The effect of a loading profile containing a high frequency (200 Hz) loading component superimposed on a low frequency (∼0.05 to 0.5 Hz) loading component on fatigue crack growth of Inconel 718 at 650°C was investigated. The high frequency sinusoidal loading was added during the upper level hold period of the low frequency trapezoidal waveform.
Fatigue crack growth tests were carried out on Inconel 718 with a constant low frequency ΔK and an increasing high frequency ΔK in such a manner that the regimes in which crack growth is low frequency cycle and high frequency cycle dominated were investigated. Particular attention was devoted to the interaction of the two loading components in the low frequency dominated crack growth regime and the dependence of the transition to high cycle domination on factors such as low frequency cycle ΔK and hold time. The effects of hold times in the range of 2 to 180 s and low frequency ΔK in the range of 15 to 40 MPa√m were investigated.
In the high cycle dominated regime, the relationship between crack growth rate and high frequency ΔK had the characteristics of threshold crack growth data. In the low cycle dominated regime the high frequency cycle retarded crack growth under certain circumstances.