Abstract
The change in operation of power plants due to the increasing use of renewable energies causes additional stresses to the components by a high number of smaller load cycles. This fact results in a demand for validated new approaches to estimate fatigue life especially for welded joints that are the weak parts within the piping. The components are operated in the LCF regime where short fatigue crack growth determines the life. Therefore, a non-linear fracture mechanics based approach is appropriate. For the development and validation of the model, an experimental campaign was performed including fatigue tests of smooth specimens with various microstructures of X6CrNiNb18-10 (AISI 347) as well as specimens containing mechanical and microstructural notches. Experiments are performed with all types of specimens with an increasing complexity from constant to variable amplitude loading, the latter also applied as pseudo-random sequence derived from measurements in power plants. The developed non-linear fracture mechanics based model uses the effective cyclic J-Integral normalized to the crack length to replace crack growth calculation by a linear damage accumulation. To consider the loading history an algorithm for the calculation of crack opening and crack closure is used. The advantages of this approach are evident when comparing calculated with experimentally determined fatigue lives. Remaining differences serve for further considerations of how to improve the life prediction for variable amplitudes.
Highlights
The fluctuating production of renewable energies compared with a cyclically varying energy consumption mainly induced by day-night-rhythm led to a flexible operation of conventional power plants
Conventional power plants have to cover the gaps in energy consumption between day-night-rhythm, but even between a windy cloudless summer day when solar and wind are able to cover the whole energy needs and a windless cloudy winter day when solar and wind do not contribute the production of energy
Without the possibility to store the surplus of energy for peak-periods, conventional power plants still need to be present
Summary
The fluctuating production of renewable energies compared with a cyclically varying energy consumption mainly induced by day-night-rhythm led to a flexible operation of conventional power plants. Welded and notched specimens are tested at 180°C which is the average of the temperatures measured during shutdown of a power plant at the component of interest (see Fig. 1). The tests under eight stage amplitude loading are performed with the amplitudes and their frequency of occurrence derived from the temperature measurement of a component during shutdown of a nuclear power plant (see Fig. 1, left). The latter sequence is superimposed with a thermal cycle of 50-350°C for the operational load TMF tests.
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