Surface versus internal fatigue crack initiation in steel: Influence of mean stress

Abstract Stress-controlled fatigue tests were run at different R ratios ( = σ min / σ max ) up to at most 3 million cycles on a 2.5%Cr–1%Mo steel (ASTM A182 F22) used in riser tubes connectors for offshore oil drilling. The fatigue lives, as well as the slope of the S–N curves were found to decrease with increasing R and the endurance limit to follow Gerber’s parabola. Surface crack initiation without any defect involved, was most often observed for R = −1, −0.5 and 0, while an R ratio of 0.25 triggered crack initiation from either surface or internal pores or chemically inhomogeneous areas, leading, in the latter case, to fish-eye patterns for relatively low numbers of cycles. A further increase in R ratio to 0.5 promoted only defect-initiated surface cracks, while no fatigue fracture was observed within 10 million cycles above R = 0.6 . These transitions in crack initiation mode are discussed based on X-ray diffraction analyses of residual stresses, elastic–plastic F.E. computations on a unit cell model containing a pore and some fracture mechanics analyses, with a particular attention to environmental effects.