Aluminium alloys are used extensively in aerospace and automotive structures due to their low weight and high specific strength, high fracture toughness, corrosion resistance, and easy machinability characteristics. In particular, AA 2024 alloy exhibits high specific strength, tensile properties, high surface finish and fatigue resistance over a wide range of temperatures. In general, AA 2024 alloy is subjected to solution treatment followed by ageing treatment to improve its tensile and fracture toughness. During actual service condition of aircraft structures, the structural components experience stress in the form of tension, compression, bending, and hydrostatic due to air medium. Also, it could experience dynamic load, especially impact due to collision with bird and other solid debris in the space. The impact due to collision is a transient process influenced by several factors such as strain, strain rate, and temperature leading to large deformations followed by failure of the materials.
The tensile and fracture toughness of AA 2024 are well reported in the literature. However, the dynamic behavior, particularly, ballistic impact behaviour of AA 2024 is scarce in the literature. Therefore, the present work is focused to analyses ballistic impact resistance of AA2024-T3 alloy using finite element analysis. LS DYNA software is used to investigate the deformation characteristics of AA 2024 alloy under varying strain, strain rate and temperature. Tensile properties of AA 2024-T3 alloy reported in the literature are used to estimate the material and damage parameters in Johnson Cook plasticity model. Johnson-Cook elasto-viscoplastic in model is used to investigate the ballistic impact resistance of target AA 2024 alloy impacted by steel projectile. The possible ranges of materials parameter such as n, m and C are assumed during impact simulation. To estimate the damage parameters, the initial fracture in the specimen is defined and the corresponding ranges in the damage parameters are estimated.
The preliminary study on impact simulation deals with failure in the target (Al alloy) adopting complete element erosion of the target elements. The target is a disc shaped geometry whereas the steel projectile is a cylindrical geometry with blunt nose shape. Eroding surface to surface contact is implemented between the target and projectile. The projectile is triggered on to the target with a velocity range of 50-305m/s. Hexahedral elements are adopted for both target and projectile. Finer mesh is used at center of the target and relatively coarser mesh is used at the outward of the target to ensure that the impact propagation along the target is captured accurately.
It is observed that the ballistic limit velocities have led to the highest energy absorption behaviour of Al alloy. Further, with increase in the target thickness, the impact resistance has increased. The strain hardening component, ‘n’ in the JC model has shown a significant effect on the Von-mises stress. Thus, with increase in ‘n’ value, the impact resistance of target has increased. The kinematic strengthening coefficient, ‘C’ did not affect the flow stress as strain rate is not considered. The residual velocities are found to increase for all the target thicknesses and projectile velocities considered due to the thermal softening coefficient ’m’
To capture the dynamic changes in the target material, the mesh density ought to be increased, which would lead to higher computation time and effort. This could be addressed by adopting node-splitting codes for element removal, which forms a scope of the future work. The ballistic limit of projectile and energy absorption characteristics of AA 2024 subjected to impact loadings will be investigated as a part of the proposed research work.