PSI - Issue 42

Paulo Mendes et al. / Procedia Structural Integrity 42 (2022) 1752–1761 1757 Paulo Mendes, Rita Dantas, José A.F.O Correia, Nicholas Fantuzzi, Abílio M.P. Jesus / Structural Integrity Procedia 00 (2019) 000 – 000 4. Results and discussion The characteristic fatigue curve of the double-side welded connection is obtained using a statistical analysis based on the ASTM E739 standard, BS ISO 12107 standard, Eurocode 3, IIW, DNVGL-RP-C203 and through a two parameter Weibull distribution and a Bayesian inference. The Weibull parameters and parameters related to the lifetime of the probabilistic model were obtained. An advantage of this model is to include the description of the fatigue limit, but stress data is required in the fatigue limit region. The value imposed for the inverse slope of the curves according to the standards (European Committee for Standardisation, 2010), (Hobbacher, 2008) and (DNVGL RP-C203: Fatigue Design of Offshore Steel Structures, 2016) is = 3 . There are two parameters that need to be defined: the confidence level (1 − α n ) and the probability of survival ( ). Eurocode 3 Part 1-9 recommends the use of (1 − ) = 75% and = 95% , DNVGL-RP-C203 provides a 97.7% probability of survival where the mean curve is determined with 75% confidence, IIW standard assumes a survival probability of at least 95%, calculated from the mean value based on two-sided tolerance limits at the 75% level. The representation of each curve was done according with equation (7. Figure 1 shows all the P-S-N curves with a probability of survival of 95% = 95% ) for the hot spot stress case comparing them all also with the design fatigue curves presented in the IIW and Eurocode 3 standards. The Eurocode S-N curve category 71 seems to limit well the data even though it is extremely conservative when compared with the experimental data. The S-N curve was chosen according with Table 8.1 on Eurocode 3 Part 1-9 (European Committee for Standardisation, 2010). As for the IIW standard (Hobbacher, 2008), Table 3.3 was taken into consideration. It is possible to conclude that the lower the confidence levels are, the less conservative are the P S-N curves comparing the P-S-N curves estimated using BS ISO 7910 guidelines. The Bayesian inference method seems to present the most suitable curve because it is the less conservative and most fitting distribution for the hot spot stress case with a probability of survival of 95%.

Figure 1 – Comparison of all the = 95% P-S-N curves from the different methods for the hot-spot stress case. Figure 2 shows all the P-S-N curves with a probability of survival of 97.7% ( = 97.7% ) for the hot-spot stress case comparing them all also with the design fatigue curve presented in DNVGL-RP-C203 (DNV GL, 2016). The assessment of these curves was done for this probability because the DNVGL-RP-C203 is the most current used design code for offshore structures, and it only presents S-N curves for a = 97.7% . For this probability of survival ( = 97.7% ), the Bayesian inference shows the most fitting distribution.