Verification of Class B S-N curve for fatigue design of steel forgings

• A specimen design for fatigue testing of forged connector was successfully developed.
• Use of BS 7608 Class B S-N curve for plain steel was verified experimentally.
• An alternative fatigue design approach for plain steel, that included the effect of applied stress ratio, was developed.
• The range of surface roughness investigated was not found to have a significant effect on fatigue performance.

Currently the fatigue design procedure for forged steel connectors in steel catenary risers (SCRs) is based on the BS 7608 Class B S-N curve. However, there is very little direct support for this. Therefore, a fatigue testing programme, including strip specimens and small-scale cylindrical machined specimens extracted from actual forged J-lay connectors, was carried out to establish a suitable fatigue design procedure. The effects of stress ratio and surface roughness on fatigue strength were investigated. On the basis of these test results, together with available published data obtained from specimens that failed in plain steel, the Class B S-N curve was verified. The method was particularly conservative for low stress ratios in the high-cycle regime and a procedure based on the use of the Goodman diagram was devised for correcting the Class B curve for mean stress. Surface roughness in the range investigated, between 3.2 and 8.1 μm in Ra, had little effect on the fatigue performance of the forged steel.