<p>It has been experimentally observed that, in the perforation of metal plates by a flat-nosed projectile, there exists a plateau phenomenon where the ballistic limit increases slightly with increasing plate thickness, which is related to a change in the mode of failure. No theoretical model has so far explained this phenomenon satisfactorily. This paper presents a combined numerical and theoretical study on the perforation of 2024-T351 aluminum plates struck by flat-nosed projectiles. First, numerical simulations are performed to investigate the failure mechanisms/deformation modes of the aluminum plates. Then, a theoretical model is proposed based on the numerical results and the experimental observations within a unified framework. The model takes into account the main energy absorbing mechanisms and the corresponding energies absorbed are determined analytically. In particular, a dimensionless equation is suggested to describe the relationship between global deformations and impact velocity. It transpires that the model predictions are in good agreement with the test data and the numerical results for the perforation of 2024-T351 aluminum plates struck by rigid flat-nosed projectiles in terms of residual velocity, ballistic limit, relationship between global deformations and impact velocity, and transition of failure modes. It also transpires that the present model can predict the “plateau” phenomenon, which shows a slight increase in ballistic limit as plate thickness increases. Furthermore, the energy absorption mechanisms are discussed on the basis of the theoretical analysis.</p>