Abstract
The influence of stress ratio on the retardation of surface fatigue crack growth of mild steel in sea water was investigated by varying the testing frequency from 0.2Hz to 10Hz and the temperature from 288K to 298K at two stress ratios of 0.32 and 0.5. For both stress ratios, there appeared to be a frequency at which the rate of crack growth showed maximum. This frequency increased with the increase of the temperature, although the maximum of the crack growth rate decreased with the increase of the temperature. Furthermore, the rate of crack growth increased with the increase of the testing frequency up to a certain frequency, but it started to decrease with the further increase of the testing frequency. At lower testing frequencies and for the two stress ratios employed here, there appeared to be a critical frequency at, or below which the rate of crack growth in sea water was so low that it could be regarded as virtually zero. This critical frequency increased with increase of the sea water temperature. At temperatures in the range between 288K and 298K, the critical frequency was higher at the stress ratio of 0.5 than that of 0.32. These observations can be explained reasonably by crack tip blunting caused by dissolution in sea water.
