The objective of this study is to provide fatigue crack growth rate data in the near threshold region to aid design and defect assessment procedure in steel offshore structures. In order to account for different steels, microstructures, environments and loading conditions, duplicate tests were carried out on three different steels, welds and simulated heat-affected zones in environments including air, seawater, seawater with cathodic protection (CP), and biologically active seawater. Additional studies were conducted to examine different H2S levels in biologically active seawater, the influence of R ratio and variable amplitude fatigue. Fatigue crack growth rates and pseudo-threshold stress intensities were generally in the order weld < parent plate < coarse grain HAZ < fine grain HAZ. These were primarily controlled by the thresholds which were considered to be influenced by residual stresses. In particular, simulated heat-affected zones produced very high delta KTh values with a great deal of scatter. Environmental effects on da/aN and threshold were in the order: air < seawater < seawater+CP < Seawater+H2S. It was considered that thresholds were controlled by closure effects from corrosion products. Thresholds in the three aqueous environments were relatively similar. Crack growth rates were considered to be dependent on hydrogen availability and effective delta K. Crack growth rates were much higher in the biologically active seawater than the other environments and appeared to be relatively independent of the H2S concentration. Variable amplitude testing using the WASH stress history resulted in a pseudo-threshold stress intensity delta Krms which was only marginally higher than those obtained in constant amplitude tests. Crack growth rate in this region averaged 3x10-10 m/cycle, again similar to constant amplitude tests. Tests were conducted on welded T joints in seawater containing H2S produced by bacterial action. The Wöhler curves were similar to joints tested in seawater at an overprotection potential of -1.1 V (Ag/AgCl). In both cases, crack initiation appeared to take 30 to 50 % of total life.
Near threshold fatigue crack growth in structural steels
Ermüdungsrißwachstum nahe des Schwellwertes in Baustählen
1998
134 Seiten, 47 Bilder, 12 Tabellen, 12 Quellen
Report
Englisch
hochfester Stahl , Schweißnaht , Wärmeeinflusszone , Meereskonstruktion , Ermüdungsriss , Rissausbreitung , Ausbreitungsgeschwindigkeit , Rissausbreitungsversuch , Spannungsintensitätsfaktor , Meerwasserkorrosion , Schwefelwasserstoff , kathodischer Schutz , Wöhler-Kurve , Schwellenspannung (mechanisch) , Risseinleitung , mikrolegierter Stahl , T-Stoß
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