Effect of Multi-Axial State of Stress on Creep Deformation Behavior of Alloy 690 Material

Alloy 690 is used in high-temperature components such as steam generator tubes and processing vessels in nuclear reprocessing plants. These components experience multiaxial stress due to loading conditions, geometry changes, and microstructural inhomogeneity during fabrication. Operating temperatures range from 600 °C to 1000 °C, subjecting the components to creep deformation under multiaxial stress. While current designs rely on uniaxial creep data, a more realistic life assessment requires understanding creep rupture behavior under multiaxial stress. This study examines the effect of multiaxial stress on creep behavior using notched specimens with various root radii (0.5 mm, 1 mm, and 2 mm) to simulate different stress states. Creep tests were conducted on smooth and notched specimens at temperatures from 800 °C to 1000 °C. The average hardening exponent was found to be 3.15 with an activation energy of 345 kJ/mol-K in this temperature range. Finite element analysis was conducted to determine stress triaxiality at the notch root which increased from 0.388 for a 2 mm radius to 0.89 for a 0.5 mm radius. Representative stresses in notched specimens were evaluated using rupture data from smooth specimen tests. Results showed that Alloy 690 exhibits notch strengthening due to its high ductility at these temperatures. A significant reduction in displacement rates was observed for notched specimens due to its slow accumulation of creep damage. These data can be used for the design and safety assessment of nuclear reprocessing components.