Abstract
Rotary bending fatigue and static fatigue tests were carried out for silicon nitride and silicon carbide ceramics at room temperature, with determining the fracture toughness values by SEVNB and SEPB methods. Two of the silicon nitride ceramics exhibit the great difference between the toughness values obtained from two methods, suggesting that there is a strong traction force in the propagating crack interface. From the observation of crack extension path, mechanisms of grain bridging and crack surface interlocking are concluded to be responsible for the strong traction. Materials, having a much greater SEPB toughness than that of SEVNB, exhibited considerable degradation in fatigue strength due to cyclic loads during rotary bending tests. This suggests that the damage in the crack surface causes degradation in fatigue strength. Analysis, assuming that cyclic loads affect the R-curve behavior, was consistent with the measured rotary bending fatigue behavior.
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