Effects of cycle frequency on corrosion-fatigue crack growth in cathodically protected high-strength steels

Submarine hulls and offshore structures are subject to cyclic stresses in sea-water, and data on crack growth rates are required for predicting the lives of such structures. The corrosion-fatigue crack growth rate of a cathodically protected, high-strength low-alloy steel weld metal (yield strength 700-900 MPa) has been measured as a function of cycle frequency at several constant K values. The fatigue crack growth rates increase in a sigmoidal fashion with increasing rise-time of the load cycles, with (i) the inflection points of the sigmoidal curves for different Delta K all falling on a common line, and (ii) the gradient at the inflection points having the same value for each K. Previously published models for predicting effects of cycle frequency on crack growth rates do not properly account for the data. A new model, based on synergistic interactions between the environment and deformation during loading, has been formulated. The new model, which has some elements of previous models, produces a good fit to the experimental data, not only for the weld metal but also for other steels of similar strengths. (C) 2010 Published by Elsevier Ltd.