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Initiation and early crack growth in VHCF of stainless steels: Experimental and theoretical analysis
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics. (Materials Engineering)
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 [en]
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: urn:nbn:se:kau:diva-47004ISBN: 978-91-7063-733-9OAI: oai:DiVA.org:kau-47004DiVA: diva2:1044141
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
List of papers
1. Fatigue initiation and strength of duplex stainless steel strip specimens in the very high cycle fatigue regime
Open this publication in new window or tab >>Fatigue initiation and strength of duplex stainless steel strip specimens in the very high cycle fatigue regime
2014 (English)In: Very high cycle fatigue 6 (VHCF6), 2014Conference paper (Refereed)
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.

Keyword
Very high cycle fatigue, initiation, ultrasonic fatigue testing, strip specimen
National Category
Other Materials Engineering
Research subject
Materials Engineering
Identifiers
urn:nbn:se:kau:diva-34633 (URN)
Conference
6th International conference on very high cycle fatigue (VHCF6), Chengdu, China
Projects
Very high cycle fatigue of stainless steels - an unknown life
Funder
Knowledge Foundation
Available from: 2014-11-21 Created: 2014-11-21 Last updated: 2016-11-09
2. Fatigue strength, crack initiation, and localized plastic fatigue damage in VHCF of duplex stainless steels
Open this publication in new window or tab >>Fatigue strength, crack initiation, and localized plastic fatigue damage in VHCF of duplex stainless steels
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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
Keyword
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:nbn:se:kau:diva-38693 (URN)10.1002/srin.201500263 (DOI)
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
3. EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steels
Open this publication in new window or tab >>EBSD and TEM analysis of plastic fatigue damage accumulation responsible for fatigue crack initiation and propagation in VHCF of duplex stainless steels
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(English)Manuscript (preprint) (Other academic)
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.

Keyword
Duplex stainless steel, High frequency testing, Crack initiation, Damage accumulation, Fatigue crack growth, Short cracks
National Category
Other Materials Engineering
Research subject
Materials Engineering; Mechanical Engineering
Identifiers
urn:nbn:se:kau:diva-47125 (URN)
Funder
Knowledge Foundation
Available from: 2016-11-07 Created: 2016-11-07 Last updated: 2016-11-09
4. Very High Cycle Fatigue of cold rolled stainless steels, crack initiation and formation of Fine Granular Area
Open this publication in new window or tab >>Very High Cycle Fatigue of cold rolled stainless steels, crack initiation and formation of Fine Granular Area
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Fatigue tests of cold rolled strip materials, a duplex stainless steel and a martensitic stainless steel, were performed using an ultrasonic fatigue test equipment operating at 20 kHz under a completely reversed tension compression load ratio R =-1. Fatigue test data (SN data) was generated in the VHCF regime and fracture surfaces of the failed specimens were investigated using a Scanning Electron Microscope (SEM). In the duple stainless steel grade, fatigue failures were found to be initiated at surface defects on the side surfaces or corners, created due to cold rolling, of the strip specimens. Features of a Fine Granular Area (FGA) were observed around the crack initiating surface defects on the fracture surfaces. In the martensitic stainless steel grade, fatigue crack initiation occurred due to aluminium-silicon oxide inclusions or surface defects created due to cold rolling of the material. In situ Focussed Ion Beam (FIB) technique was used to extract cross-sections from the FGA around the crack initiating defect on the fracture surface. Transmission Electron Microscope (TEM) investigations of the extracted cross-sections revealed FGAs in immediate vicinity of the crack initiating surface defects. The observed fine grained layers seemed to be composed of nano-sized grains and, thus, could be distinguished from the bulk material. The FGA around the surface crack initiating defects seems to have formed due to localized plastic deformation by stress concentration at the defects.

Keyword
Duplex stainless steel, Martensitic stainless steel, Fatigue, High frequency testing, Crack initiation, Fine granular area
National Category
Other Materials Engineering
Research subject
Materials Engineering; Mechanical Engineering
Identifiers
urn:nbn:se:kau:diva-47126 (URN)
Funder
Knowledge Foundation
Available from: 2016-11-07 Created: 2016-11-07 Last updated: 2016-11-09
5. Development of a distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half-plane
Open this publication in new window or tab >>Development of a distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half-plane
2014 (English)In: International Journal of Solids and Structures, ISSN 0020-7683, E-ISSN 1879-2146, Vol. 51, 2878-2892 p.Article in journal (Refereed) 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.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Cracks, Dislocation dipoles, stress intensity factors, Singular integral equations
National Category
Other Mechanical Engineering
Research subject
Materials Engineering
Identifiers
urn:nbn:se:kau:diva-34744 (URN)10.1016/j.ijsolstr.2014.04.011 (DOI)000338005200013 ()
Projects
Very high cycle fatigue of stainless steels- an unknown life
Funder
Knowledge Foundation
Available from: 2014-12-09 Created: 2014-12-09 Last updated: 2016-11-09Bibliographically approved
6. Development of the distributed dislocation dipole technique for the analysis of closure of complex fractures involving kinks and branches
Open this publication in new window or tab >>Development of the distributed dislocation dipole technique for the analysis of closure of complex fractures involving kinks and branches
(English)Manuscript (preprint) (Other academic)
Abstract [en]

This paper presents the development of the distributed dislocation dipole technique (DDDT) for the analysis of crack surface closure of crack cases involving kinks and branches. Crack cases in which closure occurs are analyzed by reformulating the Bueckner's principle taking the contact stresses at the contacting portions of the crack surfaces into account. Stress intensity factors corresponding to opening and sliding mode of deformation at the crack tips are computed. Three test cases involving kinked and/or branched cracks with at least one of the crack segments undergoing crack surface closure when subjected to remote tensile loading are analyzed. The results obtained from the DDDT are compared to those obtained from the Finite Element Method (FEM) analysis of the same crack cases. This comparison shows that the computation of stress intensity factors for the crack cases involving crack surface closure are less acurate compared to fully open crack cases. However, the stress intensity factors are still computed to an accuracy of within 2 percent if the Jacobi polynomial expansions of at least the sixth order are used to represent the crack surface opening and sliding displacements. Higher order Jacobi polynomials lead to increased accuracy.

Keyword
Cracks, Dislocation dipoles, Stress intensity factors, Singular integral equations, Crack closure, Contact
National Category
Applied Mechanics Reliability and Maintenance Aerospace Engineering Other Materials Engineering
Research subject
Materials Engineering; Mechanical Engineering
Identifiers
urn:nbn:se:kau:diva-47128 (URN)
Funder
Knowledge Foundation
Available from: 2016-11-08 Created: 2016-11-08 Last updated: 2016-11-09

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Tofique, Muhammad Waqas
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