Some components in nuclear power plants operate at high temperature with transient thermal loading and mechanical loading. It is well known that the majority of engineering metals are metallurgical unstable and various particular phenomena, such as creep or thermal ratchetting, occur at elevated temperature. In addition, creep-fatigue interaction is the critical mechanism to decrease the life of the components under cyclic loading. So as to achieve the reliable life estimation, inelastic analysis is recommended for structural components subjected to such complex loading conditions.
For the purpose of establishing inelastic analysis, three part of research should be attained; Formation of constitutive equation describing the material behavior at elevated temperature, making the FEM code to apply constitutive equation to the real structures and organizing systematic life estimation method. In the present study Chaboche viscoplastic model was selected and implemented into the UMAT subroutine of ABAQUS. In order to estimate the life of the components the concept of damage mechanics was introduced and the Chaboche model was modified.
The validity of the modified Chaboche model was examined by comparing with the experimental results in references and good agreement was achieved for material behavior and service life. The reliability of the UMAT was verified by comparing result with exact solution obtained by numerical solver, the IMSL routine. Finally, the safety assessment of LMFBR(liquid metal fast breed reactor) vessel under severe transient thermal and mechanical loading was carried out using ABAQUS in connection with the developed UMAT program. The life estimation was accomplished using the modified Chaboche model and compared with the result obtained from Creep-Fatigue damage evaluation in ASME Code Case N-47.
