DETERMINATION OF DYNAMIC FRACTURE TOUGHNESS FOR BRITTLE MATERIALS WITH A MODIFIED SHPB

The experimental procedure for determining the fracture toughness for advanced ceramic materials under quasi-static loading conditions has been well established by an ASTM standard (C 1421-99). The development of experimental methods for determining the dynamic fracture toughness for such materials faces two main challenges in experiment design that must be overcome before valid results can be obtained. Dynamic equilibrium over the entire specimen needs to be approximately achieved. This is necessary to interpret the crack tip loading state with the far-field loading conditions using relations derived under quasi-static conditions. Furthermore, the loading rate at the crack tip should be nearly constant during an experiment. This is a required condition in order to determine the loading-rate effects on the dynamic fracture toughness. A new experimental technique, based on a split Hopkinson pressure bar (SHPB), has been developed to determine the valid dynamic fracture toughness for brittle materials. A precise control of the loading pulse profile facilitates the dynamic equilibrium in the specimen and a nearly constant loading rate at the crack tip. Sensitive piezoelectric force transducers are used to monitor the dynamic equilibrium and constant loading rate on the precracked specimen. The feasibility of the new technique is demonstrated through the determination of the dynamic fracture toughness as a function of loading rate for a model brittle material