PLIM - Thermo-Mechanical and Multiaxial Fatigue caused by Cyclic Thermal Shocks

The current PLiM project (2014-2017) is funded by swissnuclear and is dealing with the development of improved advanced lifetime prediction models for thermo-mechanical fatigue (TMF) in austenitic stainless steel piping under complex thermo-hydraulic boundary conditions, and with their validation by specially designed fatigue tests under plant-relevant conditions. Specific thermo-hydraulic conditions, e.g. turbulent mixing of hot and cold water, may lead to cyclic thermal shocks (CTS) in the primary circuit piping of light water reactors. This may result in crack initiation, possibly followed by crack growth and failure of the component. In spite of the large body of scientific investigations worldwide in this area, the exact conditions which lead to thermal fatigue and associated development of crack networks, caused by turbulent mixing, are not yet fully understood, and more research is needed in both thermal-hydraulics and structural mechanics. The main questions are:

• Under which flow conditions do the thermal fluctuations appear?
• What are critical locations for crack formation in the primary loop of light-water reactors?
• What are the temperature differences ΔT and frequencies of these fluctuations?
• Which conditions (ω, ΔT, mean temperature T_m, pressure) have to be fulfilled for the initiation and growth of thermally induced cracks?
• How deep do these cracks grow? Under what conditions do they arrest?
• What are the reliable materials models for thermal fatigue life prediction?

The major goal of the current PliM project at PSI is to answer the above questions, which are addressed by experimental investigations and numerical simulations of the complex fluid dynamics and the thermal stresses induced in the piping material. The overall aim of these investigations is to acquire a better understanding of the governing mechanisms and to improve numerical tools for the prediction of components’ lifetime. For the investigation of crack initiation and growth in austenitic stainless steels due to CTS, a unique thermo-shock facility allows to load test specimens simultaneously with CTS and biaxial mechanical forces under well-defined and realistic boundary conditions. Furthermore, multiaxial high cycle fatigue experiments are carried out in order to explore the influence of multiaxial proportional and non-proportional loading on the lifetime of components. Besides the experimental work, the ongoing numerical modeling concentrates on the simulation of turbulent fluid flow and the concerning temperature fluctuations in mixing tees. However, the main effort is on the advanced description of cyclic plasticity by considering the materials microstructure and the simulation of component failure due to the cyclic thermal shocks. Therefore, crystal plasticity theories are applied in combination with the finite element method.