Investigation Of Design Criterion And Welding Repair Principle Of High Temperature Components Using Continuum Damage Mechanics Based Finite Element Method

Creep damage is one of the essential reasons for the failure of the components served at elevated temperature, which is obviously different from the failure due to insufficient strength at room temperature. Different design methodology is thus required to account for failure at high temperature. However the current design rules for high temperature components have been mostly based on the empiricism and are lack of support of scientific understanding. In light of the theory of continuum damage mechanics, design methodology and life extension repair of high temperature components are investigated through experimental study and finite element simulation. The main contributions of the paper are summarized as follows:(1) The state-of-the-art of design codes, life assessment method and damaging component repairing are reviewed. Possible improvement of current code is pointed out.(2) The modified Karchanov-Rabotnov constitutive equation has been incorporated into finite element program ABAQUS through its user subroutine -UMAT. Techniques that ensure the convergence of nonlinear numerical simulation are discussed. The program is verified by a sample problem. Further creep test verification has been carried out on a notched specimen, which suggest that the modified Karchanov -Rabotnov constitutive model can be used to interpret the creep deformation of components under multiaxial stress state.(3) The influence of the stress multiaxiality on creep life has been studied by means of damage of representative volume element. The creep life of some typical structures, such as bend pipe and T-joint, has been simulated. The numerical results show that the inelastic strain limits are sometimes overly conservative, while sometimes can be very risky. The ASME Code rules should thus be modified.(4) By use of the damage coupled finite element method, issues of the failure mode and efficiency of life extension in repair welding have been studied. The influence of groove width of welds, optimal weld repairing time, damage in full and partial repair weld have been obtained. The results show that the efficiency of life extension can not be improved by pre-repair welding for HAZ failure, on the contrary, the life is shortened. From the point of view of conservatism, the time of repair welding set at 90-95% of the maximum life is more reasonable. Wider groove of welds may result in longer life of repair welds.