Numerical studies of the residual lifetime of power plant components based on experimental results at elevated temperatures

Available online at www.sciencedirect.com Available online at www.sciencedirect.com

ScienceDirect ScienceDirect

Structural Integrity Procedia 00 (2017) 000–000 Available online www.sciencedirect.com Available online at at www.sciencedirect.com Structural Integrity Procedia 00 (2017) 000–000

ScienceDirect ScienceDirect

Procedia Structural (2017) 869–874 Structural IntegrityIntegrity Procedia500 (2016) 000–000

www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia

www.elsevier.com/locate/procedia

2nd International Conference on Structural Integrity, ICSI 2017, 4-7 September 2017, Funchal, 2nd International Conference on Structural Integrity, ICSI 2017, 4-7 September 2017, Funchal, Madeira, Portugal Madeira, Portugal

Numerical studies of the residual lifetime of power plant components based on experimental results at elevated temperatures components based on experimental results at elevated temperatures Thermo-mechanical modeling of a high pressure turbine blade of an Maria Paarmann*, Patrick Mutschler, Manuela Sander Maria Paarmann*, Patrick Manuela Sander airplane gasMutschler, turbine engine University of Rostock, Institute of structural Mechanics, Albert-Einstein-Str. 2, 18059 Rostock

XV Portuguese Conference on Fracture, 2016, 10-12 February Paço plant de Arcos, Portugal Numerical studies of thePCF residual lifetime of2016, power

University of Rostock, Institute of structural Mechanics, Albert-Einstein-Str. 2, 18059 Rostock

P. Brandãoa, V. Infanteb, A.M. Deusc*

Abstracta AbstractDepartment of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, Portugal

b the future, the energy supply will strongly fluctuate, which results in more frequently load cycles. For this reason, In IDMEC, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, In the future,to theanalyze energy fatigue supply will strongly which results in more frequently loadincycles. Forpower this reason, Portugal it is necessary crack growthfluctuate, under the aspect of reasonable load cases relevant plant c CeFEMA, Department of Mechanical Engineering, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1, 1049-001 Lisboa, it is necessary to analyze fatigue crack growth under the aspect of reasonable load cases in relevant power plant components. Portugal components. Therefore, numerical three-dimensional fatigue crack growth simulations for different power plant components Therefore, numerical three-dimensional fatigue crack growth simulations for different components have been performed using FRANC3D taking temperature transients and pressure loads as power well as plant different relevant have been performed using FRANC3D taking temperature transients and pressure loads as well as different relevant crack positions into account. The results show that the temperature gradient of thermal loadings has a large influence Abstract crack positions into account. The results show that the temperature gradient of thermal loadings has a large influence on stress intensity factors (SIF) and may lead to much higher SIF than under pure mechanical loading. onDuring stress intensity factors (SIF) and may lead temperature-dependent tocomponents much higherareSIF thanfatigue under pure mechanical loading. their operation, modern aircraft engine subjected to increasingly demanding operating conditions, In order to quantify the residual lifetime, crack growth curves for different stress especially the high pressure turbine (HPT) blades. Such conditions cause these parts to undergo different types of time-dependent In order to quantify the residual lifetime, temperature-dependent fatigue crack growth curves for different stress ratios were experimentally determined. Therefore, experiments with constant temperatures between room temperature degradation, one of which isdetermined. creep. A model using theexperiments finite elementwith method (FEM) was developed, in order to betemperature able to predict ratios were experimentally Therefore, constant temperatures between room and 600°C have been performed using C(T)-specimens cut of a decommissioned high-pressure bypass station made the600°C creep have behaviour HPT blades. Flight data records cut (FDR) a specific aircraft, provided bybypass a commercial aviation and been of performed using C(T)-specimens of afor decommissioned high-pressure made of the ferritic-martensitic steel thermal X20CrMoV12-1. The investigations show that crack growth ratestorise instation the company, were used to obtain and mechanical data for three different flight cycles. In order create the PARIS3D model ofneeded the ferritic-martensitic steel aX20CrMoV12-1. The investigations show thata(N)-curves crack growth rates rise inproperties theresults PARISregime under data were used extract from numerical in for thehigher FEM temperatures. analysis, HPTThe blade scrap wasfinally scanned, andtoits chemical composition and material were regime under higher temperatures. The data were finally used to extract a(N)-curves from numerical results obtained. The data that was gathered was fed into the FEM model and different simulations were run, first with a simplifiedin3D FRANC3D. FRANC3D. rectangular block shape, in order to better establish the model, and then with the real 3D mesh obtained from the blade scrap. The

overall expected behaviour in by terms of displacement was observed, in particular at the trailing edge of the blade. Therefore such a © 2017 2017The The Authors. Published Elsevier © Authors. Published by Elsevier B.V. B.V. can under be useful in the goalby ofElsevier predicting turbine blade life, given a set of FDR data. © model 2017 The Authors. Published B.V. Peer-review responsibility of the Scientific Committee ICSI 2017. Peer-review under responsibility of the Scientific Committee of ICSIof 2017 Peer-review under responsibility of the Scientific Committee of ICSI 2017. © 2016 thermal The Authors. bycomponents; Elsevier B.V. Keywords: loading;Published power plant residual lifetime prediction Peer-review under responsibility the Scientific Committee of PCF 2016. Keywords: thermal loading; power plantof components; residual lifetime prediction Keywords: High Pressure Turbine Blade; Creep; Finite Element Method; 3D Model; Simulation.

