Fatigue crack growth in mooring chain steel at different mean load levels

The fatigue crack growth rates of R4 mooring chain steel at different R-ratios are investigated. Crack growth experiments are performed at three different R-ratios, 0.1, 0.3 and 0.7, utilizing compact tension specimens in air environment. Huang and Moan's driving force parameter ΔKE, which relates the growth rates at different R-ratios to those obtained at R=0, is explored and adapted for R4 steel. A full-scale fatigue experiment involving a chain assembly with an identified crack is presented, where the observed crack growth was successfully modelled by the proposed crack growth model. Case studies with different mean loads are explored where the crack growth model is utilized to predict the fatigue life of D = 145 mm chain links with a crack at the crown location. The crack growth simulations are compared and discussed against published S–N curves for chain links with varying mean load. The S–N curves may give very inaccurate results when extrapolated outside of the underlying test data. The crack growth model predicts longer lives at low stress ranges and low mean loads compared to the S–N curves, indicating that current guidelines may be overly conservative in these conditions.