Thermal power plants must be operated with a much greater degree of flexibility, now and in the future, to balance the heavily fluctuating infeed of renewable energy into the power network. As a result of this significant change in the requirements of power plant components which were originally designed for static loads, a heavy impact on the fatigue load and interaction between creep and fatigue is to be expected: thermal loads are induced particularly in thick-walled components when the plant is started up and shut down, which accelerate damage to the material through an additional fatigue element.
The design and calculation of pressure-bearing parts of water tube boilers and system components is set out in the EN 12952-3 standard, but this does not take account of the interactions of creep fatigue. Assessment of the actual lifetime consumption through creep fatigue is carried out on the basis of EN 12952-4. The current approach under this standard provides for a linear accumulation of the amount of damage from creep and fatigue loads, whereby the element of creep damage is determined by the life-fraction rule and the fatigue fraction by means of a virtual stress range to be determined in accordance with EN 12952-3 and a reference lifetime curve independent of material. Another approach to evaluating the proportion of fatigue damage is based on the generalized damage accumulation hypothesis, for which a reference lifetime curve for pure fatigue depending on material and load is required.
An analysis of creep fatigue experiments on materials typically used in power plants shows that the interaction between creep and fatigue is not only dependent on material but also on load, which cannot be reflected in the phenomenological concepts mentioned above without further ado. For a reliable lifetime prediction under creep fatigue loading, a lifetime model based on fracture mechanics has therefore been developed at the Fraunhofer IWM, which can describe the propagation of cracks due to fatigue and due to creep separately and in their interaction. Application of the new lifetime model to materials typically used in power plants shows a significantly better lifetime prediction than conventional concepts. The model formulation selected allows both the lifetime until technical cracking (i.e. until a crack length of 1.5 mm) and the residual lifetime of components with cracks under creep fatigue loading to be assessed.