Stress-state redistribution and thickness-ratio dependence of ballistic resistance in ductile-front 2024-T351/7075-T651 bilayer targets under ogive-nosed projectile impact

Thin high-strength 7075-T651 aluminum plates are susceptible to radial cracking, petaling, and rear-face fragmentation under ogive-nosed projectile impact. This study investigates the influence of a ductile 2024-T351 front layer on the ballistic response and failure evolution of a thin 7075-T651 substrate. Ballistic experiments were conducted on 1.59-mm monolithic 7075-T651 targets and 2.4/1.59-mm 2024-T351/7075-T651 bilayer targets, and the results were combined with three-dimensional finite-element simulations employing material-specific constitutive and fracture models. The bilayer configuration exhibited an experimental ballistic-limit velocity of 184.8 m/s, compared with 102.4 m/s for the monolithic 7075-T651 target, corresponding to a configuration-level increase of 80.5%. At comparable normalized impact velocities, the selected failed elements in the 7075-T651 rear plate of the bilayer target exhibited a distribution shifted toward higher positive damage-averaged stress triaxiality compared with those in the monolithic target. This shift was accompanied by a more localized damaged region and petaling in the bilayer rear plate, linking the presence of the 2024-T351 front layer to changes in both stress-state histories and failure morphology. Buckingham π analysis and seven parametric simulations further established a dimensionally consistent semi-empirical relation between ballistic-limit velocity and front-to-rear thickness ratio, Vbl=153.7trR2=0.997, while the incremental ballistic benefit decreased with increasing tr. These results link ductile-front-induced stress-state redistribution with failure-mode evolution and provide a quantitative framework for thickness allocation in layered aluminum targets.