Stress-state-driven failure transition enhances ballistic resistance of 7075-T651/2024-T351 bilayer aluminum alloy targets under ogive-nosed projectile impact

Abstract Improving the ballistic resistance of existing aluminum-alloy protective structures without complete replacement remains an important engineering challenge. This study experimentally and numerically investigated monolithic 2.4-mm-thick 2024-T351 targets and 1.59 mm 7075-T651/2.4 mm 2024-T351 bilayer targets subjected to impact by 5.96 mm diameter ogive nosed projectiles. Ballistic tests using a single-stage light-gas gun and high-speed imaging were combined with ABAQUS/Explicit simulations employing the Wen–Mahmoud fracture criterion for 7075-T651 and the Modified Johnson–Cook criterion for 2024-T351. The predicted ballistic limit velocities differed from the experimental values by 3.2% and 1.8% for the monolithic and bilayer targets, respectively, while the residual-velocity trends and dominant failure patterns were also reasonably reproduced. The bilayer configuration increased the ballistic-limit velocity from 144.7 to 183.3 m/s, corresponding to an increase of 26.7%. This improvement was associated with the additional penetration resistance provided by the high-strength front plate, coupled bending of the bilayer assembly, and redistribution of the backing-plate stress state toward tension-dominated petalling. Damage-averaged stress-triaxiality and Lode-angle analyses further indicated that the front plate reduced the contribution of compression–shear-dominated damage in the 2024-T351 backing plate. Decomposition simulations attributed approximately 85% of the ballistic-limit increase to the added thickness and coupled structural response and the remaining 15% to the material-specific contribution of 7075-T651. Although the equivalent-thickness monolithic 2024-T351 target exhibited a higher ballistic-limit velocity of 214.6 m/s, the bilayer configuration remains a practical retrofit option for upgrading existing 2024-T351 protective structures when complete replacement is impractical. These findings establish a link between layer-induced stress-state redistribution and failure-mode transition and provide guidance for the retrofit design of heterogeneous aluminum-alloy protective systems.