Crack propagation tests were carried out in various environments from water to vacuum using CT specimens. The main results obtained are as follows. (1) In vacuum, cyclic load accelerated crack propagation rate faster as compared with that under static load. Cyclic fatigue can occur without the presence of vapor. This behavior is substantial cyclic fatigue in this material. (2) Degree of the acceleration in creased as partial pressure of water vapor rose. (3) These results are explained successfully by the mechanism in which the origin of 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.
