Ballistic failure processes in alumina

Abstract
This is a preliminary report in a new program of research to evaluate the basic failure processes occurring in dense ceramics during projectile penetration, and, in particular, to study the role played by mechanical constraint in inhibiting some failure modes. In this contribution we describe a method for monitoring the response of selected ceramics to a highly localized pressure pulse under conditions of controlled constraint and distinguish the effect of constraint on the three primary failure modes : 1. Crushing, comminution and erosion in the zone of high compressive stress which exists near the contact surface between the projectile and the impacted ceramic. 2. Nucleation and propagation of a cone crack, predominantly in mode II (the shear stress mode) to form a ceramic plug ahead of the advancing projectile. 3. Fragmentation associated with radial (long-term) cracks nucleated and propagated by tensile stresses in the target material surrounding the contact zone. Constraint acts primarily to prevent the formation of a cone crack, allowing higher compressive stresses to be reached in the contact zone. At the levels of constraint studied so far, radial (tensile) cracks always terminate the failure process, releasing the compressive stresses in the contact zone.