Micro-crack growth behavior and life in high temperature low cycle fatigue of blade root and disc joint for turbines

Low cycle fatigue tests were carried out at a temperature of 600 °C using a component specimen of 12%-Cr steel, which simulates a blade root and disc joint for turbines. The growth behavior of micro-cracks in the joint region of the specimens was investigated to clarify the damage mechanism of blade-root joints used in high temperature environments and to improve life assessment methods using finite element analysis. Micro-crack growth behavior similar to that in smooth bar specimens was observed in the specimens tested under conditions of relatively high total strain. Micro-cracks initiation was observed at the notch region of the specimens at an early stage. The crack growth rate increased with surface crack length. The fatigue life of the component specimens under this condition was similar to that of smooth bar specimens. Meanwhile, the component specimens tested under conditions of relatively low total strain showed a different growth behavior. No cracks were observed at the notch region and some micro-cracks were initiated at the edge of the contact region of the specimens in the early stages. Almost no increase in the crack growth rate was observed. Life of the component specimens under this condition was shorter than that of the smooth bar specimens. This might be attributed to fretting fatigue at the contact edge and to mean stresses.