Development of deformation instability during the low cycle fatigue testing of F82H steel in high temperature water.

Load-controlled low cycle fatigue tests were carried out on the reduced activation martensitic steel, F82H modified, in pure oxygen-free water at 240 degreesC, thus simulating the most viable coolant chemistry for fusion reactors. It was found that water determined cyclic life reduction as compared to the base-line data in air. Depending on the mechanical parameters employed, the fracture modes were either of the fatigue type and associated with subcritical crack nucleation and stable propagation assisted by the aqueous environment, or almost completely plastic due to the onset of deformation instability which preceded any environmentally-induced cracking process. The results presented in this paper will be discussed in terms of individual concurrent damaging mechanisms, either time or cycle dependent. Possible causative factors are also suggested for further assessments concerning the cyclic response variability observed in specimens from different lots having, a priori, the same history.