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 techniques. The indentation-strength response is shown to be sigmoidal with concave-down behavior at small indentation loads and concave-up behavior 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 cracking at large indentation loads. The model is fit to the strength data and used to deconvolute the underlying toughness variation and predict the intrinsic strength of the materials. Direct measurements of toughness using the ''long-crack'' double cantilever beam geometry are shown to overestimate the toughness variations effective during ''short-crack'' strength tests.