Abstract
In-service fatigue cracking of turbine disks of EI698 superalloy is discussed based on crack growth analyses. In the bolt joint for disks to shaft connecting there is high level of stress-state, which directed to earlier in-disks fatigue crack origination in low-cycle-fatigue regime. Fracture surface pattern such as fatigue striations were used for their spacing measurement and crack growth duration estimating. Developed disk tests on a special bench by the equivalent program to in-service cyclic loads have allowed discovering one-to-one correlation between fatigue striation spacing and crack increment in one flight. Number of fatigue striations and beach-marks calculations permitted to estimate crack growth period for the different stages of in-service disks cracking. Equivalent stress level for in-service cracked disks was calculated and compared with stress-level intested disks under stress equivalent program to in-service operated cyclic loads. Based on this result nondestructive inspection intervals were discussed and recommended for in-service disks in dependence on number of their flights at the moment of developed inspection to exclude in-flight disks fast fracture.
Highlights
A ircraft structures in-service fatigue cracking can be appeared under wide range of cyclic loads combinations [1,2,3,4]
The two remaining fractures were of a mixed type with the signs of macroscopic plastic deformation all over the fracture boundaries; such a fracture pattern is typical of superalloys subjected to short-term static loading
Expecting fatigue striations to form during a start-and-shut-down cycle of the engine, a number of the fatigue striations must be characteristic of the growth period of a fatigue crack, measured as the number of flights of an aircraft
Summary
A ircraft structures in-service fatigue cracking can be appeared under wide range of cyclic loads combinations [1,2,3,4]. Stated the synergetics concept allows to connect among themselves all experimentally demonstrated data on research of metals fatigue at different scale levels and to explain increase and decrease of dispersion of fatigue durability in process of increase of cyclic stress level at achievement of critical stress levels. Such manner of metal fatigue behavior consideration is lawful, even if in process of evolution metal undergoes only one unstable condition and has only one bifurcation area between two boundary conditions when it is not loaded (one border) and when it is completely failed (the second border). In the case of through or semi-elliptically-shaped cracks, which usually takes place on the first stage of structure cracking one can see equation for principle stresses 1 , 2 , their ratio 2 / 1 , and value of e in the form [9]:
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