Assessment of fatigue crack growth under isothermal and thermomechanical fatigue loading using a time-dependent fracture mechanics approach

The paper presents results from fatigue crack growth tests for two nickel-base superalloys MAR-M247 CC (HIP) and CM-247 LC performed under isothermal and thermomechanical fatigue (TMF) loading conditions. All presented tests were performed under load controlled conditions. Under TMF loading conditions different stress ratios and dwell times were applied. In situ observations of the damage evolution under TMF loading give insights into the complex interaction of different damage mechanisms operating under in-phase and out-of-phase loading. In order to get a unified description of the fatigue crack growth results, a mechanism based model based on the cyclic crack-tip opening displacement ΔCTOD is applied. The model accounts for time dependent effects, fatigue crack closure and inelastic deformations. The ΔCTOD approach shows an improved correlation of fatigue crack growth rates in comparison to the cyclic stress intensity factor, but does not account for the observed creep damage contribution occurring under in-phase TMF loading