On the dynamic shear resistance of ceramic composites and its dependence on applied multiaxial deformation

The high strain rate response of an AIN/AIN/AI composite manufactured by Lanxide Armor Products, has been studied by means of normal and pressure-shear plate impact experiments. A dramatic reduction in post yield shear strength, measured in these experiments, motivated the examination of the material response by using a microcracking multiple-plane model and a continuum elasto-viscoplastic constitutive model. Numerical simulations of the normal impact experiments do not support microcracking as the dominant inelastic mechanism at the early stages of inelasticity. By contrast, an elasto-viscoplastic description of the material behavior predicts the main features of the normal stress history. Nonetheless, the elasto-viscoplastic model cannot reproduce both the normal and the pressure-shear experiments with a single set of model parameters. The inadequacy of the continuum elasto-viscoplastic model seems to result from the isotropic flow assumption embodied in its formulation. The shear resistance measured in the pressure-shear experiments is adequately predicted by a multiple-plane model with a pressure and rate dependent flow mechanism. The agreement seems to hinge on the continuous shearing of the material in a micro-localized fashion; i.e. only one orientation becomes dominant and controls the inelastic shear deformation rate. This event does not occur in the normal impact configuration, 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 microcracking, the implications discussed here are valid for other mechanisms in which the inelastic deformation is pressure dependent. The actual inelastic mechanism is still unknown. Therefore, plate impact experiments specially designed for post-test examination of the specimens are needed for its proper identification.