Numerical study of the impact response and ballistic limit of AA2024 friction stir welded aircraft panels

Abstract

Friction Stir Welding is a novel environmentally friendly welding technique able to produce higher joint properties than conventional processes. It has become an attractive process for many manufacturers in the last years, with the increasing potential of replacing existing joining processes used in the aerospace industry, such as riveting, which plays a significant role in weight and cost reduction. If used to join exterior skin of aircraft components, welds represent weak regions likely to experience a number of impact events, including bird-strike and hard projectiles impact (engine debris). The aim of this project is focused on developing a finite elements model of an AA2024-T3 friction stir welded thin panel and then performing a detailed numerical study (LS-DYNA) of the impact response and ballistic performance under impact conditions with soft projectiles and different hard projectile shapes. Barlat’s anisotropic material model is used in an attempt to model the different level of anisotropic plastic behaviour present at each weld region, based on microstructural aspects of the weld. The effect of implementing material anisotropy on the ballistic limit prediction of the welded panel is assessed by carrying out a comparison with the ballistic limit distributions obtained across the weld area after assuming isotropic plastic behaviour for the whole region. Results show that the predicted ballistic limit is lower when a certain level of anisotropic plastic behaviour is implemented, independently from the projectile shape. In the case of the soft projectile impact (bird-strike), higher deflections of the panel are obtained when material anisotropy is present with respect to the case of assuming an isotropic plastic behaviour. Presence of the weld is found to produce a loss in ballistic limit in the case of the hard projectiles impact and higher panel deflections with respect to the base material when directly impacting the centre of the weld with the soft projectiles.