Dynamic Compression Strength and Hardness of Ceramics

The intrinsic compression strength of ceramics can be difficult to determine. The specimen geometry and test fixture, if not properly designed, can result in the generation of undesirable tensile stresses that can lead to misleadingly low strength values. On the other hand, the quasi-static hardness of ceramics is straightforward to measure but has received significantly less attention under dynamic loading conditions. Since compression strength, for which hardness is a proxy, is a parameter in numerous modeling and simulation packages used to predict impact performance, properly measuring it is critical. This presentation focuses on compression strength experiments using a dumbbell-shaped specimen and scaled-down dynamic hardness experiments, where the split-Hopkinson pressure bar (SHPB) is utilized to apply the dynamic loading. The dumbbell-shaped specimen was designed to increase the likelihood of fracture initiating in the gage section, which is confirmed via high speed imaging, and we will present results on a variety of ceramics under both quasi-static and dynamic strain rates. The recently developed miniature dynamic hardness technique, utilizes laser interferometry to measure forces and displacements and cleverly exploits wave reverberations in the incident and transmitted bars to achieve a single indent over a several microsecond loading time, will be presented. Knoop hardness measurements on a variety of ceramics will be presented under quasi-static and dynamic loading conditions. We show both of these techniques have the fidelity to quantify strain rate effects that are generally not possible with typical approaches.