SIGMOIDAL INDENTATION-STRENGTH CHARACTERISTICS OF POLYCRYSTALLINE ALUMINA

The fracture properties of a series of three polycrystalline aluminas are examined using indentation-strength and double cantilever beam tec hniques. The indentation-strength response is shown to be sigmoidal wi th concave-down behavior at small indentation loads and concave-up beh avior at large indentation loads. A model is developed for the general response, combining increasing toughening by ligamentary bridging at small crack lengths and increasing residual-stress relief by lateral c racking at large indentation loads. The model is fit to the strength d ata and used to deconvolute the underlying toughness variation and pre dict the intrinsic strength of the materials. Direct measurements of t oughness using the ''long-crack'' double cantilever beam geometry are shown to overestimate the toughness variations effective during ''shor t-crack'' strength tests.