Explomet 2000, June 18-22, 2000, Albuquerque, New Mexico 1 Microstructural and micromechanical aspects of ceramic/long-rod projectile interactions: dwell/penetration transitions Jerry C. LaSalvia a , Edward J. Horwath a , Edward J. Rapacki a , C. James Shih b , and Marc A. Meyers c a U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005-5069 b Ceradyne, Inc., Costa Mesa, CA 92626 c University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093 The transition from dwell to penetration by long-rod projectiles is strongly dependent upon the inelastic response of both the projectile and target materials to the stresses generated. For thick ceramic targets, the possibility for either a brittle or ductile response locally, naturally leads to both lower and upper bounds in the impact velocity for the dwell/penetration transition. The existence of these bounds was recently investigated by Lundberg and co-workers. Both bounds are governed by the inelastic region formed beneath the projectile and its extension to and/or from the free surface. The formation of the inelastic regions for the upper and lower bounds are dominated by the cumulative effects of crystal plasticity and microcracking, respectively. Evidence for the latter was obtained through detailed observations within the comminuted region of several different ceramics recovered from dwell experiments conducted by Hauver and co-workers. The dominant damage mechanism within the comminuted region appeared to be the extension of wing-cracks along grain boundaries. A wing-crack model for compressive failure of brittle materials, developed by Horii and Nemat-Nasser, enables predictions for wing-crack initiation (brittle response) and/or suppression (ductile response) depending upon the ratio of the confining stress to the applied stress σ 2 /σ 1 , the coefficient of friction μ, and a ductility parameter Δ defined as K IC /τ Y √πc, where K IC is the mode I fracture toughness (short-crack), τ Y is the shear strength of the ceramic, and 2c is the length of the pre-existing flaw. This model, when coupled with Hertz’s contact theory, provides the basis for a rational explanation for the differences in the appearance of comminuted regions in different ceramics. It also provides the basis for the derivation of an expression for the dwell/penetration transition velocity that includes the effect of mechanical properties and inference to the effect of microstructure. 1. INTRODUCTION Ceramics are attractive materials for use in armor systems because of their superior hardness and compressive strength values relative to metals. As a result, ceramics have been subjected to a multitude of ballistic and dynamic behavior investigations during the last thirty years[1-22]. One of the more promising findings of these investigations in terms of ceramic armor development has been the phenomenon known as “interface defeat” by Hauver et al.[15-16]. Interface defeat is a term used to describe the complete erosion or dwell of a long-rod projectile on a ceramic with no penetration[15-19]. However, until recently, very little had been achieved in either its application or fundamental understanding[18-19]. A major contribution to the fundamental understanding of interface defeat was recently achieved by Lundberg et al.[20-21]. Lundberg et al.[21] investigated the velocity for the transition from interface defeat to penetration (i.e. dwell/penetration transition) of both W-based and Mo subscale long- rod projectiles fired against several ceramics. For the ceramics tested and independent of the projectile material, they found two curves, an upper and lower bound, for the dwell/penetration transition. Based upon a simple analysis, they developed upper and lower bound predictions for the transition velocities. Their hypothesis was that the lower bound is determined by the critical pressure required to initiate plastic yielding below the surface in an elastic solid. Unfortunately, this is inconsistent with observations from ceramics recovered by Hauver et al.[15-16]. Most armor ceramics that have been successfully recovered exhibited a region of intense microcracking or comminution below the impact area. The shape of this region appears consistent with deviatoric stress distributions produced as a