Crack Growth in High Strength Chain Steel Subjected to Fatigue Loading in a Corrosive Environment

The present article presents the fatigue crack growth behavior of new high strength steels designated R4S grade. Eight Compact Tension (CT) specimens with 25 mm thickness were subjected to constant amplitude loading while exposed to seawater without and with cathodic protection. The Cathodic Potential (CP) was set to −890 mV and −1100 mV relative to an Ag/AgCl reference cell. Rates in air are included as a reference. The crack growth parameters were determined from a linear relation between da/dN and δK for a log-log scale. The derived figures are given in the table below. The figures for dry air and cathodic protection are valid for δK between 15 and 30 MPam^0.5. Below this range the slope m of the linear relation will change and further investigations have to be carried out for this region. The figures for free corrosion are valid for δK values from 10 to 30 MPam^0.5. The threshold value for δK is close to 5 MPam^0.5 in this case. The measured growth rates were compared with the rates for medium strength carbon manganese steels found in rules and regulation, i.e. BS7910. The present growth rates are well within the scatter band given for these steels in air and free corrosion. The present growth rates found in seawater with cathodic protection are however substantially lower than the rates given in BS7910. When a cathodic potential of −1100 mV was applied, crack closure was observed at medium levels of δK. The explanation is the formation of calcareous deposit in the wake of the crack front that gives significantly reduced growth rates and finally leads to crack closure. This finding is a surprise for a high strength steel. The results are promising and should be investigated further. Finally, a linear elastic fracture mechanics model was established to study the fatigue behavior in a stud-less link. The model was used to construct S-N curves that are consistent with experimental fatigue lives and the design curve given in the DNV rules. The present growth parameters were used in conjuction with a crack-like initial flaw with depth in the range from 0.12 to 0.20 mm. The difference found between the growth rates in dry air and in free corrosion are in accordance with tested fatigue lives for these two environments.