Fatigue analysis of low alloy steel forgings used in the subsea industry

Design S-N curves are presented for low alloy machined steel forgings. Separate S-N curves are given for air and for seawater with cathodic protection. The selection of the S-N curve is made based on the tensile strength and the surface roughness. S-N curves for four tensile strength classes and two surface roughness classes are given. The tensile strength classes cover steels with a tensile strength in the range of 517-793 MPa. The two surface roughness classes cover surface roughnesses up to Ra=6.3 μm. Separate S-N curves are given for a stress ratio of R=0 and for a high mean tensile stress. Secondly, a method for adjusting the peak stress in the notch root with respect to the stress gradient is presented. The stress gradient corrected peak stress is used with the presented S-N curves for estimating the fatigue life of fatigue tested notched specimens with typical subsea design features. All estimated fatigue lives are demonstrably conservative compared to the test results for the wide variety of notched specimens types considered.