The effect of ratcheting deformation and pre-strain on fatigue and creep-fatigue life in Mod.9Cr-1Mo steel was investigated. Uni-axial fatigue and creep-fatigue tests with pre-strain and progressive strain were performed to evaluate the effect of pre-strain and ratcheting strain on the failure cycle. In a series of tests, a specific amount of progressive strain was accumulated in each cycle. The accumulated strain as ratcheting deformation and cycles to reach the accumulated strain were varied in the tests. In the fatigue tests with pre-strain, failure lives were not declined. In the fatigue tests with progressive strain, slight reductions of failure lives were observed, however, they were within a factor of 1.5 of the failure life in normal fatigue test. The apparent relationship between failure cycles and testing parameters were not observed. Analyses of the test results indicated that the effect of ratcheting strain on the fatigue failure life is negligible as long as the parameters are within the strain limit of FR code (strains at surface less than 2%). In the creep-fatigue tests with progressive strain, the same conclusion of the fatigue tests was obtained. In both kinds of tests, maximum mean stresses during the tests were insignificant and/or generated in early cycle in the tests, and this character is considered as a reason of that the effect of ratcheting deformation on the fatigue and creep-fatigue lives are insignificant.
