be used to simulate real bird impact on real aircraft structures in the future. Thus, this PhD work can be potentially implemented into industrial research programmes to aid design and optimisation.