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

Abstract

International audienceStress-controlled fatigue tests were run at different R ratios (¼ r min =r 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 = À1, À0.5 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 ¼ 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

Mean Stress Effect on the Axial Fatigue Strength of DIN 34CrNiMo6 Quenched and Tempered Steel

“…However, Figure 8 shows a deep subsurface initiation in an inclusion, which failed about 10 6 cycles in a R = 0.05 axial test. This failure mode is prone to appear with an increase in the stress ratio in the axial loading, as reported by Gaur et al [41]. Moreover, a crack initiation beneath the surface is unlikely in rotating-bending tests due to the gradient of the stress field, characteristic of the bending load.…”

Corrosion‐fatigue behaviour of Cr–Mo steel under biaxial tension

“…The results of biaxial tension fatigue tests done in air at various biaxiality ratios were already discussed in previous papers 20,21 and are only briefly reported here, in Table 3 and Figure 4. Compared with uniaxial loading, the fatigue resistance of the material was slightly enhanced at low biaxiality ratio ( B =0.25) but slightly reduced under equibiaxial tension ( B =1).…”

“…The effects of load biaxiality on fatigue lives and damage mechanisms in air of the steel which constitutes the Clip‐Riser TM connectors were thus investigated and reported in two previous papers 20,21 . The present paper focusses on the combined effects of load biaxiality and the corrosive environment.…”

“…Several inclusions of an average size of 5 μm are present along with some less frequent but large pores and inhomogeneities (40–300 μm) (Figure 1). The detailed microstructural and texture analysis and the effect of these inclusions, pores or inhomogeneities on fatigue damage mechanisms in this material under different loading conditions has been discussed in previous papers 20,21 . Tensile tests were conducted on cylindrical specimens, 8 mm dia, at a constant strain rate of 10 −3 /s.The measured tensile properties are shown in Table 2.…”

Report of Committee III.2: Fatigue and fracture

“…High-cycle fatigue tests to determine the influence of mean stress on the initiation mechanism in a 2.5%Cr-1%Mo steel used in riser tube connectors for offshore oil drilling was conducted in (Gaur et al, 2016). Tests were run at 7 different R ratios: -1, -0.5, 0, 0.25, 0.5 (runout at 3•10 6 cycles), and 0.6 and 0.7 (runout at 10 7 cycles).…”