Abstract: The dependence of K_ic on the notch-root radius was tested with the notch radius as small as a few micrometers in a polycrystalline alumina ceramics. The results show that the fracture toughness decreases as the notch radius decreases, when the notch radius approximately equal, to or lowcr than the average grain diameter, the fracturc tonghness becomes constant, this implies that the critical notch tip radius has the same sizc as the average grain diameter. The fracture toughness of a same alumina was also tested by seven different methods, ie.e. bridge indentation, compressive cyclic fatigue, precrack before sintered and polished sharp notch methods with the SENB specimens; wedge press and precrack before sintered methods with DCB specimens; indentation method with disc. the results show that SENB specimens with polished sharp notch or compressive cyclic fatigue precrack yield the true K_ic value.

Key words: alumina, fracture toughness, sharp notch