Degradation of Strength by Cyclic Loading and its Mechanism in Fracture of Ceramic Materials

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Slow crack growth under the constant stress intensity factor was investigated for several ceramic materials with the particular attention of the effect of cyclic loading. The crack growth rate in silicon nitrides and alumina ceramics decreases with the crack extension and finally the crack extensions are arrested. On the otherhand, the Borosilicate glass doesn't show such behavior. Under constant DELTAKAPPA cyclic loading, the cracks in the ceramics continue to grow without being arrested, though some decrease in growth rate is observed in the initial stage of crack extension. The delay of crack growth and arrest were more significantly observed in coarse grained silicon nitride and alumina. From the observation of crack opening and closure in the SEM, it was shown that the bridging effect resulted the delay and arrest of the crack growth when the crack passed through the grain boundary at the coarse grain. The bridging effect due to the coarse grain depends on the length of crack extension. The fact above indicates that the relation between the crack growth rate and KAPPA in ceramics (so called the KAPPA-v relation) depends on the crack length and can not be considered to be the materials own properties.In this work, rolling contact fatigue of ceramics was also invetigated. Typical S-N diagrams were obtained for the silicon nitride and alumina ceramics. Fractographic and metallographic examinations showed the crack initiation sites located inside under the contact surface at which the shear stress amplitude took maximum value. The silicon nitride didn't show the cyclic fatigue behavior under the tension-compression loading. It indicates the possibility of the cyclic fatigue under the compressible shear stress state in brittle ceramic materials.