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Abstract: For the compact tension (CT) specimen with a mismatched weld in creep properties, an equivalent homogeneity model is presented based on the limit load analysis. In use of the proposed equivalent material model and existing limit load solutions, the modified experimental C ∗ integral estimation equations for the CT specimens with weld centre crack, fusion line crack and asymmetric crack were derived. Finite element (FE) analysis for C ∗ integral of CT specimens with a mismatched weld was performed by using the ABAQUS software. Results show that, compared with the η o factor for homogeneous CT specimens, a soft under-mismatched weld will increase the non-dimensional factor η w while a hard over-mismatched weld will decrease the η w value. It indicates that the current C ∗ estimation equation in ASTM E1457 may underestimate C ∗ for creep soft weld but overestimate it for creep-hard weld. The discrepancies between the C ∗ from ASTM E1457 and FE analysis are sensitive to the crack location in weld, to the mismatch ratio and to the weld width. Whether for under-mismatched weld (M <1) or for over-mismatched weld considered herein, the predicted C ∗ parameters from the presented formulae are very close to FE results and thus can be used to interpret the creep crack growth behaviour of welded specimens. [Copyright &y& Elsevier]