Fatigue under mixed mode loading of vibratory stress and start-stop cycling

The combination of centrifugal load and vibratory load acts on a component of turbo machinery, for example, a moving blade in a turbine. When the machine is used in frequent start-up and shut-down operation, the mixed mode fatigue evaluation should be done for such a component. The mixed mode loading is composed of the low cycle fatigue (LCF) due to the repetition of start-up and shut-down process and the high cycle fatigue (HCF) due to vibration. In a mixed mode loading, a short crack is generated by the LCF and it grows to some extent in LCF manner. At a certain crack depth, the condition for HCF crack propagation is satisfied and the transition from LCF to HCF crack propagation occurs. After the transition, the crack propagates rapidly in HCF crack propagation mode. The evaluation of the transition point has been usually done using the fracture mechanics on the basis of (ΔK_eff)_th,x which is the effective threshold stress intensity factor range of a long crack at an extremely high stress ratio condition. However, it has been shown that the (ΔK_eff)_th becomes lower than (ΔK _eff)_th in short crack region of hard material, which suggests that the transition into HCF crack propagation occurs at a shorter crack depth than that predicted on the basis of (ΔK_eff) _th,∞. The test results showed that the transition occurred at about 1/2-1/3 of the predicted life. The consideration of short crack effect on the (ΔK_eff)_th is important, especially in the case of mixed mode loading of turbo machinery.