PSI - Issue 60

D. Sen et al. / Procedia Structural Integrity 60 (2024) 44–59 Deeprodyuti Sen/ Structural Integrity Procedia 00 (2024) 000 – 000

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The variation of allowable nominal stress with flaw tip root radius for different values of axial flaw length 2c for a round bottom and flat bottom BPFF is shown in Fig. 8a and Fig. 8 b, respectively. The radial flaw depth ‘ a ’ is kept fixed as 0.6 mm. The allowable nominal stress decreases with an increase in axial flaw length. As noted earlier, the modeling of BPFF as a round bottom or flat bottom flaw does not have any significant influence on the allowable nominal stress.

a

b

Figure 9: Graphical plot of allowable nominal stress versus flaw tip root radius for a round bottom BPFF showing the safe and unsafe regions. The flaw is assumed to be located in a region near the rolled joint in a pressure tube used in 220MWe Indian PHWR. The residual stress is assumed to be (a) 1 00 MPa and radial flaw depth “ a ” is kept as 0.425 mm and (b) 150 MPa and radial flaw depth “ a ” is kept as 0.3 mm. We now analyse the effect of residual stress in the rolled joint region on the allowable nominal stress and, hence, on the maximum permissible flaw size. Two different values of residual stress are considered in our numerical calculations (i.e. 100 and 150 MPa). The allowable nominal stress for a case where a residual stress of 100 MPa is assumed in addition to the hoop stress resulting from design internal pressure is shown in Fig. 9a. The allowable stress is plotted as a function of flaw tip root radius for a range of axial flaw length 2c , the radial flaw depth ‘ a ’ is kept constant. Since the total hoop stress has increased due to the presence of residual stress, the permissible flaw sizes are much lesser compared to those shown in Fig. 8. Similar trends are also seen for debries fretting flaw (DFF) shown in Fig. 10a and Fig. 10b. In Fig. 10b, no residual stress is considered and most of the analysed flaw dimensions are in the safe zone.