Service condition for structures at elevated temperatures in nuclear power plant involves transient thermal and mechanical load levels that are severe enough to cause inelastic deformations. It is well known that most of engineering materials are metallurgically unstable and creep phenomenon would be one of most important failure mechanism of deformation. In addition, creep-fatigue damage leads to failure when cyclic loads are applied to structures at elevated temperatures. Therefore, systematic inelastic analysis is needed for design of structural components in nuclear power plant subjected to such loading conditions. For this purpose, development of constitutive equations that can predict material behavior more accurately at elevated temperatures is necessary and numerical techniques with stability and accuracy for applying proposed constitutive equations are required also.
For the purpose of establishing systematic inelastic analysis techniques, Chaboche model that is one of the proposed unified viscoplastic constitutive equations was selected and was formulated with GMR (generalized mid-point rule) time integral scheme and implemented for the UMAT program of ABAQUS in the present investigation. First, in order to verify reliability of the developed UMAT program and the validity of the adopted Chaboche model, three types of uniaxial problems such as uniaxial hardening problem, uniaxial stress relaxation problem, and uniaxial cyclic hardening problem were solved using the developed UMAT program and the 4th-order Runge-Kutta method. By comparing the results obtained from each method, the numerical stability and accuracy of the developed UMAT program were verified. In the case of uniaxial hardening test, solutions obtained from the FEM analyses using the UMAT program were compared well to the experimental results in references. Finally, concerned with Y-piece structure of LMFBR (liquid metal fast breeder reactor) vessel under severe thermal transient loads were considerated to evaluate the safety estimation. In order to determine thermal stress level, ABAQUS using the developed UMAT program was used in the present investigation based on elastic and elasto-plastic model. ASME Code Case N-47 was used for creep-fatigue damage evaluations for each case.
