Abstract
The influence of cyclic frequency and sea water temperature on surface fatigue crack growth retardation of mild steel was investigated by varying the testing frequency from 0.1Hz to 15Hz and the temperature from 278K to 308K. A particular frequency was found at which the environmental acceleration factor, (dl/dN)CF/(dl/dN)air, showed maximum. The particular frequency increased with increasing the temperature, while the maximum acceleration factor decreased with increasing the temperature. The particular frequencies at the temperature of 288, 298, and 308K were found to be 0.7, 1.0 and 10Hz, and the maximum values of the acceleration factor were 2.2, 2.0 and 1.6 respectively at a ΔK of 5MN/m3/2. The acceleration factor decreased with increasing the testing frequencies above the particular frequency and decreased with decreasing the testing frequencies below it. When testing at lower frequencies, there was a critical frequency at which or below which the rate of crack growth in sea water slowed down and finally stopped. It increased with increasing the temperature. The critical frequencies at the temperature of 298, 303 and 308K were found to be 0.15, 0.3 and 0.6Hz respectively. These phenomena can be explained by crack tip blunting by dissolution in sea water.
