THERRI - Thermal fatigue crack growth

In the face of the energy transition and the associated changes in the energy market, the technical load requirements for fossil power plants have changed fundamentally nowadays. Modern power plants must now be able to operate flexibly to compensate 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 due to increasing alternating stress is thus of enormous importance for the power plant operator, whereas the influence of creep rupture stress is reduced due to shorter dwell times at high operating temperatures.

To ensure maximum safety and smooth operation, reliable predictions on components (e.g. failure probabilities or material status) are an important step towards modern power plants. The focus is on thick-walled power plant parts in the feed water and fresh steam system, as they are subject to particularly high loads.

Fracture mechanics crack growth analysis provides an effective way of assessing high degrees of fatigue of components subject to load cycling. In addition to the rule-based fatigue analysis, this allows additional service life reserves of the component and thus service life extensions to be tapped without reducing the required safety margins

In the project THERRI (THermal Fatigue RIS Growth), this subject area was dealt with starting from experimental engineering to finite element modelling up to the development of a new set of rules. The fracture mechanical tests on the IEK-2 were carried out both in air and in an atmosphere containing water vapour (H2O/Ar) in a temperature range of 300 to 600 °C in order to test the environmental influence on the crack growth behaviour of the steels examined and thus enable the laboratory results to be transferred to practical application in power plants.

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

This means: At low temperatures, fatigue cracks mainly spread due to alternating loads, while creep cracks above 500 to 600 °C increase in importance.

An important goal of the research project THERRI 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 customized to the current or planned operation. In addition, an evaluation methodology was developed which, in contrast to the classical fatigue analysis, does not require an extended knowledge of the operating history to date. This results in a wide range of international applications, as online monitoring systems are only used to a limited extent in many thermal power plants worldwide to record service life consumption.