Very high cycle fatigue properties of bainitic high carbon–chromium steel under variable amplitude conditions

Very high cycle fatigue (VHCF) properties of bainitic 100Cr6 (SAE 52100, JIS SUJ2) steel are studied under variable amplitude (VA) loading conditions with a cumulative frequency distribution of stress amplitudes similar to a Gauss distribution. Ultrasonic VA fatigue tests are performed at different maximum stress amplitudes, σmax of the random sequence and fatigue lifetimes between 2 × 106 and 1010 cycles. 42% of the fractured specimens failed from internal Al2O3-inclusions and 58% failed from the surface, predominantly from scratches or cavities produced during grinding. If σmax is high, surface failures occur earlier than internal failures. If σmax is close to the constant amplitude (CA) mean fatigue limit, fractures from internal inclusions occur earlier. Facets formed at inclusions in VA tests are larger than in CA tests. Linear damage accumulation calculation delivers mean damage sum S = 0.44 in experiments with mean lifetimes below 4 × 107 cycles. Decreasing σmax leads to decreasing damage sums, e.g. VA experiments with mean lifetime of 9.9 × 108 cycles show S = 0.016. Considering surface and internal failures separately, the damage sums decrease with increasing VA lifetimes, more pronounced when the crack initiates at internal inclusions. When mean VA fatigue lifetimes are in the regime of 109 cycles or above, more than 99% of the stress amplitudes are below the CA mean endurance limit, and the deleterious influence of low load cycles is underestimated on the basis of CA fatigue data.