* Corresponding author. Tel.: +49-381-498-9340; fax: +49-381-498-9342. * Corresponding Tel.: +49-381-498-9340; fax: +49-381-498-9342. E-mail address:author. [email protected] E-mail address: [email protected] 2452-3216 © 2017 The Authors. Published by Elsevier B.V. 2452-3216 © 2017 Authors. Published Elsevier B.V. Peer-review underThe responsibility of theby Scientific Committee of ICSI 2017. * Corresponding Tel.: +351of218419991. Peer-review underauthor. responsibility the Scientific Committee of ICSI 2017. E-mail address: [email protected] 2452-3216 © 2016 The Authors. Published by Elsevier B.V.

Peer-review under responsibility of the Scientific Committee of PCF 2016. 2452-3216  2017 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the Scientific Committee of ICSI 2017 10.1016/j.prostr.2017.07.108

Maria Paarmann et al. / Procedia Structural Integrity 5 (2017) 869–874 M. Paarmann et al / Structural Integrity Procedia 00 (2017) 000–000

870 2

1. Introduction In future, renewable energies shall substitute fossil power stations. This will lead to a much more flexible operation of conventional power plants, which in turn leads to enhanced cyclic stresses. Therefore, fatigue and fatigue crack growth become much more, and the influence of creep fatigue becomes less important. These new load cases were not considered when the power plants were designed. To minimize risks, the influence of these loads on the power plant components has to be evaluated. For investigating fatigue crack growth the knowledge of the load in the component and the strength of the material are very important. Thus, crack growth experiments were carried out to get a reliable database to describe the fatigue crack growth at elevated temperatures. This is important, because the power plant components operate at elevated temperature and the temperature has an enormous influence on the crack growth rate, which is shown e.g by Petit, Henaff and SarrazinBaudoux (1999), Chen, Kawagoishi and Nisitani (2000) and UEMATSU et al. (2008). The load in the power plant components is determined with numerical linear-elastic simulations. The simulations are carried out exemplarily at three relevant components. For one component the results are shown in detail. The numeric simulations deliver e.g. the stress intensity factor (SIF) solution for diverse crack positions and load cases. Finally as a result, residual lifetimes can be predicted starting from an initial crack for the selected components. 2. Experiments for describing material behavior under elevated temperatures To simulate crack growth in power plant components, it is necessary to know and to describe their temperature dependent crack growth behavior. Therefore, fatigue crack growth tests were carried out with C(T)-specimens, which have been machined in different orientations from a decommissioned high pressure bypass valve made of X20CrMoV12-1. Thus, various crack orientations can be evaluated separately. Beside the orientation, the test frequency, the normalized K-gradient (Test Method for Measurement of Fatigue Crack Growth Rates, 2015), R-ratio and the temperature were varied. The main influencing factors are temperature and R-ratio. The other parameters have no or just a minor influence on the crack growth rate. For all investigated temperatures, the crack growth tests have shown that the R-ratio has almost no influence in the PARIS-regime. In the threshold range higher R-ratios lead to lower threshold values. This expected trend is observed for all investigated temperatures. However, the influence of the R-ratio decreases for increasing temperatures. Fig. 1 exemplarily shows mean curves of the crack growth data for R = 0.1 and R = 0.5 for the investigated temperatures. Both R-ratios show that the crack growth rates in the PARIS-regime increase for increasing temperatures. Further, it is noticeable that the threshold values in Fig. 1a of the elevated temperatures are significantly lower than the threshold value of the tests at room temperature. But, for higher temperatures the trend of decreasing threshold values is not existent. a) b) R = 0.1

R = 0.5

Fig. 1. (a) Mean curves of the crack growth tests for R = 0.1, and (b) for R = 0.5 (Mutschler and Sander (2016))



Maria Paarmann et al. / Procedia Structural Integrity 5 (2017) 869–874 M. Paarmann et al./ Structural Integrity Procedia 00 (2017) 000–000

