Research Papers

On the Evaluation and Consideration of Fracture Mechanical Notch Support Within a Creep-Fatigue Lifetime Assessment

[+] Author and Article Information
Christian Kontermann, Falk Müller, Matthias Oechsner

Chair and Institute of Materials Technology,
Technical University Darmstadt,
Darmstadt D-64283, Germany

Henning Almstedt

Siemens AG Power and Gas Division,
Muelheim an der Ruhr D-45478, Germany

Manuscript received June 22, 2018; final manuscript received June 26, 2018; published online October 31, 2018. Editor: Jerzy T. Sawicki.

J. Eng. Gas Turbines Power 140(12), 121014 (Oct 31, 2018) (9 pages) Paper No: GTP-18-1309; doi: 10.1115/1.4040733 History: Received June 22, 2018; Revised June 26, 2018

Changes within the global energy market and a demand for a more flexible operation of gas- and steam-turbines lead to higher utilization of main components and raise the question how to deal with this challenge. One strategy to encounter this is to increase the accuracy of the lifetime assessment by quantifying and reducing conservatisms. At first the impact of considering a fracture mechanical notch support under creep-fatigue loading is studied by discussing the results of an extensive experimental program performed on notched round-bars under global strain control. A proposal of how to consider this fracture mechanical notch support within a lifetime assessment is discussed within the second part of the paper. Here, a theoretical finite element method (FEM)-based concept is introduced and validated by comparing the theoretical prediction with the results of the previously mentioned experimental study. Finally, the applicability of the developed and validated FEM-based procedure is demonstrated.

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Fig. 1

(a) Sketch of a notched component and a specimen with identical local equivalent notch root loading, (b) trends of equivalent stresses as a function of surface distance, and (c) resulting early crack growth and resulting load cycle difference, respectively fracture mechanical notch support as a function of the technical crack depth

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Fig. 2

(a) Experimental setup, (b) specimen geometries and determined stress concentration factors respectively normalized stress gradients

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Fig. 3

Comparison of the early crack growth behavior of two differently sharp notches under identical equivalent local notch root loading

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Fig. 4

Crack initiation diagram including results of a notched test series—criterion: a = 0.2 mm

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Fig. 5

Crack initiation diagram including results of a notched test series—criterion: a = 1.0 mm

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Fig. 6

Load cycle based and purely experimentally quantified fracture mechanical notch support factor

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Fig. 7

Illustration of transient crack closure for cracks developing and growing within a stress/strain gradient field

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Fig. 8

Workflow of the developed fracture mechanics approach

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Fig. 9

On the FEM-based determination of Saxena's Ct -parameter for a tension holdtime under global strain control

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Fig. 10

Comparison of the load cycle based notch support factor evaluated by experimental measurements and theoretically determined

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Fig. 11

Boundary conditions and resulting axial stress distribution for the two states directly after opening (a) and directly after closing (b)

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Fig. 12

Predicted early creep-FCG of the valve stem notched cross section by utilizing the fracture mechanical approach as function of load cycles (a) and as function of cumulative tension holdtime (b)



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