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Modelling of fatigue crack propagation in Inconel 718 under hold time conditions
Linköping University, Department of Management and Engineering, Solid Mechanics. Linköping University, The Institute of Technology.
2014 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

In this thesis an investigation and modelling of the fatigue crack propagation in the nickel based superalloy Inconel 718, with a special emphasis on the effect of hold times, is presented. The modelling work has been concentrated on describing the hold time fatigue crack propagation by using the concept of a damaged zone in front of the crack tip, which is believed to have a lowered resistance against crack propagation.

The modelling framework is built on physically motivated parameters, which are all easy to calibrate through one specially designed test type. Later evaluation through many experimental tests has also shown that the model is capable, within reasonable scatter level to predict, the hold time fatigue crack propagation for many different temperatures and loading conditions. Further evaluation of a complex flight spectrum, with the incorporation of crack closure within the model, was also predicted with a satisfying result.

This thesis is divided into two parts. First, a background and a somewhat deeper discussion of the modelling of fatigue crack growth under hold time conditions is presented. The second part consists of ve appended papers, which describe the work completed so far in the project.

Place, publisher, year, edition, pages
Linköping: Linköping University Electronic Press, 2014. , 48 p.
Series
Linköping Studies in Science and Technology. Thesis, ISSN 0280-7971 ; 1645
National Category
Engineering and Technology
Identifiers
URN: urn:nbn:se:liu:diva-104814DOI: 10.3384/lic.diva-104814ISBN: 978-91-7519-403-5 (print)OAI: oai:DiVA.org:liu-104814DiVA: diva2:699351
Presentation
2014-03-21, A34, Hus A, Campus Valla, Linköpings universitet, Linköping, 10:15 (English)
Opponent
Supervisors
Available from: 2014-02-27 Created: 2014-02-27 Last updated: 2014-03-14Bibliographically approved
List of papers
1. High temperature fatigue crack growth behaviour of Inconel 718 under hold time and overload conditions
Open this publication in new window or tab >>High temperature fatigue crack growth behaviour of Inconel 718 under hold time and overload conditions
2013 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 48, 178-186 p.Article in journal (Refereed) Published
Abstract [en]

Inconel 718 is a frequently used material for gas turbine applications at temperatures up to 650°C. For such components, the main load cycle is typically defined by the start-up and shut-down of the engine. In this main loading cycle, hold times at high temperature are commonly present in critical components. These high temperature hold times may greatly increase the fatigue crack growth rate with respect to the number of cycles unless other beneficial factors such as for example initial overloads are present. The latter can be caused by abnormal service conditions but can also occur on a more regular basis and are then typically observed in components with strong thermal transients during engine start-up. In this paper, focus has been placed on the effect of overloads on the hold time fatigue crack growth behaviour and its subsequent description. More specifically, crack propagation in Inconel 718 has been studied at the temperatures 550°C and 650°C with and without an overload at the start of the cycle. The effect of initial overloads was found to be substantial. A simple model for describing the effect of these loading conditions has also been developed based on the concept of the damaged zone, present around the crack tip. Irregular crack fronts and unbroken ligaments left on the fracture surfaces seen in complementary microscopy studies seem to support this approach. Furthermore, the stress state in front of a crack tip in a 2D model was investigated both with and without an initial overload. The results were related to the observed crack growth retardation behaviour found in the material testing.

Place, publisher, year, edition, pages
Elsevier, 2013
Keyword
Nickel-base superalloys, fatigue crack propagation, Inconel 718, hold times, grain boundary embrittlement, crack propagation modelling, Finite Element modelling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-85931 (URN)10.1016/j.ijfatigue.2012.10.018 (DOI)000315617500020 ()
Available from: 2013-03-13 Created: 2012-12-03 Last updated: 2017-12-07Bibliographically approved
2. Modelling of high temperature fatigue crack growth in Inconel 718 under hold time conditions
Open this publication in new window or tab >>Modelling of high temperature fatigue crack growth in Inconel 718 under hold time conditions
2013 (English)In: International Journal of Fatigue, ISSN 0142-1123, E-ISSN 1879-3452, Vol. 52, 124-130 p.Article in journal (Refereed) Published
Abstract [en]

Inconel 718 is a frequently used material for gas turbine applications at temperatures up to 650 °C. The main load cycle for such components is typically defined by the start-up and shut-down of the engine. It generally includes hold times at high temperatures, which have been found to have a potential for greatly increasing the fatigue crack growth rate with respect to the number of load cycles. However, these effects may be totally or partly cancelled by other load features, such as overloads or blocks of continuous cyclic loading, and the actual crack propagation rate will therefore depend on the totality of features encompassed by the load cycle. It has previously been shown that the increased crack growth rate found in hold time experiments can be associated with a damage evolution, where the latter is not only responsible for the rapid intergranular crack propagation during the actual hold times, but also for the increased crack growth during the load reversals. In this paper, modelling of the hold time fatigue crack growth behaviour of Inconel 718 has been carried out, using the concept of a damaged zone as the basis for the treatment. With this conceptually simple and partly novel approach, it is shown that good agreement with experimental results can be found.

