Revealing the influences of strain amplitudes on hybrid stress–strain controlled creep-fatigue interaction responses for 9 %Cr steel

The hybrid stress–strain controlled creep-fatigue interaction (HCFI) tests were performed on 9 %Cr steel at strain amplitudes ranging from ± 0.25% to ± 0.6%. The strain amplitude increase brings more possibilities of cracking but fewer crack deflections, along with some differences in LABs and grain size. But for HCFI tests with high creep damage, strain amplitude plays a minor role since considerable creep strain during stress holding is converted into fatigue strain even at low strain amplitudes. Additionally, a dwell indicator (DI) benchmarked against macroscopic LCF responses is proposed to evaluate strain amplitude-induced microscopic HCFI damage. When DI is approximately equal to (or greater than) 0.9, the differences in strain amplitudes can bring significant changes in internal degradation and fracture behaviors. When DI is close to (or lower than) 0.5, HCFI damage exhibits low sensitivity to strain amplitude.