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EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steels
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). (Materials Engineering)ORCID-id: 0000-0003-1672-1235
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). (Materials Engineering)ORCID-id: 0000-0001-6029-2613
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik (from 2013). (Materials Physics)ORCID-id: 0000-0003-1711-5595
Department of Materials Engineering, Linköping University, Sweden.
Vise andre og tillknytning
(engelsk)Manuskript (preprint) (Annet vitenskapelig)
Abstract [en]

Fatigue test data (stress-life) of two duplex stainless steel grades, LDX 2101 and 2304 SRG, in the Very High Cycle Fatigue regime (VHCF) is presented. The fatigue testing is conducted using an ultrasonic fatigue testing equipment operating at 20 kHz under fully reversed tension-compression condition (R=-1). Scanning Electron Microscope (SEM) studies of the fracture surfaces and external surfaces of failed specimens is conducted. Electron Back Scattered Diffraction (EBSD) studies of the axially cut surface of the failed specimens is done to analyse grains near the external surface and crack initiation site. Analysis of accumulation of plastic fatigue damage and growth of cracks in the Crystallographic Growth Region (CGR) is carried out. Transmission Electron Microscope (TEM) analysis of thin foils cut from failed specimens of LDX 2101 is carried out to examine the effect of fatigue loading on dislocation structure. SEM studies of the CGR show features like grain boundaries and fatigue striations. The barrier effect of grain and phase boundaries on short fatigue crack propagation is observed. ECCI images and EBSD analysis show that Persistent Slip Bands (PSBs) are observed in ferrite grains in LDX 2101 grade. On the other hand, no PSBs are observed in any of the grains in 2304 SRG. The TEM observations in thin foils cut from the failed specimen of LDX 2101 show stacking faults in austenite grains. Stacking faults were observed to stop at the grain and phase boundaries.

Emneord [en]
Duplex stainless steel, High frequency testing, Crack initiation, Damage accumulation, Fatigue crack growth, Short cracks
HSV kategori
Forskningsprogram
Materialteknik; Maskinteknik
Identifikatorer
URN: urn:nbn:se:kau:diva-47125OAI: oai:DiVA.org:kau-47125DiVA, id: diva2:1044994
Forskningsfinansiär
Knowledge FoundationTilgjengelig fra: 2016-11-07 Laget: 2016-11-07 Sist oppdatert: 2019-07-03bibliografisk kontrollert
Inngår i avhandling
1. Initiation and early crack growth in VHCF of stainless steels: Experimental and theoretical analysis
Åpne denne publikasjonen i ny fane eller vindu >>Initiation and early crack growth in VHCF of stainless steels: Experimental and theoretical analysis
2016 (engelsk)Doktoravhandling, med artikler (Annet vitenskapelig)
Abstract [en]

Mechanical fatigue is a failure phenomenon that occurs due to repeated application of mechanical loads. Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Increasing numbers of structural components have service life in the VHCF regime, for instance in automotive and high speed train transportation, gas turbine disks, and components of paper production machinery. Safe and reliable operation of these components depends on the knowledge of their VHCF properties. In this thesis both experimental tools and theoretical modelling were utilized to develop better understanding of the VHCF phenomena.

In the experimental part, ultrasonic fatigue testing at 20 kHz of cold rolled and hot rolled stainless steel grades was conducted and fatigue strengths in the VHCF regime were obtained. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes. For the cold rolled stainless steels crack initiation and early growth occurred through the formation of the Fine Granular Area (FGA) observed on the fracture surface and in TEM observations of cross-sections. The crack growth in the FGA seems to control more than 90% of the total fatigue life. For the hot rolled duplex stainless steels fatigue crack initiation occurred due to accumulation of plastic fatigue damage at the external surface, and early crack growth proceeded through a crystallographic growth mechanism.

Theoretical modelling of complex cracks involving kinks and branches in an elastic half-plane under static loading was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The technique was implemented for 2D crack problems. Both fully open and partially closed crack cases were analyzed. The main aim of the development of the DDDT was to compute the stress intensity factors. Accuracy of 2% in the computations was attainable compared to the solutions obtained by the Finite Element Method.

Abstract [en]

Very High Cycle Fatigue (VHCF) is considered as the domain of fatigue life greater than 10 million load cycles. Structural components that have service life in the VHCF regime include wheels and axles of high speed trains, gas turbine disks, and components of paper production machinery. Safe and reliable design, and the longevity, of these components depends on the knowledge of their VHCF properties. The overall aim of the experimental portion of this thesis was to gain in-depth knowledge of the VHCF properties of stainless steels. Fatigue test data in the VHCF regime was generated for different stainless steel grades using ultrasonic fatigue testing. The mechanisms for fatigue crack initiation and short crack growth were investigated using electron microscopes.

Theoretical modelling of complex crack geometries involving kinks and branches was carried out by using the Distributed Dislocation Dipole Technique (DDDT). The main aim of this development was to compute the stress intensity factors and to analyse the stress state around the cracks. The results showed that accuracy of 2% was attainable compared to the solutions obtained by Finite Element Method (FEM).

sted, utgiver, år, opplag, sider
Karlstad: Karlstads universitet, 2016. s. 68
Serie
Karlstad University Studies, ISSN 1403-8099 ; 2016:50
Emneord
Very High Cycle Fatigue, Stainless steel, Ultrasonic fatigue testing, Crack initiation, Crystallographic crack growth, Distributed Dislocation Dipole Technique, Closed cracks
HSV kategori
Forskningsprogram
Materialteknik
Identifikatorer
urn:nbn:se:kau:diva-47004 (URN)978-91-7063-733-9 (ISBN)
Disputas
2016-12-19, Eva Erikssonsalen, 21A342, Karlstads Universitetet, Karlstad, 10:15 (engelsk)
Opponent
Veileder
Forskningsfinansiär
Knowledge Foundation
Merknad

Artikel 4 publicerad i avhandlingen som manuskript

Tilgjengelig fra: 2016-11-29 Laget: 2016-11-02 Sist oppdatert: 2019-10-21bibliografisk kontrollert

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