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When a long rod projectile hits a ceramic target, the projectile may sometimes dwell at the target boundary and flow radially. This dwell or interface defeat phenomenon has to do with the dynamic failure process of the ceramic target material. As ceramics are brittle materials, what is needed to model dwell, is a realistic model for dynamic failure of brittle materials. A "standard" such model is the so called JH model (which has several versions). According to JH the material accumulates damage as a function of the effective plastic strain, which is a ductile response feature. Brittle materials are not supposed to accumulate plastic strain before they're fully failed. To model dwell we therefore propose here a different failure model. We call it BSF (= Brittle Shear Failure), and it is based on the Overstress (or overload) principle. Our BSF model is rather simple, has a small number of adjustable parameters, and is readily calibrated. We implement the model in a hydro-code and demonstrate how it works for a typical example of dwell situation. In the example, a long steel rod impacts an AD995 alumina target with and without a copper buffer. [ABSTRACT FROM AUTHOR]

Unlike in metals, stress and particle velocity histories of shock waves in ceramic materials show a typical "ramping" above the Hugoniot Elastic Limit (HEL). Under the assumption that the HEL signifies the beginning of failure of the material, this "ramping" has been described by viscoplasticity or by moduli degradation. However, there is ample experimental evidence to rule out plastic flow at the HEL level of stress, and it seems improbable that the moduli would degrade significantly during the compressive phase of a plate-impact. The proposed micro-mechanical mechanism for the HEL, and for the ramping beyond the HEL, is based on the process of porous compaction due to pressure above a threshold pressure P[sub crush]. Although the pore volume of high-grade ceramics is quite small, it is sufficient to cause the ramping observed in Hugoniot measurements from the seventies. Additional evidence for the effect of porosity on the HEL stress has been given in recent years. The experimental evidence is supported by simulations, in which a simple model for porous compaction was used. These simulations suggest that porous compaction is the main micro-mechanical mechanism causing the "ramping" and other features related to the HEL. [ABSTRACT FROM AUTHOR]