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

Abstract

A B S T R A C T 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.

Predicting creep crack initiation in austenitic and ferritic steels using the creep toughness parameter and time‐dependent failure assessment diagram

“…Therefore, it is considered that mismatch effects may not be significant for the P22 weldments analysis. The influence of mismatch on the isochronous stress–strain curves of Cr–Mo steel, following the R6 approach, have been examined in 15 where the influence of geometry, crack size and time on the TDFAD was found negligible. A simplified procedure for evaluating isochronous stress–strain curves has been proposed in 13 (neglecting plasticity and assuming the elastic response of the entire weldment is equal to the PM's response) and applied to a 1Cr0.5Mo steel weldment.…”

“…4 Components are often welded and the application of the TDFAD approach for weldments has been considered. [12][13][14][15][16] However, a number of complications arise when considering weldments such as material mismatch effects that may require consideration.…”

“…The primary and secondary creep constants in Eqs. (14) and (15) are given in Table 1 for 316H stainless steel at 550 • C and P22 steel at 565 • C. For a particular time, the total strain at any stress level is given by the sum of the elastic and plastic strain and the total creep strain accumulated in that time, i.e.…”

“…Therefore, the application of methods based on CCG test data to predict real component lifetimes require validation 4 . Components are often welded and the application of the TDFAD approach for weldments has been considered 12–16 . However, a number of complications arise when considering weldments such as material mismatch effects that may require consideration.…”

Time-dependent deformation and fracture of multi-material systems at high temperature

“…Similar to those of the existing TDFAD approach for homogeneity, the redefinitions of the normalized load parameter L r and the normalized fracture parameter K r can be accomplished straightforwardly except that limit load and fracture toughness of homogeneity are replaced by those of multi-material systems [45] L r ¼ F=F Lmis (28)…”