Estimation of crack growth in the creep range during plastic cyclic loading

Crack growth investigations were performed on the creep-resistant steel 13 CrMo 4 4 in the fatigue and the creep fatigue regime, especially regarding the influence of creep damage on crack growth. To this effect, 2% creep strain was applied to the material at a temperature of 560deg C. The crack propagation rate was determined as a functionof the specimen shape, temperature, test frequency and hold times. In the case of compact tension (CT-)specimens, creep pretreatment does not affect crack growth. For center-cracked tension (CCT-)specimens, however, the creep pretreatment results in a considerable increase in the crack propagation rate. Hold times of 90 minutes at maximum loading cause an increase in da/dN due to further cavity nucleation. The hold time at which cavity nucleation might occur is evaluated. The dependency on frequency of crack growth may be evaluated by means of a linear superposition of creep and fatigue crack growth. The transition frequency above which pure fatigue crack growth occurs is calculated and the regimes of fatigue, creep and creep-fatigue interaction with environmental influences are characterized. (orig.)