Geometry effects on corrosion-fatigue in offshore structural steels

Fatigue crack growth rates were measured for two offshore steels with yield stresses of roughly 380 and 750 MPa in air and in synthetic seawater under both freely corroding and cathodically polarized conditions. Particular attention was given to the influence of testing frequency and specimen thickness. Small surface cracks were also evaluated. The higher-strength steel tends to exhibit higher fatigue crack growth rates as thickness is increased, this effect becoming more pronounced in an aggressive environment. Furthermore, the frequency dependence under cathodic polarization is more pronounced in the higher-strength steel than in the lower-strength steel. No 'chemically short crack' effect (i.e. that small cracks only grow more rapidly than long through-cracks in a corrosive environment) is observed when the 'reference' da/dN-ΔK curve for long cracks utilizes specimens with comparable dimensions.