A UNIFIED CONSTITUTIVE MODEL FOR HIGH TEMPERATURE MULTIAXIAL CREEP FATIGUE AND RATCHETING RESPONSE SIMULATION OF ALLOY 617

The study is developing a unified constitutive model for Alloy 617 which is the prime candidate material considered for intermediate heat exchanger (IHX) of next generation nuclear power plant. Alloy 617 can experience long term exposure to elevated temperatures as high as 950 degrees C, however, the ASME design code (Subsection NH) does not include design provisions for this temperature range. In addition, the Draft Alloy 617 Code Case specifies that the inelastic design analysis for temperatures above 649 degrees C must be based on unified constitutive model. Therefore, this study focuses on developing a unified constitutive model to simulate high temperature uniaxial and multiaxial creep-fatigue and creep-ratcheting responses of Alloy 617. As multiaxial response simulation is a key factor for design-by-analysis of IHX, a set of biaxial tests with varying degrees of loading non-proportionality has been performed at different steady temperatures within 25 degrees C-950 degrees C, and with different strain rates and strain ranges. From the tests, it has been observed that temperature, strain rate, strain ranges and non-proportionality of loading path greatly influences the creep-fatigue and creep-ratcheting responses of Alloy 617. Thus, development of a unified constitutive model considering dependence of these parameters is required. The current Chaboche viscoplasticity model with static recovery term can simulate uniaxial responses very well but it overpredicts biaxial ratcheting responses. Hence, a modified Chaboche model has been developed to improve biaxial ratcheting simulations. Multiaxial modeling features of non-proportionality and ratcheting are investigated. These modeling features and improved response simulations are presented in the paper.