DYNAMIC FAILURE MECHANISMS OF CERAMIC BARS - EXPERIMENTS AND NUMERICAL SIMULATIONS

Failure mechanisms in ceramics are investigated by means of bar impact experiments and numerical simulations of the wave propagation event. Stress histories are measured by embedding manganin stress gauges in the ceramic bars. The fracture event is examined by high speed photography. A violent radial expansion, in a region dose to the impact surface, followed by a cloud of debris is observed. Numerical simulations of the inelastic wave propagation event are performed with a multiple-plane microcracking model. The simulations show that when the impact stress exceeds a material threshold, the stress wave in the bar has a relatively short duration which is controlled by the rate of unconfined compressive damage. A nonzero inelastic strain rate at the wave front is required in the simulations to properly capture the measured stress attenuation with propagation distance. This feature is related to a heterogeneous material microstructure which is a common occurrence in ceramics. Furthermore, the simulations predict a radial expansion of the bar as a result of not only compressive but also tensile damage. The radial velocity histories on the bar surface are functions of wave propagation distance and damage rate. Tensile damage is induced by stress release from the rod surface and is restricted to the bar core, due to wave focusing, and to the bar free end. In the latest case, reflection of the compressive pulse produces bar spallation. The two dimensional distribution of tensile and compressive damage is assessed by means of contour plots of volumetric strain and the second invariant of the inelastic strain tenser.