PSI - Issue 51

R. Andreotti et al. / Procedia Structural Integrity 51 (2023) 37–43 R. Andreotti et al. / Structural Integrity Procedia 00 (2023) 000–000

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which the panels are constrained must themselves withstand the extreme loads generated by the ballistic impacts and propagating to the entire airframe as stress waves. Moreover, the survivability of the airframe must be guaranteed after multiple shots, each of which can happen randomly on the protection panels; therefore, the number of load cases to be simulated can easily be significant, likely in the order of magnitude of 10 2 . Finite element numerical simulation is the tool used to optimize the airframe to withstand these loads, but the load history to be applied is impossible to measure experimentally due to the extremely impulsive nature of the phenomena. Load histories to be applied are therefore usually estimated by means of complex simulations involving the detailed prediction of the interactions between bullets and target. Unfortunately, these simulations are not only difficult and expensive to conduct, but also uncertain in accuracy, due to the impossibility to fully characterize the dynamic behavior of the materials at such extremely high strain rates. In this scenario we proposed to focus on a particular experimental case: the so-called bullet splash phenomenon, which represents the typical outcome of a perfectly passed ballistic test: the bullet is completely fragmented and deflected by the target’s surface and cannot penetrate the impacted surface. We first proposed a simplified approach to model bullet splash based on treating the interaction between impactor and target as a fluid structure interaction (FSI). This approach was validated for 9x21mm full metal jacket (FMJ) handgun bullets by Andreotti et al. (2021). A further method to reduce the computational cost of ballistic impact simulations for structural integrity assessment was recently proposed and validated on the same experimental dataset by Andreotti et al. (2022) and is based on a simplified formula to estimate the load history F(t) as only depending on the initial impact velocity and the density distribution within each cross section of the bullet. Here we propose the first application of these two methods to simulate the bullet splash of rifle bullets. The scope of this paper is extending the experimental validation of the methods for a monolithic .308 copper bullet hitting a 4mm plate of high strength steel. The validation is performed by comparing the experimental back plate residual deformation field with the ones predicted by the simulations conducted with the FSI and load history approaches already validated for 9x21mm FMJ bullets. The comparison between the methods also considers the effect of different pressure distributions on the stress waves propagating from the impact epicenter and the total reaction forces in the impact direction.

Nomenclature FMJ

full metal jacket

FSI

fluid structure interaction

ALE

arbitrary Lagrangian Eulerian formulation

t

time

F(t) load history v impact velocity density ����

bullet’s section ideally intersecting the surface of the target at time t

2. Materials and methods The experimental ballistic tests were conducted by shooting monolithic copper .308 WIN. ARIETE (mass 9.6g / 148gr) bullets supplied by HASLER® (Fig. 1) hitting 500x500x4mm plates made of Creusabro® high strength steel. The shots were performed with a Remington rifle (Model 700) from 100m distance, thanks to a Swarovski 6-24x50 HABICHT optics system. The impacts were filmed by two high frame-rate cameras Phantom Veo E-310L positioned orthogonally and at 45° angle from bullets’ direction to capture the kinematics and fragmentation of the impactors. The impact velocity was measured thanks to a background calibrated checkered screen, mounted sideways to the target. The acquisition rate was 35000 fps. The experimental analysis consisted in verifying the bullets’ kinematics and their fragmentation as well as measuring the field of backplate residual displacements of the plates across the epicenter of the impacts.