常圧焼結窒化ケイ素の回転曲げ及び静疲労におけるミラー状領域の挙動

概要

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Rotary bending and static fatigue tests were carried out on sintered Si_3N_4 at room temperature to study the effect of loading conditions on the fatigue behavior. It is confirmed that 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. The assumed fatigue limit under rotary bending was also lower than that under static fatigue. However, the characters of mirror-like regions observed on the fractured surfaces after both fatigue tests were almost the same as those after the static 'test. If the fracture initiation part was near the specimen surface, the mirror-like region was roughly semicircular, and the fracture propagated radially from the region. The size of each mirror-Like region, determined from optical micrographs at a Magnification of 25, was correlated with the applied stress regardless of the loading condition. Stress intensity factor at the deepest point in the Mirror-like region was also almost constant. It is considered that the fatigue failure occurred when the size of mirror-like region reached a critical value which was dependent on the magnitude of applied stress, and that the difference of fatigue behavior under rotary bending and static fatigue was due to the subcritical crack growth behavior in the mirror-like region.
1991-07-01