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In this paper, the fatigue crack growth characteristics of PSZ ceramics with multiple cracks is described concerning with the number of cracks, crack spacing and crack length. The stress distribution around the crack tips has been analyzed in the case of four point bending test model using the finite element method. It is confirmed that the maximum principal stress value of crack tips decreases with increasing the number of crack and narrowing the crack spacing. Also, the maximum principal stress value of crack tips which lead to fracture is changed by growth of other cracks, which are independent from failure. On the other hand, crack growth rate parameter m, -logC in multiple cracks are calculated from stress intensity factor K, which is analyzed by J-integral around the crack tip. It is found that the estimated life of PSZ ceramics with multiple cracks agrees with the fatigue life which is elucidated from fatigue test.

The main objective of this study is to evaluate the strength reliability of ceramics with machining damages under cyclic loading. The conceptual algorithm for application of linear fracture mechanics is proposed to the strength reliability evaluation of ceramic component. Also the machining damages in ceramic component are converted by the multiple cracks model with same crack length and crack spacing in order to analyze. The stress intensity factor on the ceramics with multiple cracks under the cyclic loading is discussed in connection with the crack spacing and the crack propagation length with no failure. On the other hand, the fatigue crack growth parameters C and m, and the fatigue fracture toughness value Kfc of ceramic component are calculated from the result of a cyclic four point bending test. Further, statistical characteristics of the C, m and Kfc for the ceramics with multiple cracks are estimated from Weibull distribution function in the ceramics with single crack, which was discussed in the previous paper. This analytical method could be applied to predict the life of ceramics with various machining damages. Two examples of the life of PSZ ceramics with laser and grinding damages are indicated using the analytical method proposed in this paper and by the repeated four point bending test, and then the experimental result has agreed with the analytical result.