Low cycle thermomechanical fatigue of VVER-440 reactor pressure vessel steels: Investigation the fatigue kinetics and development of a life assessment model

Abstract The fatigue life of the structural materials 15Ch2MFA (CrMoV-alloyed ferritic steel) and 08Ch18N10T (CrNi-alloyed, Ti-stabilized austenitic steel) of the VVER-440 reactor pressure vessels was investigated under fully reversed total strain controlled low cycle fatigue tests. The measurements were carried out in isothermal conditions at 260 °C and with thermal-mechanical conditions in temperature range of 150 to 270 °C using the GLEEBLE 3800 servo-hydraulic thermal-mechanical simulator. Owing the nominal fatigue lifetime for different testing conditions interrupted fatigue tests were carried out to investigate the kinetics of the fatigue evolution. Microstructural evaluation of the samples was performed using transmission electron microscopy as well as X-ray diffraction, and measurement of the dislocations was completed. The course of dislocation density in relation to cumulative usage factor was similar for both materials. However, the nature and distribution of dislocations were different in the individual steels and this resulted in different mechanical behaviours. Using scanning electron microscopy the crack shapes and fracture surfaces were observed and analysed. Crack propagation was assessed in relation to the actual crack size and the loading level. A new low cycle fatigue criterion is presented based on the stored energy, which accumulates in the material during fatigue loading. The new damage parameters are based on the assumption that only the stored part of the introduced energy causes the changes in the microstructure. The proposed model is physically consistent and its prediction accuracy is higher than by the classical strain amplitude and strain energy based approaches. The low cycle fatigue behaviour investigated with the developed engineering model can provide a reference for the remaining life assessment and possible operation life extension of nuclear power plant components.