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Fatigue initiation and strength of duplex stainless steel strip specimens in the very high cycle fatigue regime
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik. (Materials Engineering)ORCID-id: 0000-0003-1672-1235
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik. (Materials Engineering)ORCID-id: 0000-0001-6029-2613
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik. (Materials Engineering)
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik. (Materials Engineering)
2014 (Engelska)Ingår i: Very high cycle fatigue 6 (VHCF6), 2014Konferensbidrag, Publicerat paper (Refereegranskat)
Abstract [en]

Fatigue studies of cold-rolled duplex stainless strip steel were performed in the very high cycle fatigue life region. The duplex austenitic-ferritic microstructure gives this grade a combination of high mechanical strength and high corrosion resistance. Fatigue properties of thin steel strips are particular due to cold rolling introducing a very fine microstructure. Crack initiation and fatigue strength are controlled by steel microstructure and alloying. The initiation and growth of the very short initial fatigue crack in very high cycle fatigue are unclear and subject to different descriptions. Fatigue test data of thin strip specimens at very high fatigue lives are scarce due to testing difficulties. For practical reasons testing must be performed at ultrasound test frequencies which involves fixturing problems. A test setup including the load chain ultrasonic horn, fixture and specimen was designed for resonance with a horse-shoe design of a screw fixture. The design of the horse-shoe fixture and the specimens along with FEM calculation of eigenfrequency are presented. Fatigue testing was performed at 20 kHz in R=-1 conditions up to fatigue life of 107 to 5*109 cycles. Fatigue strength was tested and crack initiation was studied on the fracture surface using FEG-SEM at the initiation site.

Ort, förlag, år, upplaga, sidor
2014.
Nyckelord [en]
Very high cycle fatigue, initiation, ultrasonic fatigue testing, strip specimen
Nationell ämneskategori
Annan materialteknik
Forskningsämne
Materialteknik
Identifikatorer
URN: urn:nbn:se:kau:diva-34633OAI: oai:DiVA.org:kau-34633DiVA, id: diva2:765145
Konferens
6th International conference on very high cycle fatigue (VHCF6), Chengdu, China
Projekt
Very high cycle fatigue of stainless steels - an unknown life
Forskningsfinansiär
KK-stiftelsenTillgänglig från: 2014-11-21 Skapad: 2014-11-21 Senast uppdaterad: 2019-07-10Bibliografiskt granskad
Ingår i avhandling
1. Very high cycle fatigue of duplex stainless steels and stress intensity calculations
Öppna denna publikation i ny flik eller fönster >>Very high cycle fatigue of duplex stainless steels and stress intensity calculations
2014 (Engelska)Licentiatavhandling, sammanläggning (Övrigt vetenskapligt)
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.

Ort, förlag, år, upplaga, sidor
Karlstad: Karlstads universitet, 2014. s. 30
Serie
Karlstad University Studies, ISSN 1403-8099 ; 2014:68
Nyckelord
Very high cycle fatigue, duplex stainless steel, ultrasonic fatigue testing, distributed dislocation dipole technique
Nationell ämneskategori
Maskinteknik
Forskningsämne
Materialteknik
Identifikatorer
urn:nbn:se:kau:diva-34591 (URN)978-91-7063-610-3 (ISBN)
Presentation
2014-12-17, Karlstad, 10:15 (Engelska)
Opponent
Handledare
Projekt
Very high cycle fatigue of stainless steels
Forskningsfinansiär
KK-stiftelsen
Anmärkning

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

Tillgänglig från: 2014-12-10 Skapad: 2014-11-13 Senast uppdaterad: 2019-07-10Bibliografiskt granskad
2. Initiation and early crack growth in VHCF of stainless steels: Experimental and theoretical analysis
Öppna denna publikation i ny flik eller fönster >>Initiation and early crack growth in VHCF of stainless steels: Experimental and theoretical analysis
2016 (Engelska)Doktorsavhandling, sammanläggning (Övrigt vetenskapligt)
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).

Ort, förlag, år, upplaga, sidor
Karlstad: Karlstads universitet, 2016. s. 68
Serie
Karlstad University Studies, ISSN 1403-8099 ; 2016:50
Nyckelord
Very High Cycle Fatigue, Stainless steel, Ultrasonic fatigue testing, Crack initiation, Crystallographic crack growth, Distributed Dislocation Dipole Technique, Closed cracks
Nationell ämneskategori
Annan materialteknik
Forskningsämne
Materialteknik
Identifikatorer
urn:nbn:se:kau:diva-47004 (URN)978-91-7063-733-9 (ISBN)
Disputation
2016-12-19, Eva Erikssonsalen, 21A342, Karlstads Universitetet, Karlstad, 10:15 (Engelska)
Opponent
Handledare
Forskningsfinansiär
KK-stiftelsen
Anmärkning

Artikel 4 publicerad i avhandlingen som manuskript

Tillgänglig från: 2016-11-29 Skapad: 2016-11-02 Senast uppdaterad: 2018-01-05Bibliografiskt granskad

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Tofique, Muhammad WaqasBergström, JensHallbäck, NilsBurman, Christer

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