Behaviour of mild steel under very low frequency loading in sea water

It is widely believed that the deterioration promoted by sea water is small in high stress, low cycle fatigue. Crack growth is faster than the penetration rate of the corrosive medium. This is probably true when the cyclic frequency is moderate (ca. 0.1 Hz). In ships large changes of the still water bending moment may occur when the loading condition goes from ballast to fully loaded and back. Wave bending is superimposed on the still water stresses. The absolute maxima and minima of the combined bending moments may occur only about once a week. Other very low frequency changes of stresses are connected to temperature changes (day/night) and, for offshore structures, changes of wind and wave directions. Experiments have been carried out with Fe 410 and Fe 510 at 0.05–0.0003 Hz. The crack growth curves for air and sea water remained practically parallel in a log da/dn-log ΔK plot. For simple programmed loading in sea water (one peak among 200 low stress cycles) the difference with results obtained in air was for Fe 410 about 1 : 20 in terms of da/dn; for Fe 510 it was 1 : 10. In air the peaks were very beneficial. In sea water there was no advantage.