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Fatigue strength, crack initiation, and localized plastic fatigue damage in VHCF of duplex stainless steels
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics. (Materials Engineering)ORCID iD: 0000-0003-1672-1235
Karlstad University, Faculty of Technology and Science, Department of Mechanical and Materials Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.ORCID iD: 0000-0001-6029-2613
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics. (Materialvetenskap)
Karlstad University, Faculty of Technology and Science.
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2015 (English)In: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 87, no 7, 899-910 p.Article in journal (Refereed) Published
Abstract [en]

The fatigue strength of two-duplex stainless steel grades, 2304 SRG and LDX 2101, with austenitic–ferritic microstructure is tested using ultrasonic fatigue testing equipment operating at 20 kHz. The testing is conducted in tension-compression mode with the load ratio R=-1. The fatigue strength is evaluated at 107, 108, and 109 load cycles and the estimates of fatigue strength are higher for the LDX 2101 grade. The fatigue crack initiation mechanisms are analyzed using a scanning electron microscope. The fatigue cracks, in all cases, appear to initiate due to accumulation of plastic fatigue damage at the surface. In the 2304 SRG grade, accumulation of fatigue damage occurs at the external surface of fatigued specimens in the form of extrusions at the grain/phase boundaries and in the form of individual slip lines in the austenite phase. Meanwhile, in the LDX 2101 grade accumulation of plastic fatigue damage in the form of extrusions and intrusions occurs mainly within the ferrite grain. When the crack is microstructurally short, the crack growth appears to be crystallographic in nature and the crack appears to change its direction propagating from one grain into another.

Place, publisher, year, edition, pages
Wiley-VCH Verlagsgesellschaft, 2015. Vol. 87, no 7, 899-910 p.
Keyword [en]
duplex stainless steel; ultrasonic fatigue testing; plastic fatigue damage; very high cycle fatigue; crystallographic crack growth
National Category
Metallurgy and Metallic Materials
Research subject
Materials Engineering; Mechanical Engineering
Identifiers
URN: urn:nbn:se:kau:diva-38693DOI: 10.1002/srin.201500263OAI: oai:DiVA.org:kau-38693DiVA: diva2:873311
Projects
Very High Cycle Fatigue of Stainless Steels - an unknown life
Funder
Knowledge Foundation
Available from: 2015-11-23 Created: 2015-11-23 Last updated: 2016-11-09Bibliographically approved
In thesis
1. Very high cycle fatigue of duplex stainless steels and stress intensity calculations
Open this publication in new window or tab >>Very high cycle fatigue of duplex stainless steels and stress intensity calculations
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Very high cycle fatigue (VHCF) is generally considered as the domain of fatigue lifetime beyond 10 million (107) load cycles. Few examples of structural components which are subjected to 107-109 load cycles during their service life are engine parts, turbine disks, railway axles and load-carrying parts of automobiles. Therefore, the safe and reliable operation of these components depends on the knowledge of their fatigue strength and the prevalent damage/failure mechanisms. Moreover, the fatigue life of materials in the VHCF regime is controlled by the fatigue crack initiation and early growth stage of short cracks.

This study was focussed on the evaluation of fatigue properties of duplex stainless steels in the VHCF regime using the ultrasonic fatigue testing equipment. The ultrasonic fatigue tests were conducted on the cold rolled duplex stainless strip steel and hot rolled duplex stainless steel grades. Two different geometries of ultrasonic fatigue test specimens were tested. Considerable attention was devoted to the evaluation of fatigue crack initiation and growth mechanisms using the high resolution scanning electron microscopy. The fatigue crack initiation was found to be surface initiated phenomena in all the tested grades, albeit different in each case.

The second part of this thesis work was the development of a distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half plane. Cracks with dimensions much smaller than the overall size of the domain were considered. The main goal of the development of this technique was the evaluation of stress intensity factor at each crack tip. The comparison of results from the stress intensity factor evaluation by the developed procedure and the well-established Finite Element Method software ABAQUS showed difference of less than 1% for Jacobi polynomial expansion of sixth order in the dipole density representation.

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2014. 30 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2014:68
Keyword
Very high cycle fatigue, duplex stainless steel, ultrasonic fatigue testing, distributed dislocation dipole technique
National Category
Mechanical Engineering
Research subject
Materials Engineering
Identifiers
urn:nbn:se:kau:diva-34591 (URN)978-91-7063-610-3 (ISBN)
Presentation
2014-12-17, Karlstad, 10:15 (English)
Opponent
Supervisors
Projects
Very high cycle fatigue of stainless steels
Funder
Knowledge Foundation
Note

Article III was still in manuscript form at the time of the defense.

Available from: 2014-12-10 Created: 2014-11-13 Last updated: 2016-08-16Bibliographically approved
2. Initiation and early crack growth in VHCF of stainless steels: Experimental and theoretical analysis
Open this publication in new window or tab >>Initiation and early crack growth in VHCF of stainless steels: Experimental and theoretical analysis
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
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).

Place, publisher, year, edition, pages
Karlstad: Karlstads universitet, 2016. 68 p.
Series
Karlstad University Studies, ISSN 1403-8099 ; 2016:50
Keyword
Very High Cycle Fatigue, Stainless steel, Ultrasonic fatigue testing, Crack initiation, Crystallographic crack growth, Distributed Dislocation Dipole Technique, Closed cracks
National Category
Other Materials Engineering
Research subject
Materials Engineering
Identifiers
urn:nbn:se:kau:diva-47004 (URN)978-91-7063-733-9 (ISBN)
Public defence
2016-12-19, Eva Erikssonsalen, 21A342, Karlstads Universitetet, Karlstad, 10:15 (English)
Opponent
Supervisors
Funder
Knowledge Foundation
Available from: 2016-11-29 Created: 2016-11-02 Last updated: 2016-11-29Bibliographically approved

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