ON THE DYNAMIC SHEAR RESISTANCE OF CERAMIC COMPOSITES AND ITS DEPENDENCE ON APPLIED MULTIAXIAL DEFORMATION

The high strain rate response of an AlN/AlN/Al composite manufactured by Lanxide Armor Products, has been studied by means of normal and pre ssure-shear plate impact experiments. A dramatic reduction in post yie ld shear strength, measured in these experiments, motivated the examin ation of the material response by using a microcracking multiple-plane model and a continuum elasto-viscoplastic constitutive model. Numeric al simulations of the normal impact experiments do not support microcr acking as the dominant inelastic mechanism at the early stages of inel asticity. By contrast, an elasto-viscoplastic description of the mater ial behavior predicts the main features of the normal stress history. Nonetheless, the elasto-viscoplastic model cannot reproduce both the n ormal and the pressure-shear experiments with a single set of model pa rameters. The inadequacy of the continuum elasto-viscoplastic model se ems to result from the isotropic flow assumption embodied in its formu lation. The shear resistance measured in the pressure-shear experiment s is adequately predicted by a multiple-plane model with a pressure an d rate dependent flow mechanism. The agreement seems to hinge on the c ontinuous shearing of the material in a micro-localized fashion;i.e. o nly one orientation becomes dominant and controls the inelastic shear deformation rate. This event does not occur in the normal impact confi guration, in which the amount of inelasticity is primarily controlled by the elastic compressibility of the material. These findings explain the higher sensitivity to damage and microplasticity observed in the pressure-shear testing of ceramics and ceramic composites, as well as the softer material response recorded in this configuration. Although the mechanism used in the formulation of the multiple-plane model is m icrocracking, the implications discussed here are valid for other mech anisms in which the inelastic deformation is pressure dependent. The a ctual inelastic mechanism is still unknown. Therefore, plate impact ex periments specially designed for post-test examination of the specimen s are needed for its proper identification.