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.

This study pays attention to reveal material properties that control a resistance curve for ductile crack growth (CTOD-R curve) on the basis of the mechanism for ductile crack growth, so that the R-curve could be numerically predicted only from those properties. Crack growth tests using 3-point bend specimens with a fatigue pre-crack are conducted for two steels that have different ductile crack growth resistance, whereas both steels have the same mechanical properties in terms of strength and work hardening. Observation of crack growth behaviors provides that different mechanisms between ductile crack initiation from fatigue pre-crack and subsequent growth process can be applied. It is shown that two types of ductile properties of steel associated with ductile damage can mainly influence CTOD-R curve; one is a resistance of ductile crack initiation estimated with critical local strain for ductile cracking from a surface of notch root, and the other one is a stress triaxiality dependent ductility obtained with circumferentially notched round-bar specimens. The damage model for numerically simulating the R-curve is proposed taking the above two ductile properties into account, where the ductile crack initiation from crack-tip is in accordance with local strain criterion, and the subsequent crack growth triaxiality dependent damage criterion. The proposed model accurately predicts the measured different R-curves between two steels used that have the same 'strength properties', and also the stress triaxiality dependence of R-curve.

From the standpoint of evaluating Type IV creep damage in the fine-grained heat affected zones (FGHAZ) of welded joints, an analysis method combining continuum damage mechanics (CDM) and a cavity nucleation model is proposed and applied to the creep testing of simulated-FGHAZ notched bars of mod. 9Cr-1Mo steel at 650℃. The Perrin-Hayhurst CDM model is adopted, which considers both softening by precipitate coarsening and damage by creep cavities. For the cavity nucleation model, a proposal by Gonzalez and Cocks is employed, which considers the randomness of grain-boundary-facet orientations in a polycrystalline material and gives a nucleation rate that is a function of the creep strain rate and a tri-axiality factor. The critical value of the damage parameter, corresponding to the initiation of micro cracks due to the coalescence of creep cavities, is expressed in terms of a critical value of the number density of creep cavities as determined from grain-boundary-resistance model simulations by the present authors. Creep rupture experiments have been conducted for circumferentially notched bar specimens with two kinds of notch acuities. The applicability of the combined CDM and cavity nucleation model is demonstrated by comparing the distribution of creep cavities observed experimentally with the simulation results. The final rupture life of the circumferentially notched bar specimens was also predicted to within a factor of two.

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