THERRI

Determination of parameters for the evaluation of thermal fatigue crack growth in power plants

Background / Objectives

In the wake of the energy transition and the associated changes in the energy market, the technical load requirements for fossil-fuel power plants have changed fundamentally. Modern power plants must now be able to operate flexibly to compensate for the residual load, i.e., the load that cannot be provided by renewable energies.

Due to the increased and therefore necessary number of start-up and shut-down processes and the associated temperature and internal pressure fluctuations, the technical material is subjected to greater stress than ever before. Fatigue damage resulting from the increasing cyclic loading thus becomes enormously important for the power plant operator, whereas the impact of creep stress is diminishing due to the shorter residence times at high operating temperatures.

To ensure maximum safety and smooth operation, reliable predictions of components (e.g., failure probabilities or material status) are an important step toward modern power plants. This particularly focuses on thick-walled power plant components in the feedwater and main steam systems, as they are subject to particularly high levels of stress.

The THERRI project

An effective method for assessing high levels of fatigue in components subjected to alternating loads is fracture mechanics crack growth analysis. This approach, beyond the standard-based fatigue analysis, allows for the identification of additional service life reserves and thus extended service life without compromising the required safety factors.

In the  THERRI (Thermal Fatigue Crack Growth) project,  this topic was addressed from experimental technology and finite element modeling to the development of a new set of standards. The fracture mechanics tests at IEK-2 were conducted both in air and in a water vapor-containing atmosphere (H2O/Ar) in the temperature range of 300 to 600 °C to examine the environmental influence on the crack growth behavior of the steels under investigation and thus enable the transfer of laboratory results to practical applications in power plants.

Fatigue and creep cracks in power plant applications

Fatigue cracks occur due to frequent load changes, while creep cracks occur at consistently high temperatures.

This means that at low temperatures, fatigue cracks mainly propagate due to alternating loads, while above 500 to 600 °C, creep cracks become more important.

An important objective of the THERRI research project was to determine the lower temperature limit of the relevant influence of creep cracks.

Another goal of the research project was to determine load-dependent inspection intervals tailored to current or planned operation. Furthermore, an evaluation methodology was developed that, unlike traditional fatigue analysis, largely does not require knowledge of the previous operating history. This opens up a wide range of international applications, as online monitoring systems for recording lifetime consumption are used only to a limited extent in many thermal power plants worldwide.