: Recent experimental work within the U.S. Army Research Laboratory has identified the formation of nanoscale-sized intragranular amorphous bands leading to a marked reduction in ballistic performance of boron carbide (B4C). This pressureinduced amorphization has been examined through application of the Born stability criterion that imposes restrictions on the relative magnitudes of the elastic constants of a stable crystal. The analysis has been conducted for B4C as a function of structural polytype using ab initio solid-state density functional methods and the results of the pressure evolution of the B4C elastic constants are reported. It is shown that the C-C-C polytype, a minority phase in the B4C lattice, fails at a pressure of =20 gigapascals less than the other polytypes tested in this survey, indicating that it may serve as one of the initial points of failure upon impact.

: The characteristic features of many armor-related ceramic materials are the anisotropy on the micro-scale level and the very limited, though non-vanishing, plasticity due to limited number of the planes for plastic slip. This work in progress is targeted toward revising phenomenological models of such materials with limited number of slip planes. In order to emphasize the key ideas of our approach, we illustrate the approach using the simplest possible model, the so-called deck-of-cards model.

Several Edge-on Impact (EOI) tests on transparent glasses and polycrystalline ceramics have shown that failure fronts have an extremely rough morphology, including the appearance of spikes. A simple thermodynamic theory has been used to interpret the observed morphological instability of failure fronts. For the case of isotropic phases, the instability criterion can be obtained in explicit form.

: When a high-speed projectile impacts a brittle material, such as glass or polycrystalline ceramic, severe damage and fragmentation is normally observed before projectile penetrates. AlON is a material being considered for a variety of transparent armor, sensor window, and radome applications. It is a polycrystalline ceramic that fulfills the requirements of transparency and requisite mechanical properties for transparent armor against armor piercing ammunition. AlON has a cubic, spinel crystal structure (Fd3m) that can be processed to transparency in a polycrystalline microstructure. It differs from glasses which do not have any periodic crystalline order, but it is akin to polycrystalline opaque ceramics, such as aluminum oxide. AlON was recently investigated by Strassburger, Patel, McCauley, and Templeton (2006) using EOI test in two different optical configurations. In the first, a regular transmitted-light shadowgraph set-up was used to observe wave and damage propagation. In the second, a modified configuration was used, where the specimens were placed between crossed polarizers, and the photoelastic effect was utilized to visualize the stress waves. Pairs of impact tests at approximately equivalent velocities were carried out in transmitted plane light (shadowgraphs) and crossed polarized light. AlON and fused silica specimens were impacted using solid cylinder steel projectiles with velocities ranging from 270 to 925 m/s.

Abstract : This report outlines the results of the effort to suggest a theoretical method of describing brittle ceramic armor. We analyze thermodynamics and kinetics of brittle fracture by combining the Gibbs approach to the study of heterogeneous thermodynamic systems, the Griffith energetic criterion for crack growth, and the Onsager irreversible thermodynamics. The central object of the study is the exchange between accumulated elastic energy and "chemical" energy accumulated in cohesive bonds of material particles. One of the main thrusts of the research is development of a continuum model describing dwell-defeat transition for a metallic projectile hitting ceramic armor. As compared with known theories of fracture, we describe the morphology of the damaged zone. This makes our approach somewhat similar to the Gibbs theory of heterogeneous systems.