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Fatigue crack propagation in a ductile superalloy at room temperature and extensive cyclic plastic flow
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering. Siemens Ind Turbomachinery AB,Finspång, Sweden.
Siemens Ind Turbomachinery AB, Finspång, Sweden.
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
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2015 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 80, 40-49 p.Article in journal (Refereed) Published
Abstract [en]

Fatigue crack propagation experiments under both force and displacement control have been performed on the wrought superalloy Haynes 230 at room temperature, using a single edge notched specimen. The force controlled tests are nominally elastic, and the displacement controlled tests have nominally large plastic hysteresis at the beginning of the tests, but saturates towards linear elastic conditions as the crack grows. As some tests are in the large scale yielding regime, a non-linear fracture mechanics approach is used to correlate crack growth rates versus the fracture parameter Delta J. It is shown that crack closure must be accounted for, to correctly model the crack growth seen in all the tests in a unified manner. For the force controlled small scale yielding tests the Newman crack closure model was used. The Newman equation is however not valid for large nominal cyclic plasticity, instead the crack closure in the displacement controlled tests is extracted from the test data. A good agreement between all tests is shown, when closure is accounted for and effective values of Delta J are used.

Place, publisher, year, edition, pages
ELSEVIER SCI LTD , 2015. Vol. 80, 40-49 p.
Keyword [en]
Fatigue crack propagation; Haynes 230; Large scale yielding; Cyclic J-integral or Delta J; Crack closure
National Category
Applied Mechanics
Identifiers
URN: urn:nbn:se:liu:diva-122045DOI: 10.1016/j.ijfatigue.2015.04.006ISI: 000360596500005OAI: oai:DiVA.org:liu-122045DiVA: diva2:885495
Note

Funding Agencies|Siemens Industrial Turbomachinery AB, Finspang, Sweden

Available from: 2015-12-18 Created: 2015-10-19 Last updated: 2016-04-21
In thesis
1. Modelling and experimental evaluation of non-linear fatigue crack propagation in a ductile superalloy
Open this publication in new window or tab >>Modelling and experimental evaluation of non-linear fatigue crack propagation in a ductile superalloy
2016 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Fatigue life evaluation is an important part in the design process of an industrial gas turbine. The fatigue life can be divided into crack initiation and crack propagation, and not to address the crack propagation part usually yields a non-conservative and overcomplicated design. Historically a lot of attention has been directed towards the crack initiation, but the crack propagation part in an industrial gas turbine context has not been given the same attention due to limitations in theoretical modelling, lack of test possibilities and that the design requirements have been fulfilled within the initiation life. However, with the need to reduce service down time and to improve performance, the crack propagation life needs to be further accounted for. As an example, cracks that emerge from notches or other stress concentrations grow under non-elastic conditions, which cannot be modelled with linear theories.

In this Licentiate of Engineering thesis a non-linear approach is put forward in which the plastic contribution in fatigue crack propagation is addressed and accounted for. The theoretical background is not new, but the finite element implementation done was, to the author knowledge, not available. This numerical post processing tool can calculate the non-linear ΔJ value for an arbitrary 2D-geometry. It was used to produce an expression for a non-linear geometry factor used in a simple expression for estimation of ΔJ in a test evaluation context. Room temperature tests were performed on a single notch specimen, under both displacement and force control. The latter were carried out in order to show the behaviour under small scale yielding conditions, while the displacement controlled testing was to show large scale yielding at the beginning of the tests. It was shown that all the test results could be collected in a Paris law type plot with ΔJ if the crack closure effect is taken into account. Furthermore, a study was performed where both a linear and a non-linear approach are applied on the displacement controlled tests. It was concluded that for the studied test series, the linear fatigue fracture parameter ΔK underestimates the crack growth behaviour if the elasto-plastic stresses from the tests are used, hence yielding non-conservative results.

Since this project focuses on non-linear crack propagation at thermo-mechanical conditions a crack length description is put forward, which simplifies and increases the accuracy of crack length measurements in fatigue crack propagation tests. It has also been shown that irrespectively of the crack initiation location in a single edge notch specimen the data fall on one curve, meaning that no care has to be taken regarding this aspect when evaluating crack length with the modified compliance method put forward in Paper III.

This Licentiate of Engineering thesis consists of two parts, where Part I gives an introduction to the subject, while Part II consists of three papers.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2016. 23 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1747
National Category
Materials Engineering Other Materials Engineering Applied Mechanics
Identifiers
urn:nbn:se:liu:diva-127347 (URN)978-91-7685-773-1 (Print) (ISBN)
Presentation
2016-05-20, ACAS, A-huset, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Supervisors
Available from: 2016-04-21 Created: 2016-04-21 Last updated: 2016-04-21Bibliographically approved

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