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3-D analysis of fatigue crack behaviour in a shot peened steam turbine blade material

3-D analysis of fatigue crack behaviour in a shot peened steam turbine blade material
3-D analysis of fatigue crack behaviour in a shot peened steam turbine blade material
Serial mechanical sectioning and high resolution X-ray tomography have been used to study the three-dimensional morphology of small fatigue cracks growing in a 12 Cr tempered martensitic steam turbine blade material. A range of surface conditions has been studied, namely polished and shot peened (with varying levels of intensity). In the polished (unpeened) condition, inclusions (alumina and manganese sulphide) played an important role in initiating and controlling early fatigue crack behaviour. When fatigue cracks initiated from an alumina stringer, the crack morphology was normally dominated by single stringers, which were always in the centre of the fatigue crack, indicating its primary role in initiation. Manganese sulphide inclusion groups however seemed to dominate and affect the crack path along both the surface and depth crack growth directions. The more intensely shot peened condition did not however evidence inclusion or stringer affected fatigue crack initiation or growth behaviour; sub-surface crack coalescence being clearly observed by both serial sectioning and CT (computed tomography) imaging techniques at a depth of about 150 ~ 180 ?m. These sub-surface crack coalescences can be linked to both the extent of the compressive residual stress as well as the depth of the plastic deformation arising from the intense shot peening process. Shot peening appears to provide a different defect population that initiates fatigue cracks and competes with the underlying metallurgical defect populations. The most beneficial shot peening process would in this case appear to “deactivate” the original metallurgical defect population and substitute a known defect distribution from the shot peening process from which fatigue cracks grow rather slowly in the strain hardened surface layer which also contains compressive residual stresses. A benefit to fatigue life in bending, even under Low Cycle Fatigue (LCF) conditions, has been observed in these tests if a sufficiently severe shot peening condition is applied in a constrained notch configuration.
low cycle fatigue, shot peening, residual stress and strain hardening, 3 dimension small crack morphology, serial sectioning, x-ray micro-computed tomography
0921-5093
91-103
He, Binyan
0c154394-797d-42ad-b812-3fbbaea0238a
Katsamenis, Orestis
8553e7c3-d860-4b7a-a883-abf6c0c4b438
Mellor, Brian
2b13b80f-880b-49ac-82fe-827a15dde2fe
Reed, Philippa A.S.
8b79d87f-3288-4167-bcfc-c1de4b93ce17
He, Binyan
0c154394-797d-42ad-b812-3fbbaea0238a
Katsamenis, Orestis
8553e7c3-d860-4b7a-a883-abf6c0c4b438
Mellor, Brian
2b13b80f-880b-49ac-82fe-827a15dde2fe
Reed, Philippa A.S.
8b79d87f-3288-4167-bcfc-c1de4b93ce17

He, Binyan, Katsamenis, Orestis, Mellor, Brian and Reed, Philippa A.S. (2015) 3-D analysis of fatigue crack behaviour in a shot peened steam turbine blade material. Materials Science and Engineering: A, 642, 91-103. (doi:10.1016/j.msea.2015.06.082).

Record type: Article

Abstract

Serial mechanical sectioning and high resolution X-ray tomography have been used to study the three-dimensional morphology of small fatigue cracks growing in a 12 Cr tempered martensitic steam turbine blade material. A range of surface conditions has been studied, namely polished and shot peened (with varying levels of intensity). In the polished (unpeened) condition, inclusions (alumina and manganese sulphide) played an important role in initiating and controlling early fatigue crack behaviour. When fatigue cracks initiated from an alumina stringer, the crack morphology was normally dominated by single stringers, which were always in the centre of the fatigue crack, indicating its primary role in initiation. Manganese sulphide inclusion groups however seemed to dominate and affect the crack path along both the surface and depth crack growth directions. The more intensely shot peened condition did not however evidence inclusion or stringer affected fatigue crack initiation or growth behaviour; sub-surface crack coalescence being clearly observed by both serial sectioning and CT (computed tomography) imaging techniques at a depth of about 150 ~ 180 ?m. These sub-surface crack coalescences can be linked to both the extent of the compressive residual stress as well as the depth of the plastic deformation arising from the intense shot peening process. Shot peening appears to provide a different defect population that initiates fatigue cracks and competes with the underlying metallurgical defect populations. The most beneficial shot peening process would in this case appear to “deactivate” the original metallurgical defect population and substitute a known defect distribution from the shot peening process from which fatigue cracks grow rather slowly in the strain hardened surface layer which also contains compressive residual stresses. A benefit to fatigue life in bending, even under Low Cycle Fatigue (LCF) conditions, has been observed in these tests if a sufficiently severe shot peening condition is applied in a constrained notch configuration.

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Accepted/In Press date: 25 June 2015
e-pub ahead of print date: 30 June 2015
Published date: 26 August 2015
Keywords: low cycle fatigue, shot peening, residual stress and strain hardening, 3 dimension small crack morphology, serial sectioning, x-ray micro-computed tomography
Organisations: Engineering Mats & Surface Engineerg Gp

Identifiers

Local EPrints ID: 379530
URI: http://eprints.soton.ac.uk/id/eprint/379530
ISSN: 0921-5093
PURE UUID: fda40767-e8b0-4b62-8d96-4274ac9d072a
ORCID for Orestis Katsamenis: ORCID iD orcid.org/0000-0003-4367-4147
ORCID for Philippa A.S. Reed: ORCID iD orcid.org/0000-0002-2258-0347

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Date deposited: 27 Jul 2015 12:53
Last modified: 15 Mar 2024 03:38

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Author: Binyan He
Author: Brian Mellor

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