A modified time-dependent failure assessment diagram for cracks in mismatched welds at high temperatures

In this work, the case of cracks located in a weld with mismatched creep properties from that of the surrounding base material is addressed. A weighted-average method for both over- and undermatched creep performance of the weld and base materials is presented, and an equivalent isochronous stress–strain curve (EISSC) is then defined. Accordingly, a time-dependent failure assessment curve is constructed using the method of R6 Option 2. This curve now depends not only on the mechanical properties of both weld and base materials but also on the weld geometry and crack size. The application of such an approach to an internal, circumferentially defected welded cylinder under axial tension has been studied. Results indicate that the dependence of the modified time-dependent failure assessment curves (TDFACs) on the geometry, crack size and service time is not significant and nearly collapses into one curve. The general failure assessment curve from isochronous data of austenitic steel, proposed by Ainsworth, is non-conservative for some cases and thus may not be applicable for Cr-Mo steel.