PSI - Issue 79

Christoph Bleicher et al. / Procedia Structural Integrity 79 (2026) 239–247

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Bertoglio (2016) summarized literature and own fatigue data values for NiAlCu alloys and evaluated all available fatigue data in a commonly evaluated S-N curve and proposed a slope k under air of around 10 for fatigue results with tensile mean loads and with corrosion of about k = 4.4. A knee point could not be identified. These findings are in good agreement with the findings under alternating loading in this paper. But it is worth mentioning that the specimens in Bertoglio (2016) and as far as determinable also from the other publications were not subjected to a pre-corrosion in seawater. Moreover, the slope k is lower and thus less affected than Bertoglio (2016). So, the investigated material shows a better fatigue behavior. For the investigated materials and the two load ratios, it was not possible to detect a knee point. This effect is well known for metallic material cyclically loaded under corrosive media as e.g. reported in Duarte (2018) for aluminum alloys (EN AW-6082 T6 and EN AW-7075 T73). The scatter bands found for Cu3 in seawater and air have a maximum value of T σ = 1:1.28 for R σ = 0. According to Bleicher (2015) one needs to take into account values for the scatter band of up to T σ = 1:1.30 for a safe lifetime assessment for larger cast components. The derived value for T σ under alternating loading are comparably low and seem to be not on the conservative side.

Fig. 5. Comparison of fatigue strength under ambient air and sea water for R σ = -1.

4.2. Cyclic material behavior under corrosion and different test frequencies The application of aluminum bronze alloy in ship propellers is subjected to fatigue under very high cycles. Alt hough, the evaluation of an entire and final product is usually not possible in its larger dimensions, at least specimen tests should be as efficient as possible. Efficiency comes with testing speed and increased test frequencies open the possibility to proof the integrity of a material far within the VHCF regime. During the investigations DNV and the project partners agreed on a test frequency of 15 Hz to ensure enough time for the medium to exert its corrosive effect. Nonetheless, during the project the influence of a reduced (f = 5 Hz) and an increased frequency (f = 45 Hz) was examined. The results are displayed in Fig. 6 in form of specimens of “Batch 7”. An influence of the test frequency could not be shown. Both specimens with lower and higher test frequency mix well with the results for f = 15 Hz. Changes in the resulting scatter band, the slope k and the nominal stress amplitude at N lim are small, as well.