| Aero-engine turbine disk worked in elevated temperature, high pressure and heavyload, so it calls for very strict requirement of material. Nickel-base superalloy GH4133Bis the first choice of material used in aero-engine turbine disk for its high strength, wellpermanence and well fatigue properties. Turbine disk failure mode mainly shows fatiguefracture. Most of turbine disk fatigue life cost in fatigue crack initiation and short crackpropagation in the service process. Hence the research on fatigue crack intiation andshort crack propagation is the key and basis of turbine disk damage tolerance design.In this paper, GH4133B superalloy using in turbine disc of aero-egine is selected asthe research subject, the fatigue crack initiation and crack growth tests under diffrerntstress ratio are carried out in air at room temperature, the microscopic morphologies onthe fracture surface of specimen are analyzed using a scanning electron microscopy, andthe mechanism of fatigue crack initiation, short crack propagation as well as transitionfrom short to long crack behavior is explored. The results are given as following.1. The fatigue crack intiation and short crack propagation law on the specimensurface of GH4133B superalloy at various stress ratii are investigated using OLYMPUSBX51M optical microscope. The results show that fatigue cracks initiate in the PSBs onthe edge of notch, and short cracks propagate along the PSBs in early period. Crackbranching and micro-crack convergence appear during the period of the short crackpropagation, which leads to the great scatter of test data, and short crack rate shows awave form of acceleration and deceleration. For smooth circular notched CT specimens,fatigue life is mainly spent in the regime of crack initiation and short crack propagationduring service, while long crack propagation life take a small portion of total fatigue life.2. Chosing effective stress intensity factor rangeΔ Keffas a control parameter todescribe short fatigue crack propagation behavior, a short crack propagation equation isconstructed, and the experimental data obtained from the short fatigue crack propagationtests under different stress ratio are analyzed using a nonlinear regression procedure. It isshown that the short fatigue crack propagation equation characterized byΔ Keff, can welldescribe short fatigue crack behavior that firstly decelerates and then accelerates withincreasing fatigue cycles at the stage of short fatigue crack propagation in the GH4133Bsuperalloy. The behavior of short fatigue crack mostly happens at low stress ratio, andshort crack growth characterization becomes weak with increasing stress ratio. Under the condition of constant stress ratio, the crack closure effect decreases with increasingmaximum load.3. Adopting an approach of quasi-harmonic function analysis, a crack propagationequation at the stage of transition from short to long crack is constructed. Using thequasi-harmonic crack propagation equation, a series of nonlinear regression analysis forthe experimental data obtained from the fatigue crack propagation tests under differentstress ratio are carried out. The results indicate that the theoretical curves consistent withthe test data, and the constructed quasi-harmonic crack propagation equations can welldescribe the behavior that crack growth rate accelerates and decelerates in a wave formas the fatigue cycle increases at the transition from short to long crack. Comparing withshort crack growth character, the crack growth rate fluctuates with small amplitude atthe transition from short to long crack. At a constant stress ratio, as the maximum loadincreases, the sensitivity of crack growth rate on crack tip closure effect and materialmicrostructure decreases at the transition from short to long crack, GH4133B Superalloyshows higher crack growth rate, and the rate fluctuates with small amplitude.4. The fatigue fracture surface of circular notched CT specimen of GH4133Bsuperalloy with fatigue crack initiation and short fatigue crack test at various stress ratiiare analyzed using a JSM-6360scanning electron microscopy, and the fractographicmorphologies at the fatigue source region, short crack growth region and transition zonefrom short to long crack. The results suggest that the fatigue crack mainly initiates alongthe carbide inclusions or nickel-rich two-phase inclusions. In the short crack growthregion, the early fatigue striations on cleavage steps and the second cracks near cleavagesteps can slow down short fatigue crack growth rate in a certain degree. At the stage oftransition from short to long crack, the microstruture and crack closure effect at crack tiphave significant influence on crack propagation, the local plastic slip deformation at thecrack tip subjects to great resistance, so a few of twin bands can be observed on thefracture surface. The short crack propagation law in GH4133B superalloy that shortcrack propagation rate is higher at the stage of early crack growth, and then it decreaseswith increasing fatigue cycles is illustrated in the view of microscopic morphology, andthe mechanism of fatigue crack growth in a wave form of acceleration and decelerationwith a lower crack growth rate at the transition from short to long crack is explained. |
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