Modeling Defects in Transparent Ceramics to Improve Military Armor

Army Research Laboratory, Adelphi, Maryland

Tuesday, December 01 2009

The dominant materials solution used for ballistic transparency protection of armored tactical platforms in commercial and military applications is low-cost glass backed by polycarbonate. Development of next-generation ceramics is critical to offering enhanced protection capability and extended service performance for future armored windows to the soldier. Among the potential ceramic materials considered for armor — sapphire, edge-form-growth sapphire, magnesium aluminate spinel, aluminium oxynitride — one was selected for the current pursuit: magnesium aluminate spinel (MgAl₂O₄).

Finite element modeling has progressed substantially in the ability to predict failure of materials under extreme dynamic loading conditions. One of the limitations of predictive models is lack of a complete dynamic materials properties database, which is needed for materials models for each of the materials in the simulations. In order to compensate for parameters whose dynamic values were extrapolated from their static or quasistatic properties, baseline experiments are often used to recalibrate the models.

The objective of this work was to study the effect of various shape defects, located in the interior and on the surface of spinel, on the failure of the transparent material.

Coupons for ballistic testing consisted of laminated layers of spinel bonded using Huntsman 399 polyurethane adhesive to a Bayer polycarbonate. To reduce variables, the backing layer thickness was fixed at 12.7 cm of polycarbonate. The ceramic striking material for this investigation was 11 mm. The bonding layer is typically 1 mm. Experimental samples were evaluated only to attain penetration velocity to confirm the model parameters. However, the experimental results were also used to compare the actual cracking pattern with that produced from the simulation. In addition, square cuts of 1.5 × 5 mm, and cones of 4-mm diameter and 4-mm height, were introduced into the surface of the spinel. The density of the surface defects varied and represented a 2% and 4% mass loss of the solid spinel. The internal defects represented a 4% mass loss of the solid spinel.

The ballistic behavior of a model identical to the actual target geometry, which consisted of spinel, polyurethane (PU), and polycarbonate (PC), and impacted by a surrogate projectile, was simulated using the nonlinear ANSYS/AUTODYN commercial package. The material models used were obtained from the AUTODYN library. The 2D modeling laminated target consisted of panels of spinel, polyurethane, and polycarbonate of 900 cm² cross-sectional area. The defects were filled with air at one atmospheric pressure. Due to the lack of the strength and failure material models of the spinel, these were obtained by modifying the existing at the AUTODYN materials library alumina (Al₂O₃) strength and failure model by using existing experimental ballistic data.