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Modelling and experimental evaluation of non-linear fatigue crack propagation in a ductile superalloy
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, Faculty of Science & Engineering.
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: urn:nbn:se:liu:diva-127347ISBN: 978-91-7685-773-1 (Print)OAI: oai:DiVA.org:liu-127347DiVA: diva2:921931
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
List of papers
1. Fatigue crack propagation in a ductile superalloy at room temperature and extensive cyclic plastic flow
Open this publication in new window or tab >>Fatigue crack propagation in a ductile superalloy at room temperature and extensive cyclic plastic flow
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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
Keyword
Fatigue crack propagation; Haynes 230; Large scale yielding; Cyclic J-integral or Delta J; Crack closure
National Category
Applied Mechanics
Identifiers
urn:nbn:se:liu:diva-122045 (URN)10.1016/j.ijfatigue.2015.04.006 (DOI)000360596500005 ()
Note

Funding Agencies|Siemens Industrial Turbomachinery AB, Finspang, Sweden

Available from: 2015-12-18 Created: 2015-10-19 Last updated: 2016-04-21
2. Comparison between linear and non-linear fracture mechanics analysis of experimental data for the ductile superalloy Haynes 230
Open this publication in new window or tab >>Comparison between linear and non-linear fracture mechanics analysis of experimental data for the ductile superalloy Haynes 230
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2016 (English)In: Journal of engineering for gas turbines and power, ISSN 0742-4795, E-ISSN 1528-8919, Vol. 138, no 6, 062101-1-062101-7 p.Article in journal (Refereed) Published
Abstract [en]

With increasing use of renewable energy sources, an industrial gas turbine is often a competitive solution to balance the power grid. However, life robustness approaches for gas turbine components operating under increasingly cyclic conditions are a challenging task. Ductile superalloys, as Haynes 230, are often used in stationary gas turbine hot parts such as combustors. The main load for such components is due to nonhomogeneous thermal expansion within or between parts. As the material is ductile, there is considerable redistribution of stresses and strains due to inelastic deformations during the crack initiation phase. Therefore, the subsequent crack growth occurs through a material with significant residual stresses and strains. In this work, fatigue crack propagation experiments, including the initiation phase, have been performed on a single edge notched specimen under strain controlled conditions. The test results are compared to fracture mechanics analyses using the linear ΔK and the nonlinear ΔJ approaches, and an attempt to quantify the difference in terms of a life prediction is made. For the tested notched geometry, material, and strain ranges, the difference in the results using ΔKeff or ΔJeff is larger than the scatter seen when fitting the model to the experimental data. The largest differences can be found for short crack lengths, when the cyclic plastic work is the largest. The ΔJ approach clearly shows better agreement with the experimental results in this regime.

Place, publisher, year, edition, pages
ASME Press, 2016
National Category
Applied Mechanics Other Materials Engineering
Identifiers
urn:nbn:se:liu:diva-126576 (URN)10.1115/1.4031712 (DOI)000374713500010 ()
Note

Funding agencies: Siemens Industrial Turbomachinery AB, Finspang, Sweden

Available from: 2016-03-30 Created: 2016-03-30 Last updated: 2016-06-10

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