Influence of small defects and nonmetallic inclusions on the high and very high cycle fatigue strength of an ultrahigh‐strength steel

Abstract

The high and very high cycle fatigue (VHCF) properties of ultrahigh-strength Ck45M steel processed by thermomechanical rolling integrated direct quenching were investigated. S-N tests with smooth and small drilled holes containing specimens as well as near-threshold fatigue crack growth measurements were performed up to 2 Â 10 10 cycles using ultrasonic-fatigue testing technique. The fatigue strength of smooth specimens is mainly determined by the size of nonmetallic inclusions. For surface defects larger than 80 μm, the fatigue limit can be correlated with a constant threshold-stress intensity factor. The ffiffiffiffiffiffiffiffiffi area p -parameter model adequately predicts the fatigue limit for internal defects and for surface defects with sizes between 30 and 80 μm. VHCF failures from smaller surface defects occur at stress amplitudes below the predicted fatigue limit. The long-crack threshold in ambient air is close to the effective threshold stress intensity factor. In optically dark areas at interior inclusions, cracks grow at mean propagation rates of 10 À15 m/cycles.

Improve fatigue and mechanical properties of high carbon bearing steel by a new double vacuum melting route

“…Both for the CSR steel and for the CSR + VAR steel are about 5.0 MPa·m 0.5 , which is consistent with the previous research 8 . The high cycle (10 6 –10 7 ) and ultra‐high cycle (>10 7 ) fatigue tests of the SAE52100 steel show that the driving force motivating crack to propagate is about 5.0 MPa·m 0.5 regardless of high cycle or ultra‐high cycle tests 34,35 . This confirms that is the intrinsic characteristics of the steel and not affected by the stress amplitude and the fatigue cycle.…”

“…8 The high cycle (10 6 -10 7 ) and ultra-high cycle (>10 7 ) fatigue tests of the SAE52100 steel show that the driving force motivating crack to propagate is about 5.0 MPaÁm 0.5 regardless of high cycle or ultra-high cycle tests. 34,35 This confirms that ΔK th is the intrinsic characteristics of the steel and not affected by the stress amplitude and the fatigue cycle. Due to this, for the RBF tests of the same material under the same stress amplitude, the crack size corresponding to ΔK th should be a constant.…”

Effects of Non-Metallic Inclusions and Mean Stress on Axial and Torsion Very High Cycle Fatigue of SWOSC-V Spring Steel

“…It can be seen that interior inclusions are smaller than √area = 125 µm for axial loading and √area = 196 µm for torsion loading can be considered as small cracks. Similar transition sizes between small and large defects (short and long cracks) are found for other high-strength steels [28,58].…”

Mean-stress sensitivity of an ultrahigh-strength steel under uniaxial and torsional high and very high cycle fatigue loading

“…The S-N curves exhibit clear knee points between 1 Â 10 5 and 2 Â 10 5 cycles. Only one specimen tested under fully reversed axial loading failed in the VHCF regime, which might be explained by environmental effects: The observability of nonpropagating cracks at small surface defects after testing at stress amplitudes below the fatigue limit, as demonstrated in a prior study 24 performed at R = À1, suggests that these arrested cracks can further grow after a sufficient number of load cycles in a chemically active F I G U R E 3 S-N diagrams for (A) uniaxial and (B) torsional test results at different load ratios(data obtained under uniaxial loading at R = À1 were already reported in Schönbauer et al 24 ) [Colour figure can be viewed at wileyonlinelibrary.com] environment (as ambient air for ultrahigh-strength steels). This effect is most pronounced at negative load ratios since nonpropagating cracks can be hardly observed at high tensile mean stresses (where once initiated cracks tend to further propagate until failure due to a lower contribution of crack closure).…”

“…However, at fully reversed tensioncompression loading, failure from the surface was also observed in the VHCF regime (see Figure 3A). These data were evaluated in a previous study, 24 where it has been shown that fatigue failure occurred even at σ a /σ w = 0.73 from a small surface pit at N f = 6.48 Â 10 10 cycles. At positive load ratios, failure from surface inclusions and pits were not identified, which can be explained by the absence of nonpropagating cracks at stress amplitudes below the fatigue limit at high tensile mean stresses.…”