In this study, a combined low and high cycle fatigue (CCF) life prediction model, which considers the crack closure effect (CCE) of micro-defects, is proposed based on the continuous damage mechanics. The CCF life prediction model is decomposed into three sub-models: the low cycle fatigue (LCF), high cycle fatigue (HCF) under the maximum stress of LCF (HCFLM), and their coupled damage models. The CCE is considered by taking one CCE parameter into the HCFLM sub-model. The experimental CCF data of K403 full-scale turbine blades under different vibration stresses is used to verify the accuracy of the proposed model to compare with other life prediction models. The prediction life from the proposed model falls within the 2 times of scatter band compared with the experimental results. Further, there are the different damage evolution forms at different vibration stresses. When the vibration stress is below 64.48MPa, the CCF damage mainly is caused by the LCF damage. However, while the vibration stress is higher than 64.48MPa, the HCFLM damage plays a major role in the CCF damage accumulation, and it is predicted that the CCF damage of the first stage serration on the K403 turbine blades is mainly from LCF.

The effect of pneumatic shot peening (PSP) on the fatigue properties of K403 turbine blades has been investigated under the high and low cycle combined fatigue (CCF) load with two types of blades: untreated blades and PSP treated blades. It is found that there is a threshold vibration stress which should be 194MPa. The PSP has a positive effect on the CCF life of blades mainly due to the compressive residual stress resulting in the reduction of the number of crack sources and propagation rate when vibration stresses are below the threshold vibration stress. However, the PSP treatment has no or negative effect when vibration stresses are above the threshold value. The compressive residual stress is released along with the microstructure changes of K403. Meanwhile, the microstructure changes, reflected in the precipitation of the lamellar MC carbides and σ phases, can accelerate the process of crack initiation and propagation.