Abstract
Understanding of crack growth behaviour is necessary to predict accurate fatigue lives. Out-of-phase thermomechanical fatigue crack propagation tests were performed on FB2 steel used in high-temperature steam turbine sections. Testing results showed crack closure where the compressive part of the fatigue cycle affected crack growth rate. Crack closing stress was observed to be different, and had more influence on the growth rate, than crack opening stress. Crack growth rate was largely controlled by the minimum temperature of the cycle, which agreed with an isothermal crack propagation test. Finite element models with stationary sharp cracks captured the crack closure behaviour.
Highlights
As power production turbines to a greater extent supports renewable energies, the turbines are required to manage flexible operation with faster loading ramps [1,2]
The crack propagation behaviour was investigated for the 9–12 % Cr martensitic steel FB2 which is commonly used at the high-temperature section of steam turbines
A difference was seen between the crack opening stress and the crack closing stress which was more pronounced at short crack lengths and at high me chanical strain ranges
Summary
As power production turbines to a greater extent supports renewable energies, the turbines are required to manage flexible operation with faster loading ramps [1,2]. The frequent start-ups with fast ramping rates put several components under large thermal stresses due to temperature gradients, especially in thick-walled components at the high- and intermediate-pressure turbine sections [1,3]. Under such loading conditions, i.e. thermomechanical fatigue (TMF) loading, the turbine component’s life need to be determined with ac curate and less conservative fatigue life prediction models to allow for more start-ups. The controlled growth of cracks provide a useful tool to extend operation and avoid unnecessary waste of resources within safe limits This emphasises the need for crack propagation models. The main focus is on the inner section of the steam turbine casing, where an out-of-phase (OP) type of TMF loading is present
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