Characterization of creep-fatigue crack growth behavior under trapezoidal waveshape using C t -parameter

Abstract

An experimental study of creep-fatigue crack growth behavior of 1.25Cr-0.5Mo steel at 538°C (1000°F) under trapezoidal loading waveshapes has been carried out on compact type specimens. In creep-fatigue crack growth experiments, hold times ranged from 0 seconds to 24 hours with intermediate conditions of 10 seconds, 98 seconds, 10 minutes and 15 minutes. Time-dependent crack growth rate during the hold period, (da/dt)avg, is correlated with (C t )avg. The values of (C t )avg are estimated using the equation recently proposed for elastic-cyclic plastic-secondary creeping (EL-CPL-SC) materials. The (da/dt)avg vs. (C t )avg data fall on a single trend which matches with the trend of da/dt vs. C t creep crack growth data for the same material. A model is proposed for predicting both the creep crack growth behavior and the creep-fatigue crack growth behavior. The model is suitable for assessing the residual life and/or the safe inspection intervals of high-temperature components such as steam headers. Transition of crack tip damage patterns from oxidation to creep cavitation across the range of hold times examined is also discussed.