Very-high-cycle fatigue behavior of a structural steel with and without induced surface defects

Abstract

a b s t r a c tFatigue tests via rotary bending were performed on the specimens with and without induced surface defects for a structural steel of medium carbon content to investigate the effect of surface defects on fatigue behavior in high-cycle fatigue and very-high-cycle fatigue (VHCF) regimes. The S-N data showed that induced surface defects substantially degraded the related fatigue strength. For the specimens without surface defects failed in VHCF regime, crack initiated from the interior of specimens with inclusions or matrix inhomogeneities as crack origin and the initiation regions were of different extents of rough surface. The observations on the profile samples cut from crack initiation regions revealed that the region was a layer of nanograins for the case of inclusion as crack origin, and was without grain refinement for the case of grain boundary as crack origin. For the specimens with induced surface defects, crack initiated from surface defects and the initiation region was without grain refinement. The characteristics of crack initiation were carefully examined and the effect of surface defects on fatigue strength degradation was analyzed by available models.

Nanograin layer formation at crack initiation region for very‐high‐cycle fatigue of a Ti–6Al–4V alloy

“…b) and thus suggests that both equiaxed and lamellar microstructures can form nanograins. Together with the results of nanograin layer formation in the FGA of high‐strength steels, we may presume that the type of microstructure is not a dominating factor to the process of nanograin formation during crack initiation of VHCF.…”

“…5b) and thus suggests that both equiaxed and lamellar microstructures can form nanograins. Together with the results of nanograin layer formation in the FGA of high-strength steels 38,46 , we may presume that the type of microstructure is not a dominating factor to the process of nanograin formation during crack initiation of VHCF. Figure 7a and b presents the high magnification bright field images (BFI) of the NG layer, showing a more distinct morphology of nanograins in the NG layer of A1-RA and A2-RA.…”

Crack growth rates and microstructure feature of initiation region for very‐high‐cycle fatigue of a high‐strength steel

“…The new results on structural steels 31 and titanium alloys 25 have again validated the NCP model. 24,26 Note that the reported results of FGA being a nanograin layer under negative stress ratios were all subjected to constant amplitude (CA) cycling.…”

Very High Cycle Fatigue of Butt-Welded High-Strength Steel Plate

“…Previous studies determined that differences exist between fatigue and fracture behaviors according to the test regime. Test regimes are divided into three segments: low cycle (~10 4 ), high cycle (~10 7 ), and very high cycle (~10 9 ) [2][3][4][5].…”

“…A gigacycle regime was reached in approximately 912 h during a test involving a frequency of 300 Hz. Hence, some researchers used an ultrasonic fatigue tester to expedite experiments [6][7][8]. Figure 1 shows the setup of an ultrasonic fatigue test using a frequency of 20 kHz.…”