Abstract
The rotary bending test was carried out on sintered Si3N4 at room temperature in the range from 104 to 109 stress-cycles with an Ono's rotary bending fatigue testing machine operating at 3400 cycles per minute. Mirror-like regions were observed on the fracture surfaces after the test. The mirror-like region was roughly semicircular, and the fracture propagated radially from the mirror-like region. The size of mirror-like region was determined from optical micrographs at a magnification of 25. The size of mirror-like region was correlated with the stress amplitude and the number of cycles to failure. The failure was shown to occur when stress intensity factor at the deepest point of the mirror-like region reached fracture toughness of the material. It was considered that the mirror-like region was restricted to the subcritical crack growth under the cyclic stress, and the failure occurred when its size reached a critical value which is dependent on the stress amplitude.
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