Effects of Water Flow Rate on Fatigue Life of Carbon Steel in Simulated LWR Environment Under Low Strain Rate Conditions

The flow rate of water flowing on a steel surface is considered to be one of the important factors strongly influencing the fatigue life of the steel, because the water flow produces difference in the local environmental conditions. The effect of the water flow rate on the fatigue life of a carbon steel was thus investigated experimentally. Fatigue testing of the carbon steel was performed at 289°C for various dissolved oxygen contents (DO) of less than 0.01 and 0.05, 0.2, and 1 ppm, and at various water flow rates. Three different strain rates of 0.4, 0.01, and 0.001 %/s were used in the fatigue tests. At the strain rate of 0.4 %/s, no significant difference in fatigue life was observed under the various flow rate conditions. On the other hand, at 0.01 %/s, the fatigue life increased with increasing water flow rate under all DO conditions, such that the fatigue life at a 7 m/s flow rate was about three times longer than that at a 0.3 m/s flow rate. This increase in fatigue life is attributed to increases in the crack initiation life and small-crack propagation life. The major mechanism producing these increases is considered to be the flushing effect on locally corrosive environments at the surface of the metal and in the cracks. At the strain rate of 0.001 %/s, the environmental effect seems to be diminished at flow rates higher than 0.1 m/s. This behavior does not seem to be explained by the flushing effect alone. Based on this experimental evidence, it was concluded that the existing fatigue data obtained for carbon steel under stagnant or relatively low flow rate conditions may provide a conservative basis for fatigue life evaluation. This approach seems useful for characterizing fatigue life evaluation by expressing increasing fatigue life in terms of increasing water flow rate.