Numerical simulation of low-cycle fatigue test of welded 9Cr-1Mo steel at 550 °C

The integrity of the nuclear reactor components must be guaranteed under operating and emergency conditions at all times. All possible degradation mechanisms should be evaluated by the structural integrity analysis in the design stage and later during maintenance. Fatigue is one of the most common degradation mechanisms. It weakens the properties of the material, which has a significant impact on the lifetime of the components. Experimentally, fatigue is investigated by the determination of the S-N curve, otherwise called the fatigue curve. However, experimental determination of the S-N curve is slow and requires a lot of time and money and, at some times, difficult to implement because of specific environment requirements. Therefore, numerical simulations of fatigue tests can be useful in research of specimen behavior under cyclic loading. The fatigue testing matrix can be developed on the results of fatigue modeling using the finite element method (FEM). The aim of this work is the numerical and experimental investigation of the low-cycle fatigue behavior of welded 9Cr-1Mo steel specimens tested at 550 degrees C, which corresponds to the operating temperature of Generation IV nuclear reactors. For this purpose, high temperature, low-cycle fatigue tests under strain control and numerical simulations were conducted. The finite element (FE) code Caste3M was selected for numerical simulations. Results of the experimental tests were used to determine the material parameters that describe the material behavior under cyclic loading and to validate the FE model. To capture the material's cyclic softening behavior, an elastic-plastic model with kinematic and isotropic stress hardening was employed. The load correction algorithm was applied to the model to compensate for strain drift. The hysteresis loops of the stress-strain dependency and stress-cycle number curves were determined as the results at several loading levels. Additionally, the distribution of accumulated strain damage in the model was determined to predict the initiation area of fatigue crack. A comparison of the numerical simulations and the experimental data showed a good agreement, confirming that the developed finite element model with the selected material model is suitable for the simulation of low-cycle fatigue behavior of the welded specimens.