871 3

3. Numerical crack growth simulations on power plant components On the basis of the experiments, power plant components were simulated numerically. Therefore, uncracked models were built in ABAQUS to import them in FRANC3D. In this program quasi-static crack growth simulations were executed starting at a user defined initial crack front. The examined components are a ball-shaped part, a turbine bypass valve and a boiler circulation pump (Fig. 2). Within the scope of this paper, the results for the ball-shaped part are exemplarily presented. There are two relevant positions for crack growth initiation (Fig. 2a). Both were identified from XFEM-simulations on the one hand under constant inner pressure and on the other hand under transient thermal loading (Schulz et al., 2014). Crack initiation position F results from the inner pressure, while position E follows from transient thermal loading. The simulated scenarios (sc.) contain an emergency shutdown (from 545°C to 50°C) within three minutes as a worst case situation (scenario 1.1). Moreover, the influence of the temperature gradient was investigated by decreasing the temperature from 545°C to 50°C within thirty minutes (scenario 1.2) and decreasing the temperature from 545°C to 300°C also within three minutes (scenario 1.3). The inner pressure of 26.6 MPa was simulated as steady state scenario 2. Superimposed thermal and mechanical loading was also investigated (see (Paarmann and Sander, 2016)). a)

Initiation position F

b)

c)

Initiation position E

Fig. 2. (a) Model and crack initiation positions in the ball-shaped part; (b) model of the simulated turbine bypass valve; (c) modelled sector of the boiler circulation pump

All simulations started with a semi-circular crack, which had an initial crack depth a0 = 5 mm. The cyclic stress intensity factors (SIF) were calculated for all scenarios with a stress ratio of R = 0. The investigations on both crack initiation positions lead to similar results. In Fig. 3 the results of pure thermal loadings normalized on scenario 2 are shown for crack initiation position F. It can be recognized that pure mechanical loading (scenario 2) leads to significant smaller stress intensity than pure thermal loading within a short shutdown duration (scenario 1.1 and 1.3). The slight differences between scenario 2 and 1.2 also underline the large influence of the temperature gradient on crack growth. While the SIF of scenario 2 (see (Paarmann and Sander, 2016)) rises, the SIF of thermal influenced scenarios decreases over the progressing crack growth. It originates from a drop of the stresses over the wall thickness due to a decreasing temperature. A comparison of scenario 2 and 1.2 also shows that beginning at a crack depth of about 27 mm the temperature leads at the deepest point of the crack front to smaller SIF than under pure mechanical loading.

Maria Paarmann et al. / Procedia Structural00 Integrity (2017) 869–874 M. Paarmann et al / Structural Integrity Procedia (2017)5000–000

4872

a) 60

c) 60

b) 60

KI / KI,Sc2

Sc 1.1 Sc 1.2

40

40

20

20

20

0

0

0

10

20 a [mm]

30

40

Sc 1.3

10

20 a [mm]

30

10

20 a [mm]

30

Fig. 3. Comparison of cyclic SIF over crack growth for different temperature transients on (a) the surface point in the sphere, (b) the deepest point of crack front and (c) the surface point in the connector

In order to investigate the influence of the crack shape, a circumferential crack was inserted as initial crack front for scenario 1.1. Due to numerical problems the simulations of the semi-elliptical cracks stop at a crack depth of 32.8 mm. The cyclic SIF of the circumferential crack is about 25% higher than the SIF of the semi-elliptical crack on its deepest point (A). On the other hand the SIF of the surface point C of the semi-elliptical crack front is nearly constant and larger than the SIF of the circumferential crack starting at a = 15mm. This results from higher thermal loadings at smaller crack depth. The comparison underlines that cracks grow faster in the width than in the depth direction and crack growth gets slower with increasing wall thickness depending on the loading. 80 semi-elliptical A

KI / KI,Sc2

60

semi-elliptical C circumferential

40 20 0 10

20

30

40

50

60

a [mm] Fig. 4. Comparison of cyclic SIF over crack depth of semi-elliptical and circumferential crack shapes on position E for scenario 1

4. Lifetime prediction of the ball-shaped part based on experimental and numerical results With the experimental and numerical results from chapters 2 and 3 lifetime predictions are achievable. Therefore, the cyclic SIF ΔK in the cases of scenarios 1 and 2 was defined with a stress ratio of R = 0. For scenario 2, Fig. 5 shows exemplarily the necessary temperature dependent crack depth for crack growth. It becomes obvious that only for position F crack growth starts at the initial crack size. For position E the temperature dependent threshold values ΔKth are exceeded for crack depths between 13 mm and 17 mm. For that reason a(N)-curve were only determined for position F starting at the initial crack size a0 = 5 mm. Moreover, investigations for the steady state scenario 2 at different temperatures were made to test the influence of constant temperature on the residual lifetime of an actual use case. Fig. 6 shows a(N)-curves for the experimentally analyzed temperatures.



