Influence of the Inclusion Type on the Threshold Value of Failure in the VHCF-Regime of High-Strength Steels

The very high cycle fatigue behaviour of high-strength steels is mostly affected by cracks which were initiated at non-metallic inclusions inside the fatigue specimens and hence under the surface. By separating the data points in the S-N-diagram by the chemical composition of the inclusion at which a crack is initiated the authors recognized that the lifetime depends strongly on the inclusion type. The authors further deduce that the chemical composition of an inclusion as well as its geometry influences the mechanism by which such an inclusion triggers crack initiation. Thus, titanium nitride and homogeneous calcium oxide inclusions have been observed to show fractures caused by the stress concentration in the inclusion. The broken inclusions then present sharp notches in the steel matrix and cause further damage from there. In contrast, aluminium calcium oxide inclusions decay or detach from the steel matrix during loading on account of a rather low interface stress. This detachment process results in holes in the steel matrix, which act as logical starting points of fatigue cracks. Both processes described above occur at different stress intensity factors and lead to failure before an ultimate number of cycles of 10^9. Furthermore, it was possible to determine threshold values of failure for each inclusion type by stressing run out specimens on a higher stress level. These threshold values are in accordance with those of specimens which failed during one-level stressing. The fatigue tests were performed with 100Cr6 in martensitic and bainitic condition. Tension/compression tests at a load ratio of R = -1 were conducted on an ultrasonic fatigue testing facility. Failures were only initiated at non-metallic inclusions. The fracture surfaces were analyzed by scanning electron microscopy and by energy dispersive X-ray spectroscopy.