Abstract—There is a critical stress rateKcfor each of the three ZrO2ceramics tested. Fracture is controlled by the materials polycrystal fracture toughness,Kcp, if the stress rate is less thanKc. Otherwise, fracture is controlled by the single crystal fracture toughness,Kcs. The crack growth parameters determined by dynamic fatigue experiments can only represent macrocrack growth behaviour although the fracture of specimens in experiments originates from small surface flaw