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

Stress-controlled fatigue tests were run at different R ratios (=_σmin/_σmax) 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.