PSI - Issue 75

Fabrice Deleau et al. / Procedia Structural Integrity 75 (2025) 392–418 Emmanuel Persent, Deleau Fabrice, Guillaume Coudouel, Guillaume Perrin/ Structural Integrity Procedia (2025)

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The dye penetrant results show cracking on the front lug, but no apparent cracking on the rear lug. These results are consistent with those obtained by Acoustic Emission. The difference in damage observed between the two lugs is consistent with the difference in activity observed in Acoustic Emission. It might also be interesting to take a closer look at the rear lug, which likely show some level of damage as detected by both strain gages and EA monitoring. At the end of this monitoring session, it is clear that Acoustic Emission is an effective and accurate method of detecting the onset of cracking in i-Clip lugs, at least as effective as monitoring using strain gages, but certainly more adapted to industrial-scale equipment instrumentation 5.3. Global fatigue tests investigation • Fatigue results analysis, Basquin model For the test on prototype No. 987, the service life predicted by the API 16F methodology (principal main stress variation) and Basquin model (Table 2) is quite low, at 138,123 cycles (with i-Clip material SN curve), at due to the high stress range evaluation of 866 MPa (Table 3). To account for the experimentally observed starting damage at 2,450,000 cycles, the stress range must be reduced to 238 MPa. However, this methodology would result in a significantly oversized product design. The DNV methodology (Von Mises stress variation with mean stress correction) is more optimistic but lacks conservatism, as the estimated stress range reaches 669 MPa, which is below the potential endurance limit identified up to 709 MPa (considering i-Clip material SN curve). No failure should be anticipate using the DNV approach (Fig. 26). Our proposed methodology, which considers the Von Mises stress variation along the main principal stress direction in accordance with DNV C203 and a specific mean stress correction, defines a corrected stress range that falls between the other two approaches, approximating 800 MPa. Material plasticity must be taken into account to ensure representativeness, especially regarding the stress range value. According to the i-Clip material SN curve, damage initiation is predicted at 426,340 cycles. This result is relatively conservative when compared with: - A crack initiation phase estimated at 1,650,000 cycles (based on strain gauge and EA analysis). - Damage initiation observed at 2,450,000 cycles. - Crack observation at 2,644,550 cycles (end of test). It is worth noting that the prototype remains fully functional and easy to handle throughout the testing process.

Fig. 26: Analysis of small-sized prototype n°987