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Development of a distributed dislocation dipole technique for the analysis of multiple straight, kinked and branched cracks in an elastic half-plane
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics. (Materials Engineering)
Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Physics. (Materials Engineering)
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. Vol. 51, 2878-2892 p.
Keyword [en]
Cracks, Dislocation dipoles, stress intensity factors, Singular integral equations
National Category
Other Mechanical Engineering
Research subject
Materials Engineering
Identifiers
URN: urn:nbn:se:kau:diva-34744DOI: 10.1016/j.ijsolstr.2014.04.011ISI: 000338005200013OAI: oai:DiVA.org:kau-34744DiVA: diva2:769899
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
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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