Fatigue crack propagation in turbine disks of EI698 superalloy
In-service fatigue cracking of turbine disks of EI698 superalloy is discussed based on crack growthanalyses. In the bolt joint for disks to shaft connecting there is high level of stress-state, which directed to earlierin-disks fatigue crack origination in low-cycle-fatigue regime. Fracture surface pattern such as fatigue striationswere used for their spacing measurement and crack growth duration estimating. Developed disk tests on aspecial bench by the equivalent program to in-service cyclic loads have allowed discovering one-to-onecorrelation between fatigue striation spacing and crack increment in one flight. Number of fatigue striations andbeach-marks calculations permitted to estimate crack growth period for the different stages of in-service diskscracking. Equivalent stress level for in-service cracked disks was calculated and compared with stress-level intesteddisks under stress equivalent program to in-service operated cyclic loads. Based on this result nondestructiveinspection intervals were discussed and recommended for in-service disks in dependence on numberof their flights at the moment of developed inspection to exclude in-flight disks fast fracture.