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Very high cycle fatigue of duplex stainless steels and stress intensity calculations
Karlstads universitet, Fakulteten för hälsa, natur- och teknikvetenskap (from 2013), Institutionen för ingenjörsvetenskap och fysik. (Materials Engineering Group)ORCID-id: 0000-0003-1672-1235
2014 (engelsk)Licentiatavhandling, med artikler (Annet vitenskapelig)
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.

sted, utgiver, år, opplag, sider
Karlstad: Karlstads universitet, 2014. , s. 30
Serie
Karlstad University Studies, ISSN 1403-8099 ; 2014:68
Emneord [en]
Very high cycle fatigue, duplex stainless steel, ultrasonic fatigue testing, distributed dislocation dipole technique
HSV kategori
Forskningsprogram
Materialteknik
Identifikatorer
URN: urn:nbn:se:kau:diva-34591ISBN: 978-91-7063-610-3 (tryckt)OAI: oai:DiVA.org:kau-34591DiVA, id: diva2:762810
Presentation
2014-12-17, Karlstad, 10:15 (engelsk)
Opponent
Veileder
Prosjekter
Very high cycle fatigue of stainless steels
Forskningsfinansiär
Knowledge Foundation
Merknad

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

Tilgjengelig fra: 2014-12-10 Laget: 2014-11-13 Sist oppdatert: 2019-07-10bibliografisk kontrollert
Delarbeid
1. Review of VHCF studies, short crack models and theory of vibrations
Åpne denne publikasjonen i ny fane eller vindu >>Review of VHCF studies, short crack models and theory of vibrations
(engelsk)Manuskript (preprint) (Annet vitenskapelig)
Abstract [en]

This literature review loosely connects different concepts of very high cycle fatigue which are of significance within the scope of this thesis work. The ultrasonic fatigue testing equipment and its basic principle of operation is discussed in detail. The phenomenon of damping in the light of theory of vibrations is outlined. The significance of the understanding of damping phenomenon in the ultrasonic fatigue testing is highlighted. On the other hand, SN fatigue data obtained by several researchers for different materials are reviewed. In addition, the fundamental mechanisms responsible for fatigue crack initiation are also summarized.

 

Considerable attention is focussed on the mathematical models developed for the prediction of growth behaviour of short fatigue cracks. The basic concepts, and models, which led to the development of the distributed dislocation dipole technique, have also been reviewed.

Emneord
Very cycle high fatigue, crack initiation, short cracks, vibration, damping
HSV kategori
Forskningsprogram
Materialteknik
Identifikatorer
urn:nbn:se:kau:diva-34634 (URN)
Prosjekter
Very high cycle fatigue of stainless steels - an unknown life
Forskningsfinansiär
Knowledge Foundation
Tilgjengelig fra: 2014-11-21 Laget: 2014-11-21 Sist oppdatert: 2019-07-03bibliografisk kontrollert
2. Fatigue initiation and strength of duplex stainless steel strip specimens in the very high cycle fatigue regime
Åpne denne publikasjonen i ny fane eller vindu >>Fatigue initiation and strength of duplex stainless steel strip specimens in the very high cycle fatigue regime
2014 (engelsk)Inngår i: Very high cycle fatigue 6 (VHCF6), 2014Konferansepaper, Publicerat paper (Fagfellevurdert)
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.

Emneord
Very high cycle fatigue, initiation, ultrasonic fatigue testing, strip specimen
HSV kategori
Forskningsprogram
Materialteknik
Identifikatorer
urn:nbn:se:kau:diva-34633 (URN)
Konferanse
6th International conference on very high cycle fatigue (VHCF6), Chengdu, China
Prosjekter
Very high cycle fatigue of stainless steels - an unknown life
Forskningsfinansiär
Knowledge Foundation
Tilgjengelig fra: 2014-11-21 Laget: 2014-11-21 Sist oppdatert: 2019-11-28bibliografisk kontrollert
3. Fatigue strength, crack initiation, and localized plastic fatigue damage in VHCF of duplex stainless steels
Åpne denne publikasjonen i ny fane eller vindu >>Fatigue strength, crack initiation, and localized plastic fatigue damage in VHCF of duplex stainless steels
Vise andre…
2016 (engelsk)Inngår i: Steel Research International, ISSN 1611-3683, E-ISSN 1869-344X, Vol. 87, nr 7, s. 899-910Artikkel i tidsskrift (Fagfellevurdert) 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.

sted, utgiver, år, opplag, sider
Wiley-VCH Verlagsgesellschaft, 2016
Emneord
duplex stainless steel; ultrasonic fatigue testing; plastic fatigue damage; very high cycle fatigue; crystallographic crack growth
HSV kategori
Forskningsprogram
Materialteknik; Maskinteknik
Identifikatorer
urn:nbn:se:kau:diva-38693 (URN)10.1002/srin.201500263 (DOI)000385610500010 ()
Prosjekter
Very High Cycle Fatigue of Stainless Steels - an unknown life
Forskningsfinansiär
Knowledge Foundation
Tilgjengelig fra: 2015-11-23 Laget: 2015-11-23 Sist oppdatert: 2017-12-06bibliografisk kontrollert
4. Development of a distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half-plane
Åpne denne publikasjonen i ny fane eller vindu >>Development of a distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half-plane
2014 (engelsk)Inngår i: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 51, s. 2878-2892Artikkel i tidsskrift (Fagfellevurdert) Published
Abstract [en]

A distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half plane has been developed. The dipole density distribution is represented with a weighted Jacobi polynomial expansion where the weight function captures the asymptotic behaviour at each end of the crack. To allow for opening and sliding at crack kinking and branching the dipole density representation contains conditional extra terms which fulfil the asymptotic behaviour at each endpoint. Several test cases involving straight, kinked and branched cracks have been analysed, and the results suggest that the accuracy of the method is within 1% provided that Jacobi polynomial expansions up to at least the sixth order are used. Adopting even higher order Jacobi polynomials yields improved accuracy. The method is compared to a simplified procedure suggested in the literature where stress singularities associated with corners at kinking or branching are neglected in the representation for the dipole density distribution. The comparison suggests that both procedures work, but that the current procedure is superior, in as much as the same accuracy is reached using substantially lower order polynomial expansions.

sted, utgiver, år, opplag, sider
Elsevier, 2014
Emneord
Cracks, Dislocation dipoles, stress intensity factors, Singular integral equations
HSV kategori
Forskningsprogram
Materialteknik
Identifikatorer
urn:nbn:se:kau:diva-34744 (URN)10.1016/j.ijsolstr.2014.04.011 (DOI)000338005200013 ()
Prosjekter
Very high cycle fatigue of stainless steels- an unknown life
Forskningsfinansiär
Knowledge Foundation
Tilgjengelig fra: 2014-12-09 Laget: 2014-12-09 Sist oppdatert: 2017-12-06bibliografisk kontrollert

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