Creep-fatigue life prediction of notched structure after an advanced surface strengthening treatment in a nickel-based superalloy at 650C

The elucidation of creep-fatigue damage mechanisms is still controversial for high-temperature structures after surface strengthening treatments, which serves as a critical foundation for the development of an accurate life prediction method. In this work, a numerical procedure is constructed for the prediction of creep-fatigue life improvement, where a dual-scale modeling approach is proposed to integrate important strengthening factors and microstructure features. The macro-scale finite element (FE) simulation aims to investigate the cyclic deformation behavior in a notched structure by using a viscoplastic constitutive model. The initial stress field is predetermined based on the experimental residual stress. The micro-scale FE analysis is employed to investigate the local damage evolution occurring at the notched root by combining size-dependent crystal plasticity with grain boundary cavity model. The cycle-by-cycle deformation histories are extracted from the macro-scale FE model and subsequently are utilized as boundary conditions in the micro-scale FE one. From the experimental perspective, the submerged micro-abrasive waterjet peening (SMA-WJP) process is carried out for creep-fatigue life improvement of the notched structure. Results shows that the notched structure treated by the SMA-WJP process forms an obvious plastic layer with the depth of 20 mu m and residual stress with the maximum value of -926 MPa. The predicted numbers of cycles to crack initiation agree with the creep-fatigue experimental ones before and after SMA-WJP. In detail, the surface residual stress and plastic layer are unable to suppress the cavity nucleation on the grain boundaries of internal material. As a consequence, the creep-fatigue life improvement is diminished as the hold time increases, which can be accurately predicted by the developed numerical procedure.