Ratchetting strain as a damage parameter in controlling crack growth at elevated temperature

Progressive increase in tensile strains near a crack tip has been observed from finite element studies of stationary and growing cracks (Zhao, 2004, 2008) [1,2] under cyclic loading conditions. In this work, the significance of such a phenomenon was further explored. In particular, stress-controlled experiments were carried out to evaluate the uniaxial ratchetting response of a nickel-based superalloy, and the material parameters were re-calibrated using both strain-controlled and stress-controlled experimental data. An additional kinematic hardening term was introduced in the viscoplastic constitutive model and the models were utilised via a user-defined subroutine to study near crack tip ratchetting behaviour of a single edge notch tension (SENT) model geometry at elevated temperature. Loading modes near the crack tip were examined, together with the influence of particular constitutive models on the mechanistic response of the crack tip. The crack tip deformation was found to be predominantly strain-controlled, where the mean ratchetting strain seems to be more relevant to crack growth than the strain range. The former was used as a measure of crack tip damage to correlate crack growth rates at selected loading conditions.