Fatigue under Mixed Mode Loading of Vibratory Stress and Start-Stop Cycling
Yoshiyuki KONDO, Chu SAKAE, Masanobu KUBOTA and Hiroki KITAHARA
Abstract: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) cycling due to the repetition of start-up and shut-down process and the high cycle fatigue (HCF) stress 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 (ƒ¢Keff)th,‡, 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 (ƒ¢Keff)th becomes lower than (ƒ¢Keff)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 (ƒ¢Keff)th,‡. The test results showed that the transition occurred at about 1/2-1/3 of the predicted crack depth. The consideration of short crack effect on the (ƒ¢Keff)th is important, especially in case of mixed mode loading of turbo machinery. Key Words:Fatigue, Fatigue crack propagation, Mixed mode loading, High cycle fatigue, Low cycle fatigue, Short crack, Threshold stress intensity factor