Fatigue Crack Threshold of Bearing Steel at a Very Low Stress Ratio

Fracture mechanics‐based criteria for fatigue fracture of rolling bearings under the influence of defects

“…Figure 10A,B displays the crack-closing behaviour for non-propagating cracks in round-bar specimens under unloading processes, at R of −1 and −3, whereby a precrack was introduced via an ΔK-decreasing test. Figure 10C,D also exhibits the results of stress-constant tests at R of −5 and −10, based on data obtained during the authors' previous study, 40 in which a pre-crack was introduced via the ΔP-constant test. Apart from the curves, the thick short lines pinpoint the crack-closure starting points during the unloading process, determined as point of 0.1% deviation from the straight vertical line in the curve's higher-stress regime.…”

“…The diagram generally consists of two different lines corresponding to crack-opening/closing processes, with the latter's knee point viewed as the crack-closure point. 41,42 Essentially, the diagram was fixed so the straight line in an upper part of the curve could become upright, enabling the crack-closing point to be determined precisely, 40 as is later depicted in Figure 10. Both the effective stress range, Δσ eff , and effective SIF range, ΔK eff , were calculated using the following equations:…”

“…When crack-propagation halted, crack-closure behaviour was investigated using the elastic unloading compliance method. 40 Two strain guages (0.2 mm long and 1.4 mm wide) were attached to the guage section: the centre of one guage was set precisely at the crack centre, while the other was placed on the smooth section beyond the crack. The strain difference (so-called subtracted strain) was then plotted as a function of nominal stress.…”

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

“…This is a surprising observation since self-arrested cracks are rarely observed in ultrahigh-strength steels. 26,27 The presence of nonpropagating cracks after cyclic loading at subcritical loads (i.e., at or below the fatigue limit stress amplitude) is a clear indication that fracture mechanics approaches might be used to predict the fatigue strength in the presence of small defects. This is due to the fact that the fatigue limit is determined by the critical condition for crack propagation rather than for crack initiation, which means that the fatigue limit can be correlated with the threshold-stress intensity.…”

An efficient stress intensity factor evaluation method for interacting arbitrary shaped 3D cracks

“…This is because the behavior of interacting small defects at the fatigue limit varies between materials due to differences in the characteristics of non-propagating cracks. For example, it has been suggested that defect orientation [22], stress ratio [23,24] and microstructure [1] can influence the size and location of nonpropagating cracks. However, notwithstanding other factors, a non-propagating crack usually tends to appear as a result of plasticity-induced crack-closure [25].…”