Quantitative measurement of persistent slip band profiles and crack initiation

Hunsche, Axel; Neumann, Peter

doi:10.1016/0001-6160(86)90192-6

Cited by 191 publications

(63 citation statements)

References 22 publications

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“…Direct microscopic observations also revealed that the PSB-matrix interface is also a preferential site where fatigue cracks may initiate. It is due to the fact that the strains in PSBs are highly inhomogeneous and localized at the PSB-matrix interface [84,85]. In some cases, fatigue cracks can also start at grain boundaries, twin boundaries, or inclusions which impinge on the PSBs because of the local plastic incompatibility and stress concentration [86][87][88].…”

Section: Crack Initiationmentioning

confidence: 99%

Surface Integrity and Fatigue Performance of Nickel-based Superalloys

Chen¹

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Gas turbines are widely used in e.g., power generation, and aero-industries. Due to global warming, the demand for more efficient gas turbines has increased. A way to achieve this is by increasing the operating temperature of gas turbines. Therefore, nickel-based superalloys have been developed to withstand these extreme temperatures and loads, especially in the hot sections.Today, the way of operating land-based gas turbines is changing. Instead of running for long periods of time, the operation is becoming more flexible, with everincreasing cyclic loads and number of start and stop cycles. To handle the increased stress and cycles, component resistance to fatigue failure needs to be improved.Surface integrity is critical to fatigue performance, since fatigue cracks are normally initiated at surfaces. Nickel-based superalloys are difficult-to-machine materials, primarily due to their high strength, high tendency for work-hardening, and low thermal conductivity. The machining process changes the surface integrity of the alloys which can result in worse fatigue resistance.The work presented in this Ph.D. thesis was conducted in collaboration with Siemens Industrial Turbomachinery AB in Finspång, Sweden. Surface integrity changes which are induced during machining and their effects on fatigue performance have been studied on alloy Inconel 718. Inconel 718 is a widely-used nickel-based superalloy for high temperature applications in modern gas turbines.Broaching, milling, and wire electrical discharge machining, related to component manufacturing in turbo machinery industries, were included in this study. Surface irregularity and defects induced by machining provide preferential sites for fatigue crack initiation which influence the fatigue performance of the alloy. If compressive residual stresses are induced during machining, they benefit the fatigue life by retarding fatigue crack initiation away from surface regions. Shot peening was performed on machined Inconel 718, by which high compressive residual stresses are deliberately induced. It results in increased fatigue performance. The high iv temperatures in gas turbines generally deteriorate the surface integrity. For instance, recrystallization often occurs in the highly plastically-deformed surface layer. Microstructural degradation, in a form of α-Cr precipitates, have also been observed in the deformed surface and sub-surface microstructure when subjected to thermal exposure. Oxidation at elevated temperatures was found to degrade the surface integrity and thereby also the fatigue performance. Fatigue cracks are preferably initiated at oxidized surface carbides, if thermal exposure has been made prior to the test, and a reduced fatigue life is often obtained. It is even worse when high temperatures relax the beneficial compressive residual stresses induced by shotpeening and thereby lowering the fatigue resistance. This increases the interest for research regarding how to optimize the shot peening process in order to enhance the thermal stability o...

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Section: Crack Initiationmentioning

confidence: 99%

Surface Integrity and Fatigue Performance of Nickel-based Superalloys

Chen¹

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“…6,[20][21][22][23] Essmann et al 6) have proposed crack initiation in the grain body of single phase fcc metals occurs inside slip bands. In their proposed micro-mechanism for crack initiation inside a grain body these investigators first considered that persistent slip bands (PSBs) lead to protrusions (intrusions and extrusions) on specimen surface during cyclic loading.…”

Section: Mechanism Of Crack Initiation In the Investigatedmentioning

confidence: 99%

Fatigue Crack Initiation in an Interstitial Free Steel

et al. 2005

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The initiation of small cracks has been studied on dumble-shaped plate type specimens of an interstitial free (IF) steel at the load ratio of Rϭ0 under varied cyclic stress amplitudes between 0.6 and 1.0 of yield stress. Nucleation of cracks were observed at ferrite-ferrite grain boundary (FFGB) as well as inside ferrite grain body (FGB), but the former location was found to be the preferred one. The average length of FFGB cracks is found larger than that of the FGB cracks for identical cyclic loading conditions. The formation of slip bands inside the grain body, slip band impingement at grain boundary and elastic-plastic incompatibility synergistically influence the event of crack initiation in the investigated steel. In addition, nucleation of voids inside slip bands was found to be a new phenomenon associated with the above possibilities to affect the nucleation of cracks in the investigated steel.KEY WORDS: fatigue; crack initiation; small crack; interstitial free steel.

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“…In polycrystalline nickel, the volume fraction of PSBs within a grain and the volume fraction of grains occupied by PSBs, increase with increasing plastic strain range (8) . As a result of repeated loading, the PSB density saturates and localized cyclic plastic straining in PSBs is observed, followed by crack initiation along the PSBs (9,10) . Furthermore, onset of crack growth occurs from the initiated crack along the PSBs without affecting other regions.…”

Section: Introductionmentioning

confidence: 99%

Effect of Plastic Strain Range on Prediction of the Onset of Crack Growth for Low-Cycle Fatigue of SUS316NG Studied using Ultrasonic Back-Reflection

Nurul

Arai

Araki

2010

JMMP

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Strain range controlled low-cycle fatigue tests were conducted using ultrasonic method in order to investigate the effect of plastic strain range on the remaining life of austenitic stainless steel SUS316NG before the onset of crack growth in its early stages of fatigue. It was found that the decrease in ultrasonic back-reflection intensity from the surface of the material, caused by the increase in average dislocation density with localized plastic deformation at persistent slip bands (PSBs), starts earlier with increase in the plastic strain range. The amount of decrease in ultrasonic back-reflection before the onset of crack growth increases for larger plastic strain range. The difference in the cumulative plastic strains at the onset of crack growth and at the onset of decrease in the ultrasonic back-reflection remained constant over the range of tested plastic strain. This result can be used to predict the remaining life before the onset of crack growth within the plastic strain range used in this study. In addition, we present and evaluate another method to predict damage evolution involving ultrasound attenuation caused by PSBs.

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Quantitative measurement of persistent slip band profiles and crack initiation

Cited by 191 publications

References 22 publications

Surface Integrity and Fatigue Performance of Nickel-based Superalloys

Surface Integrity and Fatigue Performance of Nickel-based Superalloys

Fatigue Crack Initiation in an Interstitial Free Steel

Effect of Plastic Strain Range on Prediction of the Onset of Crack Growth for Low-Cycle Fatigue of SUS316NG Studied using Ultrasonic Back-Reflection

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