Fatigue Behavior of Sintered Si3N4 under Rotary Bending and Static Fatigue

Abstract

Rotary bending and static fatigue tests were carried out at room temperature on sintered Si3N4 whose diameters were 8 and 6 mm. An Ono’s rotary bending fatigue testing machine was used on the operating condition of 3420 cycles per minute. The rotary bending test was performed within the range 104 to 109 stress-cycles. Besides, the static fatigue test was performed for less than 107 seconds using the non-rotating fatigue machine. The fatigue behavior under rotary bending was different from that under static fatigue. The value of n under rotary bending was much smaller than that under static fatigue, in the expressions of σn N = constant and σn t = constant, and the assumed fatigue limit for the former was lower than that for the latter. However, both assumed fatigue limits showed the similar relationships between the strengths and the specimen sizes. The characteristic appearance of mirror-like regions on the fractured surfaces after both fatigue tests were almost the same as those after the static test. The size of each mirror-like region was found to have stronger correlation with the applied stress than with the loading condition. Stress intensity factor at the deepest point in the mirror-like region was also nearly constant. No remarkable difference between fatigue fractured morphology and static fractured one could be found even under microscopic observation. The existence of the fatigue limit, the basic character of rotary bending strength, the size effect on the assumed fatigue limit and the fatigue fracture features are discussed on the basis of the results.