Maria Paarmann et al. / Procedia Structural Integrity 5 (2017) 869–874 M. Paarmann et al./ Structural Integrity Procedia 00 (2017) 000–000

a)

4

b) 4

3

3

873 5

K / Kth,RT [ ]

K / Kth,RT [ ]

point A

2 ΔKth,500

1

ΔKth,400 ΔKth,300

0

10

20

30 a [mm]

ΔKth,600

2 1 0

40

point C

10

20

30 40 a [mm]

50

Fig. 5. Crack growth start at front point A and C depending on temperature for (a) position F and (b) position E

The curves are based on the FORMAN/METTU equation and the parameters determined from the experimental data. Another base of the lifetime prediction is the SIF-solution numerically determined in FRANC3D (chapter 3). In the current case the SIF for the largest crack size does not exceed the critical SIF. Nevertheless, the curves underline faster crack growth on the surface point than on the deepest point of the crack front. The figure also shows that high temperatures result in a smaller number of cycles N at the critical crack size ccrit of T = 600°C. For room temperature for example N is about thirteen times higher than at T = 600°C. It underlines the importance of investigating temperature dependent residual lifetime in power plant components. a)

60

RT

300°C

400°C

500°C

600°C

a [mm]

50 40 30 20 10 0

0

0.5

1

1.5

2 N[]

2.5

2 N[]

2.5

3

3.5

4 x 10

6

b) 60

ccrit

c [mm]

50 40 30 20 10 0

0

0.5

1

1.5

3

3.5

4 x 10

6

Fig. 6. Comparison of temperature dependent crack growth curves in the ball-shaped part for scenario 2 at (a) the deepest crack front point A and (b) the surface point C of position F

874 6

Maria Paarmann et al. / Procedia Structural Integrity 5 (2017) 869–874 M. Paarmann et al / Structural Integrity Procedia 00 (2017) 000–000

5. Conclusion Because of the strong fluctuation of the energy supply, the fossil power plants are cyclically loaded. For assure a safe life, residual lifetime calculations must be performed. On the example of a ball-shaped part, numerical crack growth simulations were made using FRANC3D. Next to mechanical loading, pure thermal loading with different temperature gradients were applied. The results show a large dependency of the SIF from this gradient. Moreover, the thermal loading leads to stresses over the wall thickness, which results in a decreasing SIF over the crack depth. As opposed to this, the SIF of pure mechanical loading rises with crack propagation. The numerical results were also used for lifetime prediction. Therefore, experiments were made to determine temperature dependent crack growth parameters using the FORMAN/METTU equation. On the example of pure mechanical loading the influence of temperature was investigated by applying the parameters for different temperatures. Based on high threshold values, crack growth would not start until high crack lengths. Moreover, the results show a higher crack growth rate for high temperatures. Acknowledgements The authors thank the German Ministry for Economic Affairs and Energy for funding the joint project THERRI (English: Determination of characteristic values for estimating thermal fatigue crack growth in power plants, German: Ermittlung von Kennwerten zur Bewertung thermischen Ermüdungsrisswachstums in Kraftwerken) Further, the authors greatly acknowledge the support by the partners TÜV NORD SysTec GmbH & Co. KG, the chair of Technical Thermodynamics at the University of Rostock, the KNG power plant Rostock and the research institute Jülich. References Chen, Q., Kawagoishi, N. and Nisitani, H. (2000) ‘Evaluation of fatigue crack growth rate and life prediction of Inconel 718 at room and elevated temperatures’, Materials Science and Engineering: A, 277(1-2), pp. 250–257. doi: 10.1016/S0921-5093(99)00555-9 Mutschler, P. and Sander, M. (2016) ‘Investigation of fatigue crack growth in a power plant steel under elevated temperatures’, Procedia Structural Integrity, 2, pp. 801–808. doi: 10.1016/j.prostr.2016.06.103 Paarmann, M. and Sander, M. (2016) Numerische Untersuchungen zur Rissausbreitung in Kraftwerkskomponenten für betriebsnahe Belastungsszenarien. Petit, J., Henaff, G. and Sarrazin-Baudoux, C. (1999) ‘Mechanisms and Modeling of Near-Threshold Fatigue Crack Propagation’, Fatigue Crack Growth threshold, Endurance Limits, and Design, ASTM STP 1372. Schulz, A. et al. (2014) Prüfkonzepte für ermüdungsführende Komponenten unter den Bedingungen eines flexiblen Kraftwerksbetriebes (SIC – Smart Inspection Concept), 46. Kraftwerkstechnisches Kolloquium 2014. Oktober 2014. Test Method for Measurement of Fatigue Crack Growth Rates (2015). West Conshohocken, PA: ASTM International. UEMATSU, Y. et al. (2008) ‘Effect of temperature on high cycle fatigue behaviour in 18Cr–2Mo ferritic stainless steel’, International Journal of Fatigue, 30(4), pp. 642–648. doi: 10.1016/j.ijfatigue.2007.05.004