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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Figure 10: Graphical plot of allowable nominal stress versus flaw tip root radius for a Debris Fretting Flaw (DFF) 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) 100 MPa and radial flaw depth “ a ” is kept as 0.475 mm and (b) 0 MPa and radial fl aw depth “ a ” is kept as 0.575 mm. The variation of allowable nominal stress with crack length for different values of flaw tip root radius was also studied and the numerical results are shown in Fig. 11a,b. As expected, allowable nominal stress saturates as the flaw length increases. It is worth noting that the threshold peak stress is independent of flaw length. The saturation tendency observed for the allowable nominal stress is, therefore, a consequence of stress concentration factor.

a

b

Figure 11: Graphical plot of allowable nominal stress versus axial flaw length ‘ c ’ showing the safe and unsafe regions. The flaw is assumed to be located in a region away from rolled joint (no residual stress) in a pressure tube used in 220MWe Indian PHWR. The radial flaw depth for (a) flat bottom BPFF with radial flaw depth “ a ” is kep t as 0.625 mm and (b) Debries Fretting Flaw with radial flaw depth “ a ” is kept as 0.525 mm Comparing Fig 11 ‘a’ and ‘b’, we can see that the allowable nominal s tress for DFF decays more steeply than BPFF. This is due to the difference between stress concentration factor of BPFF and DFF. No closed-form solution of ‘ k t ’ was available for DFF in CSA (2016). Derivation of closed- form solution of ‘ k t ’ for DFF was carried out