Modelling of creep rupture of ferritic/austenitic dissimilar weld interfaces under mode I fracture

Dissimilar metal welded structures (DMWs) have been used extensively in conventional and nuclear power generation plants. Evaluation of creep rupture properties of DMWs is critical to the structural integrity assessment. Failure of DMWs can occur in the base metal, the heat-affected zone (HAZ), or the dissimilar interface between the two welded materials, depending on the operating stress and temperature. The primary focus of this work is on interface failure in systems consisting of a ferritic steel (P91 or P22) and an Inconel filler material, which has an austenitic structure. A planar damage zone is introduced within a finite element (FE) framework to model the response of the interface. A traction-separation constitutive law with a Kachanov-type damage accumulation relationship is employed to describe the interface response, with the material parameters calibrated against available creep rupture data in which failure occurred at the dissimilar weld interface. It is found that the difference in damage accumulation along the interface of different DMW systems can be attributed to the mismatch in creep properties of the continuum materials either side of the interface. Diversion of the crack path into the HAZ is also captured as a result of damage accumulation in the heat affected zone (HAZ). The relationship between the empirical damage accumulation model and the major microstructural features that are responsible for interface failure is also discussed.