Evaluation of Crack Initiation and Propagation Lives from Internal Defects and Lower Limit of Failure Life in High Temperature Fatigue on Ni-Based Single Crystal Superalloy
Masato YAMAMOTO, Akito NITTA and Takashi OGATA
Abstract:High temperature fatigue life assessment is one of the important issues in Ni-based superalloys for a gas turbine blade. Generally speaking, however, a superalloy with higher strength has larger scatter in fatigue life at a given cyclic loading condition in comparison with conventional steels and alloys. In this study, focusing on the behaviors of fatigue crack initiation and propagation at the internal defects of eutectic gĠ phase in a Ni-based single crystal superalloy (MDSC-7M), a cause of the scatter in the fatigue life of the superalloy was discussed by conducting a series of fatigue tests at 1173K. In particular, the propagation of fatigue cracks initiating at the internal defects was characterized quantitatively by means of the beach-mark method. As a result, the fatigue crack propagation rate could be correlated well with the stress intensity factor range, including a small crack of 100-150mm. It was also found that the crack propagation life (I.e., the number of cycles to failure after crack initiation) was more than 40% of the total life (I.e., the number of cycles to failure). The scatter in the fatigue life of the material tested seemed to be mainly caused by the scatter in the crack initiation life. Finally, the lower limit of the scatter could be successfully predicted with the crack growth simulation conducted for the maximum eutectic gĠ phase which should be in a fatigue test specimen. Key Words:Single crystal superalloy, High temperature fatigue, Crack initiation, Crack propagation, Internal defect, Life prediction