Numerical Determination of the Strain Control Correction Factor for Fatigue Testing in VVER Environment

Abstract ÚJV Řež, a. s. possesses its own autoclaves, which are utilized for the purpose of conducting low-cycle fatigue tests in VVER environment. Most low-cycle fatigue tests performed in autoclaves are strain controlled. In contrast to conventional low-cycle fatigue tests conducted in air, it is usually not possible to use direct extensometer on the working part of the specimen. This is due to the risk of corrosion and initiation of fatigue cracks from the extensometer contact points, but also due to the design limitations of the autoclaves, the high temperature and the specimen being placed in a pressurized medium (mostly water). For these reasons, the principle of strain control by a Linear Variable Displacement Transducer (LVDT), which is clamped on the shoulders of the specimen, is used instead. Since the LVDT sensor does not allow direct measurement of the deformation on the working part of the specimen, this principle requires the determination of the strain control correction factor to ensure that the working part of the specimen is subjected to the required deformation amplitude, which is typically the control parameter for low-cycle fatigue testing. This principle entails a certain degree of inaccuracy in the required deformation amplitude on the working part of the specimen. The accuracy of the determination of the correction factor affects the proper conduct of the fatigue tests and consequently the results of the fatigue life assessment of the tested materials. The correction factor can be determined experimentally at room temperature in ambient air by cycling the specimen with direct extensometer or by using a video extensometer. However, direct experimental determination of the correction factor is not possible at elevated temperatures. Another way to determine the correction factor is using numerical simulations. Different organizations use different methods to determine the correction factor, also known as shoulder to gauge ratio (S2G). For example, the Incefa Scale project. This paper presents the methodology for determining the correction factor by using finite element method (FEM) simulations applied to a specific autoclave device for low-cycle fatigue testing in a VVER environment. However, the methodology is applicable to other autoclaves and materials. The correction factor is described in general terms, including what the factor depends on and what influences it. The calculation was performed using the Abaqus FEM software with a custom UAMP subroutine that was developed to achieve the required deformation on the working part of the specimen. The methodology is based on constitutive elasto-plastic material models, which have been calibrated using available experimental data from low cycle fatigue tests at various strain amplitudes for different materials. The elasto-plastic material model affects the total strain distribution in the working part and transition regions of the specimen, and therefore has a direct influence on the correct determination of the correction factor. For this reason, a correct definition of the elasto-plastic material model is crucial. The definition of the elasto-plastic behavior was determined separately for each material tested. This results in different correction factors for different materials. The correction factors determined by FEM simulations were compared with the experimental measurements obtained using the video extensometer. This comparison validated the methodology of the FEM approach. The described methodology demonstrates the feasibility of numerically determining the correction factor for material properties at elevated temperatures, where direct experimental determination of the correction factor is not possible.