THE WEDGE MECHANISM OF FATIGUE CRACK-GROWTH IN SILICON-NITRIDE

It has recently been shown by the authors that the rate of fatigue crack growth in silicon nitride is truly a cyclically controlled process rather than simply a manifestation of static fatigue. However, this cyclical process differs from that observed in metals in that the basic process of fatigue crack growth takes place during the unloading portion of the cycle. In this phase a wedge effect due to crack closure develops, and this wedge effect leads to the formation of microcracks along the crack front. The linking of these microcracks during both the closure portion as well as the rising portion of the load cycle is responsible for crack advance. In the present paper this work is reviewed and extended to include consideration of other recently obtained fatigue crack growth results for silicon nitride.