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Characterisation of strain localisation processes during fatigue crack initiation and early crack propagation by SEM-DIC in an advanced disc alloy

Characterisation of strain localisation processes during fatigue crack initiation and early crack propagation by SEM-DIC in an advanced disc alloy
Characterisation of strain localisation processes during fatigue crack initiation and early crack propagation by SEM-DIC in an advanced disc alloy
Fatigue failure processes in metallic materials are closely related to the evolution of strain localisation under cyclic loading. Characterisation of this strain localisation is important in understanding the mechanisms of fatigue crack initiation and propagation, and provides critical validation data to develop appropriate crystal plasticity models for prediction of these processes. In this study, strain localisation during fatigue crack initiation and early crack propagation in an advanced Ni-based superalloy for turbine disc application has been characterised at the grain level with a sub-micron resolution by digital image correlation on SEM images using secondary γ′ themselves as the speckle pattern. The obtained full-field strains have been analysed in global coordinates associated with the applied loading direction and in terms of the local coordinates associated with individual slip bands. Deformation arising from in-plane and out-of-plane dislocation slip can be identified by a combination of shear strain ɛxy and transverse strain ɛyy in the local slip band coordinates in combination with EBSD analysis. Cracks preferentially initiate from slip/strain bands adjacent and parallel to twin boundaries and then propagate along the slip/strain bands, leading to the onset of significant transverse strain ɛyy in the local band coordinates as a consequence of crack opening. Crack propagation is closely related to strain accumulation at the crack tip which is determined by the grain orientation and grain size. Transverse strain ɛyy in local slip band coordinates together with the inclination angle between dislocation slip direction on an activated {111} plane and the slip trace of this {111} plane at the specimen surface is proposed to be a cracking indicator/fracture criterion.
Ni-based superalloys, Strain localisation, Digital image correlation, Fatigue cracking, Transverse strain
128-144
Jiang, Rong
1e3a7913-c354-4ecb-879c-526cb6fe5fcd
Pierron, Fabrice
a1fb4a70-6f34-4625-bc23-fcb6996b79b4
Octaviani, Sari
1ac4dc6b-f783-4060-8792-136537c6f7f3
Reed, Philippa
8b79d87f-3288-4167-bcfc-c1de4b93ce17
Jiang, Rong
1e3a7913-c354-4ecb-879c-526cb6fe5fcd
Pierron, Fabrice
a1fb4a70-6f34-4625-bc23-fcb6996b79b4
Octaviani, Sari
1ac4dc6b-f783-4060-8792-136537c6f7f3
Reed, Philippa
8b79d87f-3288-4167-bcfc-c1de4b93ce17

Jiang, Rong, Pierron, Fabrice, Octaviani, Sari and Reed, Philippa (2017) Characterisation of strain localisation processes during fatigue crack initiation and early crack propagation by SEM-DIC in an advanced disc alloy. Materials Science & Engineering: A, 699, 128-144. (doi:10.1016/j.msea.2017.05.091).

Record type: Article

Abstract

Fatigue failure processes in metallic materials are closely related to the evolution of strain localisation under cyclic loading. Characterisation of this strain localisation is important in understanding the mechanisms of fatigue crack initiation and propagation, and provides critical validation data to develop appropriate crystal plasticity models for prediction of these processes. In this study, strain localisation during fatigue crack initiation and early crack propagation in an advanced Ni-based superalloy for turbine disc application has been characterised at the grain level with a sub-micron resolution by digital image correlation on SEM images using secondary γ′ themselves as the speckle pattern. The obtained full-field strains have been analysed in global coordinates associated with the applied loading direction and in terms of the local coordinates associated with individual slip bands. Deformation arising from in-plane and out-of-plane dislocation slip can be identified by a combination of shear strain ɛxy and transverse strain ɛyy in the local slip band coordinates in combination with EBSD analysis. Cracks preferentially initiate from slip/strain bands adjacent and parallel to twin boundaries and then propagate along the slip/strain bands, leading to the onset of significant transverse strain ɛyy in the local band coordinates as a consequence of crack opening. Crack propagation is closely related to strain accumulation at the crack tip which is determined by the grain orientation and grain size. Transverse strain ɛyy in local slip band coordinates together with the inclination angle between dislocation slip direction on an activated {111} plane and the slip trace of this {111} plane at the specimen surface is proposed to be a cracking indicator/fracture criterion.

Text
Characterisation of Strain localisation in an disc alloy-accepted manuscript - Accepted Manuscript
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Accepted/In Press date: 23 May 2017
e-pub ahead of print date: 24 May 2017
Published date: June 2017
Keywords: Ni-based superalloys, Strain localisation, Digital image correlation, Fatigue cracking, Transverse strain
Organisations: Engineering Mats & Surface Engineerg Gp, Engineering Science Unit, Education Hub

Identifiers

Local EPrints ID: 410533
URI: http://eprints.soton.ac.uk/id/eprint/410533
PURE UUID: 1314a977-0afa-4a04-bcd0-322cd5fbb4b0
ORCID for Fabrice Pierron: ORCID iD orcid.org/0000-0003-2813-4994
ORCID for Philippa Reed: ORCID iD orcid.org/0000-0002-2258-0347

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Date deposited: 09 Jun 2017 09:03
Last modified: 06 Jun 2024 04:01

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Contributors

Author: Rong Jiang
Author: Fabrice Pierron ORCID iD
Author: Sari Octaviani
Author: Philippa Reed ORCID iD

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