Short-fatigue-crack growth in a nickel-base superalloy at room and elevated temperature

Healy, Joseph; Grabowski, L.; Beevers, C.J.

doi:10.1016/0142-1123(91)90005-j

Cited by 35 publications

(13 citation statements)

References 5 publications

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“…At room temperature, fatigue cracks have been found to initiate from slip bands [8,9,10,11], twin boundaries [10], grain boundaries [12], at carbides [13], due to cracking of inclusions/precipitates [2], or at other features from materials processing, such as pores [14]. The small fatigue cracks that are formed immediately after initiation tend to exhibit anomalously high, irregular growth rates when compared to large cracks at similar ΔK [2,15,16].…”

Section: Introductionmentioning

confidence: 99%

Effects of microstructure on room temperature fatigue crack initiation and short crack propagation in Udimet 720Li Ni-base superalloy

Pang

Reed

2008

International Journal of Fatigue

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An assessment of the effects of microstructure on room temperature fatigue crack initiation and short crack propagation in a Ni-base superalloy is presented. The assessment was carried out on microstructural variants of U720Li, including as-received U720Li, U720Li-LG (large grain variant) and U720Li-LP (large intragranular coherent γ′ variant). Fatigue tests were carried out at room temperature using a 20Hz sinusoidal cycling waveform on plain bend bars. Tests were conducted in 3-point bend under load control with an R-ratio of 0.1. A maximum load of 95% σ y was used in all tests. Room temperature fatigue crack initiation was noted to occur due to slip band cracking and from porosity on or just beneath the surface in all materials. Crack propagation was noted to be highly faceted (due to planar slip band cracking) immediately after crack initiation followed by a transition to a flatter Stage II type crack path as crack length increases.U720Li-LP was noted to show the longest fatigue lifetime, followed by U720Li-LG while U720Li shows the shortest life. The longer lifetime of U720Li-LP was linked to a higher resistance to both fatigue crack initiation and short crack propagation. U720Li and U720Li-LG show approximately similar crack initiation resistance although U720Li-LG showed slightly improved short crack growth resistance. The observations have been rationalised in terms of the microstructural characteristics of the materials, and it is believed that larger grain size, larger coherent γ′ precipitate size and higher volume fractions of both coherent and primary γ′ precipitates will improve overall fatigue * Now at Dept. of Materials Science & Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK lifetimes in PM Ni-base alloys which exhibit planar slip characteristics at room temperature.

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Section: Introductionmentioning

confidence: 99%

Effects of microstructure on room temperature fatigue crack initiation and short crack propagation in Udimet 720Li Ni-base superalloy

Pang

Reed

2008

International Journal of Fatigue

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“…Partly this is due to the experimental difficulties of observing short cracks at high temperature. For a variety of superalloys, high temperature crack initiation has been observed at slip bands [16,17], inclusions [18], pre-cracked carbide particles [19] and preferentially oxidised carbides [20][21][22][23]. In some superalloys, primary carbides have script-like morphologies, which react to form oxide intrusions from which fatigue cracks can grow.…”

Section: Introductionmentioning

confidence: 99%

Short crack initiation and growth at 600°C in notched specimens of Inconel718

Connolley¹,

Reed²,

Starink³

2003

Materials Science and Engineering: A

113

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The natural initiation and growth of short cracks in Inconel ® 718 U-notch specimens has been studied at 600 o C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred microns in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life. IntroductionNickel-base superalloys like Inconel ® 718 (IN718) are used extensively for turbine discs and other components in industrial and aerospace gas turbines. Since turbine discs are safety-critical components, considerable effort is expended in determining their safe operating life. The lifing procedure adopted must enable the reliable prediction of the safe life of the disc, without being overly conservative. Once engines are in service, regular inspection of the engines and refinements of lifing models may enable life-extension programs to be considered. In the traditional predicted safe life philosophy, the life is declared based on laboratory specimen and component testing. In recent years a damage-tolerant approach to lifing has been adopted, in which it is accepted that components contain defects or inhomogeneities from which cracks can initiate. Assuming an initial flaw size c o , the fatigue life is determined as the number of cycles required to propagate the crack to some critical size c c . The initial size c o can be based on the maximum defect size present in the material, or on the minimum defect size detectable by nondestructive testing (NDT) [1]. Selection of c c is based on knowledge of the fracture toughness of the material, the limit load, the maximum allowable strain or maximum permitted compliance change for a particular component. Damage-tolerant lifing requires accurate information on fatigue crack initiation and growth rates. Fatigue lives are determi...

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“…(e) Waspaloy, data from [36]. (f) Waspaloy, data from [37]. Figure 6: Calculations (uncertainty ranges) for titanium and aluminium alloys compared with measurements (points) due to [38,39].…”

Section: Discussionmentioning

confidence: 99%

“…Calculations for three nickel-base superalloys Udimet 700, Inconel 718 and Waspaloy; their detailed compositions can be found in [33][34][35][36][37]. Fig.…”

Section: Ni Ti and Al Alloysmentioning

confidence: 99%

“…5 compares the model and the experimental data using the inputs listed in Table 2. The calculations are represented with the uncertainty range and the reported measurements [33][34][35][36][37] as points. The results are fascinating since the model correctly estimates the Paris slopes, although it marginally overestimates the fatigue behaviour (we have checked that this overestimation is not explained by modulus variations between the different materials).…”

Section: Ni Ti and Al Alloysmentioning

confidence: 99%

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Fatigue crack growth rate model for metallic alloys

Dimitriu

Bhadeshia

2010

Materials & Design

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A model has been created to allow the quantitative estimation of the fatigue crack growth rate in steels as a function of mechanical properties, test-specimen characteristics, stress-intensity range and test-frequency. With this design, the remarkable result is that the method which is based on steels, can be used without modification, and without any prior fatigue test, to estimate the crack growth rates in nickel, titanium and aluminium alloys. It appears therefore that a large proportion of the differences in the fatigue crack growth rate of metallic alloys can be explained in terms of the macroscopic tensile properties of the material rather than the details of the microstructure and chemical composition.

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Short-fatigue-crack growth in a nickel-base superalloy at room and elevated temperature

Cited by 35 publications

References 5 publications

Effects of microstructure on room temperature fatigue crack initiation and short crack propagation in Udimet 720Li Ni-base superalloy

Effects of microstructure on room temperature fatigue crack initiation and short crack propagation in Udimet 720Li Ni-base superalloy

Short crack initiation and growth at 600°C in notched specimens of Inconel718

Fatigue crack growth rate model for metallic alloys

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