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Effect of oxidation on high temperature fatigue crack initiation and short crack growth in Inconel 718

Record type: Conference or Workshop Item (Paper)

Fatigue tests at 600°C in air were performed on U-notch specimens of wrought IN718, using l-l-l-l and 1-20-l-l trapezoidal cycles. SEM examination and acetate replication was used to study crack initiation and growth. In support of the fatigue tests, thermal exposure experiments were performed on unstressed specimens to study surface and sub-surface primary carbide oxidation. Compared to a 1 second dwell, a dwell time of 20 seconds at maximum load had a beneficial effect on fatigue lifetime. Polishing the U-notch to remove broaching marks also had a beneficial effect on fatigue life. Multi-site crack initiation along the root of the U-notch was observed in fatigue specimens. Many of the cracks initiated at bulge-like features, associated with the oxidation of sub-surface primary carbides. Crack initiation was first observed as early as 12% of the total fatigue life, with further crack initiation occurring as tests progressed. Short cracks in the U-notches grew at a constant rate, except when interactions between parallel cracks resulted in crack arrest or coalescence. Significant crack coalescence occurred towards the end of the fatigue life, producing a dominant defect which propagated rapidly to failure. Surface eruptions and localised surface deformation were observed in fatigue and unstressed thermal exposure specimens, demonstrating a considerable volume expansion when primary (Nb,Ti)C carbides oxidised. It is proposed that the misfit strains due to primary carbide oxidation were superimposed on the plastic strain field in the U-notch due to external loading. This would create local strains high enough to cause rupture of the matrix, hence initiating a fatigue crack.

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Citation

Connolley, T., Starink, M.J. and Reed, P.A.S., (2000) Effect of oxidation on high temperature fatigue crack initiation and short crack growth in Inconel 718 Pollock, T.M., Kissinger, R.D., Bowman, R.R., Green, K.A., McLean, M., Olson, S. and Scbirra, J.J. (eds.) In Proceedings of the 9th International Symposium on Superalloys. The Minerals, Metals & Materials Society (TMS)., pp. 435-444.

More information

Published date: 2000
Venue - Dates: 9th International Symposium on Superalloys (Superalloys 2000), 2000-09-17 - 2000-09-21

Identifiers

Local EPrints ID: 21501
URI: http://eprints.soton.ac.uk/id/eprint/21501
ISBN: 0873395062
PURE UUID: 7c67a38e-833c-4233-826f-4b65c0898511
ORCID for P.A.S. Reed: ORCID iD orcid.org/0000-0002-2258-0347

Catalogue record

Date deposited: 19 Feb 2007
Last modified: 17 Jul 2017 16:26

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Contributors

Author: T. Connolley
Author: M.J. Starink
Author: P.A.S. Reed ORCID iD
Editor: T.M. Pollock
Editor: R.D. Kissinger
Editor: R.R. Bowman
Editor: K.A. Green
Editor: M. McLean
Editor: S. Olson
Editor: J.J. Scbirra

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