International Journal of Fatigue 158 (2022) 106740 Available online 17 January 2022 0142-1123/© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Accounting for crack closure effects in out-of-phase TMF crack growth with extended hold times Jordi Loureiro-Homs a, * , Per Almroth b , Frans Palmert b , David Gustafsson b , Kjell Simonsson a , Robert Eriksson a , Daniel Leidermark a a Division of Solid Mechanics, Link¨ oping University, Sweden b Siemens Energy, Finspång, Sweden A R T I C L E INFO Keywords: Crack propagation CM 247 Thermomechanical fatigue Turbine blade Crack closure Compliance Node release Analytical crack closure model ABSTRACT The crack growth behaviour of the alloy CM 247 LC is investigated for out-of-phase TMF and isothermal tests at the same temperature as the minimum temperature in the TMF tests. The results suggest that it is possible to characterise crack growth behaviour if experimental corrections for crack closure are accounted for. The repli- cation of these experimental tests using a numerical FE-solver results in similar crack growth behaviour, sug- gesting that the main mechanism in place is plasticity-induced crack closure. A pragmatic analytical model to characterise crack closure including hold time effects is proposed. The comparison of the response from this model with the experimental and numerical results suggests that the proposed analytical model is capable to approximate crack closure effects for cases where substantial creep deformation is to be expected. 1 Introduction The international efforts to reduce greenhouse gas emissions has focused mostly on the implementation and development of renewable energy sources such as solar and wind power. However, to always meet the demand of electricity these often need to be complemented with supplementary energy production or reliable energy storage solutions. For the former issue, gas turbines offer the flexibility needed to sup- plement demand when natural resources are not available [1,2]. Furthermore, gas turbines are not limited to fossil fuels and can also run with a variety of green fuels, including hydrogen and syngas, which can be sourced from power-to-fuel cycles creating in this way a CO2-neutral cycle [3]. There are though many engineering challenges ahead, as the coupling with intermittent sources requires a different operation strat- egy for gas turbines: increased cyclic capabilities, faster start-ups, increased efficiency, and reliability, among others. For turbine components, these new requirements are often related to higher inlet temperatures, larger thermal gradients and extended cyclic capabilities. Under such loading conditions, the life of a turbine component is usually limited by thermomechanical fatigue (TMF). To accommodate for the new requirements, turbine components are required to operate close to their end-of-life, and this often means that crack propagation stages, in addition to crack initiation, need to be included. The control of the behaviour of a growing crack provides in- formation that can be used to inform design and maintenance decisions, extending this way the operation life of the components. All this requires the development of robust TMF crack propagation testing methodolo- gies [4–6] and numerical models that are readily usable for industrial applications. Since the early seventies, when Elber [7,8] showed the influence of crack closure in fatigue crack growth, a great amount of work has been done highlighting the importance of this phenomenon and has been used to rationalise the behaviour of fatigue crack growth under the influence of different load ratios and load levels. Previous work on nickel-based superalloys suggests a strong dependency on crack closure to explain different load ratios under in-phase loading (IP) conditions [9]. Simi- larly, under out-of-phase loading conditions, the influence of crack closure can explain to a large degree the behaviour of crack growth [10,11]. Crack closure has been shown to explain crack growth behav- iour even for cases with prolonged hold times in IP conditions [12]. Since the use of numerical FE-based approaches to model crack closure are prohibitive and seldomly practical in industrial applications, several analytical models have been proposed over the years which enable the estimation of plasticity-induced crack closure within the * Corresponding author. E-mail address: [email protected] (J. Loureiro-Homs). Contents lists available at ScienceDirect International Journal of Fatigue journal homepage: www.elsevier.com/locate/ijfatigue https://doi.org/10.1016/j.ijfatigue.2022.106740 Received 7 July 2021; Received in revised form 10 December 2021; Accepted 11 January 2022
Accounting for crack closure effects in out-of-phase TMF crack growth with extended hold times
May 23, 2023
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