PSI - Issue 68

M. Álvarez et al. / Procedia Structural Integrity 68 (2025) 272–278

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M. Álvarez et al. / Structural Integrity Procedia 00 (2025) 000–000

3.3. SN-curves in corrosive environment constructed by analytical models In this research, a phenomenological approach was used to model the degradation of fatigue life in presence of seawater environment, allowing to graphically reflect fatigue properties degradation as a decrease in S-N curves. Proposed models by Okenyi et al. (2023), Adasooriya et al. (2020) and Bandara et al. (2015) were used to construct S-N curves for corroded material and compare them against experimental CF results and design curves (Fig. 6)

Fig. 6. S-N curves for corroded S355 developed with the selected analytical methods.

According to the corrosion-based fatigue model proposed by Okenyi et al. (2023), construction of S-N curves in Fig. 6 was made through input parameters obtained from pitting characterization and roughness of specimens. Minimum and maximum corrosion S-N curves correspond to the specimens with minimum and maximum pit size, respectively. In this case, non-conservative predictions were obtained above 300k cycles. Adasooriya et al. (2020) method manipulates the slope of S-N curve based on Basquin’s law [Basquin (1910)]] on the assumption of linear variation of the logarithms of fatigue strength for uncorroded and corroded materials. Proposed S-N curve for corroded S355 gave conservative estimations above 50k cycles. Conservative calculation to the fatigue strength in corrosive environment was estimated although endurance limit ratio of corroded and base material ( σ ∞,corr / σ ∞ ) proposed by the author corresponds to immersion, which is less severe than wet-dry cycling. Bandara et al. (2015) proposed a similar method that uses a linear increase of the difference between the non corrosive and corrosive fatigue strength. This procedure developed a conservative S-N model below 10 7 cycles. 4. Conclusions This research presented a study of the effect of wet-dry conditions during a combined CF process, and established a comparison between experimental results, conservative design curves from standards and S-N curves for degraded material developed with analytical models published in literature, obtaining the following conclusions: • Life reduction was observed when CF testing was performed in artificial seawater environment • Application of wet-dry cycling during CF tests led to a higher decrease in fatigue life. This result suggests a more severe corrosion process during wet-dry cycling. • Using uncorroded S355 steel S-N curve in air as reference, this degradation of fatigue properties was estimated in the range of 34-69% in wet condition, and 61-74% in wet-dry cycling. • Bandara et al. (2015) and Adasooriya et al. (2020) methods, based in S-N curve slope manipulation for corrosive environment, led to less conservative predictions than design curves extracted from DNV-GL RP-C203 and BS 7608:2014.