Issue 24

A. A. Shanyavskiy, Frattura ed Integrità Strutturale, 24 (2013) 13-25 ; DOI: 10.3221/IGF-ESIS.24.03 13 Special Issue: Russian Fracture Mechanics School Fatigue crack propagation in turbine disks of EI698 superalloy A.A. Shanyavskiy State Centre for Civil Aviation Flights Safety, Airport Sheremetievo-1, PO Box 54, Moscow region, Chimkinskiy State, 141426, Russia shananta@mailfrom.ru A BSTRACT . 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 in- tested disks under stress equivalent program to in-service operated cyclic loads. Based on this result non- destructive 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. K EYWORDS . Nickel-based superalloy; Crack initiation; Crystallographic facet; Fatigue striations; Crack growth period; Stress equivalent; Non-destructive inspection. I NTRODUCTION ircraft structures in-service fatigue cracking can be appeared under wide range of cyclic loads combinations [1-4]. Because of difference in structures loading conditions from one flight to another it can be effective to use material reaction for describing damage in-time accumulation in each of them [5]. This reaction on the external loading can be considered as material property pre-venting crack occurring and growth. Discussed property has not only mechanical but physical sense, and in the case of aircraft structure fatigue cracking, when Mode I crack opening is dominant, the discussed process can be described, for instance, applicably to durability, based on bifurcation diagram [6,7], Fig.1. The bifurcation diagram allows describing fatigued metals behavior based on uniform synergetical methodological principle applicably to systems which evolution occurs far from the equilibrium position. 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 consequently 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). Because of complicated external cyclic loading and environment deterioration, material re-action there has to be considered based on the stress equivalent value which has difference in dependence on a structure and its stress-state [8]. In the case of through or semi-elliptically-shaped cracks, which usually takes place on the first stage of structure cracking one can see simply equation for principle stresses 1  , 2  , their ratio 2 1 /     , and value of e  in the form [9]: A

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