Influence of Water Vapor on Crack Propagation Behavior of Sintered Silicon Nitride under Cyclic Load : On the Substantial Cyclic Load Effect

Crack propagation tests were carried out in various environments from water to vacuum using compact tension specimens. The main results obtained are as follows: (1) In vacuum, the crack propagation rate under cyclic load is faster than that under static load. Cyclic fatigue can occur without the presence of water vapor. This behavior is substantial cyclic fatigue in this material. (2) The degree of crack propagation rate acceleration increased as partial pressure of water vapor rose. (3) These results are explained by the mechanism in which the origin of the cyclic effect is the decrease in the stress shielding effect by cyclic load. In this respect, the only necessary condition for cyclic fatigue is not stress corrosion cracking due to water vapor but load cycling.