Effects of aggregate content and gradient configuration on functionally graded <scp>UHPC</scp> targets under high‐velocity projectile penetration: A <scp>3D</scp> meso‐scale numerical investigation

Abstract A three‐dimensional meso‐scale numerical modeling approach was developed to investigate the penetration resistance of functionally graded ultra‐high performance concrete (FG‐UHPC) under normal impact of high‐velocity projectiles. A meso‐scale model incorporating UHPC mortar, randomly oriented fibers, randomly distributed aggregates and interfacial transition zones (ITZs) was established using a custom‐built algorithm code, enabling the simulation of target failure evolution and heterogeneous crack propagation under high‐velocity penetration. The reliability and accuracy of the meso‐scale model were validated through projectile penetration and uniaxial compression experiments. Based on the validated model, quantitative analyses were conducted to evaluate the effects of gradient layer thickness ratio, aggregate content, aggregate size, fiber volume fraction, and projectile velocity on the damage patterns and penetration resistance R of FG‐UHPC targets. The results demonstrated that compared with conventional UHPC target, FG‐UHPC exhibited a reduction in depth of penetration (DOP) by approximately 29.9%–41.0% and in crater area by 61.2%–71.0%. The penetration resistance R increased monotonically with increasing aggregate content and size, with a diminishing marginal effect at higher values, whereas the influence of gradient layer thickness ratio and fiber volume fraction was less significant. During penetration into FG‐UHPC targets, projectiles underwent severe erosion and trajectory deflection, with a mass loss ratio ranging from 15.9% to 70.5%, which is in sharp contrast to the 3.5% mass loss in UHPC targets. The superior penetration resistance of FG‐UHPC targets strengthened with steel ball aggregates is attributed to the synergistic effects of projectile erosion, local strengthening and confinement, and trajectory deflection. Furthermore, an improved semi‐empirical formula was proposed to predict the penetration resistance R by decoupling the penetration resistance contributions of the matrix and the steel ball aggregates.