THE EXPERIMENTAL-VERIFICATION OF THE SHIELDING EFFECT IN ZIRCONIA

The toughening mechanism of CeO2-stabilized ZrO2 (CeO2-ZrO2) was discu ssed on the basis of the observation of resistance R curve and the str ess intensity factor K-tip at the crack tip. A compression splitting t est method, compressing the top of the double cantilever beam specimen , was applied to achieve slow crack growth of CeO2-ZrO2. The laser cau stic method was also applied to observe K-tip. The results showed that K infinity was increased with increasing crack extension (R curve beh aviour). Crack extension started around K infinity of 3 MPa m(1/2), an d K infinity increased to 7 MPa m(1/2), where K infinity became satura ted. On the contrary, K-tip exhibited a constant value in the range 2 similar to 3 MPa m(1/2) during the crack extension. From observed K in finity and K-tip, the contribution Delta K of transformation toughenin g was examined on the basis of the relation K infinity = K-tip + Delta K With these results, the shielding effect, which is the relaxation m echanism of stress concentration at the crack tip, was experimental ve rified.