Development and use of an anisotropic damage model for the high-temperature life assessment of a P91 weldment

This paper describes the development of a finite element (FE) continuum damage mechanics methodology to deal with anisotropic creep and anisotropic damage for a P91 weld metal. The method employs a second-order damage tensor, together with a novel rupture stress approach to define the evolution of this tensor. The method takes advantage of the transverse isotropic nature of the weld metal, to achieve a reduction in the number of material constants required from test data (and hence tests) to define the damage evolution. Hill's anisotropy potential theory is employed to model the secondary creep. An FE implementation of the model is applied to the creep life assessment of an internally pressurized axisymmetric P91 pipe weldment. The study also considered the effect of various material mismatch ratios in the various zones of the weldment. Here material mismatch is considered to be a combination of the difference in uniaxial minimum strain rates and in the uniaxial rupture times.