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Ballistic Impact Analysis on the Advanced Combat Helmet (ACH)—Testing and Simulation

L. B. TAN, K. M. TSE, H. P. LEE, V. B. C. TAN

Abstract



An accurate Advanced Combat Helmet (ACH) finite element model for the effective simulation of projectile-helmet interactions, especially brought on by ballistic impacts, has been developed at the National University of Singapore (NUS). Ballistic tests are performed to impact the combat helmets so as to calibrate the finite element model. A high speed camera is used to capture the dynamic impact sequence of the projectile with the helmet. The paper presents a novel methodology in employing Computed Tomography (CT) to obtain finite element geometry of the combat helmet and in their post-test damage assessments. The CT X-ray technique allows interior intra- and inter-laminar structural damage to be observed and compared with the damage prediction from our micromechanics-based helmet damage model that investigates the extent of matrix compression and tensile damage, as well as fiber buckling and tension failure. There is good correlation between experimental data and simulation results for the deformation and damage characteristics of the Kevlar® ACH up to the ballistic limit of the helmet which included projectile penetration and perforation into the ACH. At 297.6 m/s (the actual projectile speed whereby helmet perforation occurs), simulation shows perforation of two element layers with the flattening of the outermost element layer. With an impact velocity of 315 m/s, simulation shows that the projectile is able to penetrate more than half in-thickness into the laminate. As the helmet laminate began to deform, increasing surface area friction is encountered between the projectile and the contacting laminate. Energy and momentum is taken up to overcome the friction generated between the projectile surface and the perforated laminated layers as the projectile deforms and goes through it. The perforated helmet laminates eventually arrest the projectile’s impending motion as it loses its kinetic energy, and is “stucked” together with the laminate. For the simulation with projectile impact velocity of 325 m/s, the projectile penetrated through the entire thickness of the Kevlar® helmet with a residual forward velocity of around 50 m/s. Hence, from simulation, the ballistic limit (or helmet perforation velocity) of the ACH, using the steel spherical projectile, is between 297.6 m/s and 325 m/s. This translates to about 9% more speed and 17 % more kinetic energy required by the projectile for helmet perforation than experiment (based on a single test at 297.6 m/s on the helmet specimen). The discrepancy is reasonable considering that the physical helmet has been shot multiple times before the calibration tests and the limited number of ballistic tests conducted due to limited samples.

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