1988 Volume 37 Issue 7 Pages 432-437
The influence of natural sea water on surface fatigue crack growth behavior of mild steel was investigated at four different testing frequencies, namely 0.33, 0.25, 0.17 and 0.09Hz in terms of the rate of crack growth and from the observation of the cross section and of the fractured surface of specimens. At 0.33Hz the correlation between the stress intensity factor range and the rate of crack growth followed Paris's law at the growth rate between 4×10-7 and 1.5×10-6mm/cycle in air and at the growth rate between 3×10-7 and 1×10-6mm/cycle in sea water. The rate of crack growth in sea water was found to be 1.6 times faster at the maximum than that in air within these ranges of growth rate. When testing at lower frequencies the rate of crack growth in sea water was found to be nearly equal to that at 0.33Hz at the early time of crack growth, but from a certain crack depth it slowed down with exposure time and finally stopped. This tendency for crack growth to decline in sea water increased with the decrease of frequencies. These phenomena can be explained by crack tip blunting by dissolution in sea water.
