窒化ケイ素の疲労き裂成長とき裂閉口に及ぼす破砕粉のくさび効果

抄録

Fatigue crack growth tests of silicon nitride Si₃N₄ were carried out under four point bending load using the square bar specimens of 12×16×40mm at room temperature. A pre-crack was introduced by the bridge indentation (BI) method. K-decreasing tests under cyclic load with the stress ratio R=0.1 and 0.6 and also under static load were first carried out, and after observing the cease of the crack growth, K-increasing tests were performd excepting the case under static load. Crack closure was detected on most specimens by the elastic compliance method, and furthermore, SEM observations of crack paths were made to see what had happned during the crack growth.
The threshold and the region of steady crack growth were observed more clearly under cyclic load, and the effect of load cycling obviously existed, which became more evident when the maximum stress intensity factor K_max approached the threshold. The wedge effect caused by fine fragments left on the crack surface played an important role to the crack closure behavior of each specimen and it was suggested that the crack growth rate was controlled by both of the effective stress intensity range ΔK_eff and the effective mean stress intensity factor K_{m, eff'} at least as a first approximation.