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Modelling Back Face Deformation of Woven Layered Composite Targets under Oblique Impact

Body armour is the only protection a dismounted soldier has against projectiles or fragments in case of combat. Perforation is prevented in body armour as the kinetic energy of the projectile is transformed to deformation work in the armour material. This dynamic material response upon impact is especially crucial for helmets, as it acts directly on the human head. One potential threat nowadays a foot soldier faces during missions is the 7.62x39 mm projectile fired from a rifle. Helmets are not designed to withstand a direct impact of such a projectile, which is launched at an initial velocity of vi=720+/10 m/s. Under an obliquity angle of θ>65 degrees (NATO) projectile ricocheting is observed. The aim of the ongoing project is to promote the projectile ricochet off helmets to increase the likelihood of projectile deflection and the survivability of the wearer. The focus of this paper is the target back-face deformation (BFD) upon oblique high velocity impact. Experiments were conducted on projectile impact on plane aramid plates. These plates have the same material properties, such as layer number, as used for manufactured helmets – other ballistic helmet materials are covered in future research. Upon impact, the dynamic BFD of the aramid targets was measured, using digital image correlation (DIC). Additionally, the experiments were repeated to capture the projectile trajectory through the target thickness, using X-ray cinematography. The BFD and trajectory results are used for the qualitative comparison of a numerical model, defined within the LS-DYNA® explicit Lagrangian solver. Model components, the projectile and target plate are defined using fully integrated hexahedral elements. The projectile deformation is represented by *MAT_JOHNSON_COOK and its failure criterion; and the target plate is represented by *MAT_COMPOSITE_DAMAGE. The projectile and the composite target are in a symmetric contact defined by *CONTACT_ERODING_SURFACE_TO_SURFACE. The aim of this paper is an investigation on the most suitable modelling approach for a numerical validation of the BFD response obtained from the DIC measurements. This work is a first step to implementing experimentally and numerically achieved BFD data in a LS-DYNA® Finite element (FE) head model, using a head injury criteria (HIC).

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