Place, publisher, year, edition, pages
Elsevier, 2013
Keyword
Nickel-base superalloys, fatigue crack propagation, Inconel 718, hold times, grain boundary embrittlement, crack propagation modelling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-85933 (URN)10.1016/j.ijfatigue.2013.03.004 (DOI)000318831300013 ()
Available from: 2012-12-03 Created: 2012-12-03 Last updated: 2017-12-07Bibliographically approved
3. A load history dependent model for fatigue crack propagation in Inconel 718 under hold time conditions
Open this publication in new window or tab >>A load history dependent model for fatigue crack propagation in Inconel 718 under hold time conditions
2014 (English)In: Engineering Fracture Mechanics, ISSN 0013-7944, E-ISSN 1873-7315, Vol. 118, 17-30 p.Article in journal (Refereed) Published
Abstract [en]

Modelling of high temperature fatigue crack growth in Inconel 718 under the interaction of fast cyclic loading and hold times at maximum load has been conducted. A model, based on the concept of a damaged zone in front of the crack tip has been applied for three different temperatures, 550, 600 and 650 ◦C, with good agreement for both calibration and validation tests. A statistical evaluation of 22 tests in total was also conducted, which shows that the developed model gives a reasonable scatter factor at a probability of failure of 0.1 %.

Place, publisher, year, edition, pages
Elsevier, 2014
Keyword
Fatigue crack propagation, Inconel 718, Hold time effects, Crack growth modelling, Statistics
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-104809 (URN)10.1016/j.engfracmech.2014.02.005 (DOI)000334819600002 ()
Available from: 2014-02-27 Created: 2014-02-27 Last updated: 2017-12-05Bibliographically approved
4. Modelling of fatigue crack growth in Inconel 718 under hold time conditions - application to a flight spectrum
Open this publication in new window or tab >>Modelling of fatigue crack growth in Inconel 718 under hold time conditions - application to a flight spectrum
2014 (English)In: Advanced Materials Research, ISSN 1662-8985, Vol. 891-892, 759-764 p.Article in journal (Refereed) Published
Abstract [en]

Gas turbine operating cycles at high temperatures often consist of load reversals mixed with hold times; the latter occurring either as cruise for aero engines or at continuous power output for land based turbines, but also at low frequency loading conditions, e.g. slow “ramp up” of engine thrust. The hold time conditions cause the crack to grow by intergranular fracture due to material damage near the crack tip, thus rapidly increasing the crack growth rate. Since the damaged zone will affect the crack propagation rate due to cyclic loadings as well, the complete load history of a component therefore has to be considered. The crack propagation model presented in this paper is based on the damaged zone concept, and considers the history effect in the form of damaged zone build up during hold times, and subsequent destruction as the crack propagates onwards by rapidly applied load reversals. By incorporating crack closure for handling different R-values, an aero engine component spectrum is evaluated for a surface crack at 550 C. The result shows a good correlation to model simulation, despite the complexity of the load spectrum.

Place, publisher, year, edition, pages
Trans Tech Publications Inc., 2014
Keyword
Fatigue crack propagation, Inconel 718, Hold time effects, Crack growth modelling
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-104812 (URN)10.4028/www.scientific.net/AMR.891-892.759 (DOI)000337767700118 ()
Available from: 2014-02-27 Created: 2014-02-27 Last updated: 2016-05-18Bibliographically approved
5. Evaluation and prediction of crack length in a Ni-based superalloy for sustained loading
Open this publication in new window or tab >>Evaluation and prediction of crack length in a Ni-based superalloy for sustained loading
2014 (English)Manuscript (preprint) (Other academic)
Abstract [en]

Crack length evaluations for sustained loading have been conducted on Inconel 718 surface crack specimens for 550 and 650 ◦C. Hold times, e.g. slow ramp up of engine thrust, sustained load as continuous power output for stationary gas turbines or as cruise for aero engines, have been seen to result in material damage at the crack tip, causing the crack to grow by intergranular fracture and resulting in a strong increase of the crack growth rate. Here, sustained load tests as well as specially designed hold time tests are used to measure crack length and the size of the damaged zone (material damage) in front of the crack tip by comparing potential drop (PD) predictions with visible post-mortem beach marks on the fracture surface. Fracture surfaces were also investigated with scanning electron microscope (SEM), showing what the damaged zone most likely consists of. In addition, by applying a crack propagation model based on the damaged zone concept, crack length and damaged zone evaluations were conducted. The results show that the damaged zone must be taken into consideration, when evaluating post-mortem fracture surfaces if the test has included hold time loads, both when setting up post-mortem calibration curves and when evaluating crack propagation models.

Keyword
Fatigue crack propagation, Inconel 718, Sustained load, Hold time effects, Crack length evaluation
National Category
Engineering and Technology
Identifiers
urn:nbn:se:liu:diva-104813 (URN)
Available from: 2014-02-27 Created: 2014-02-27 Last updated: 2014-02-27Bibliographically approved

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