Frequency interactions in high-temperature fatigue crack growth in superalloys

The influence of high frequency loading on the subsequent low frequency crack growth behavior in nickel-based alloy 718 in laboratory air environment at 923 K has been investigated through the use of a sequential high/low frequency load waveform. The parameters that have been examined include the crack growth rate, fracture surface morphology, and slip line density at and below the fracture surface. Results of this study indicate that prior application of high frequency loading results in reduction of the subsequent low frequency crack growth rate. An attempt is made to interpret this type of modification as being a result of the crack tip condi- tioning through the increase in the slip line density during the high frequency part of the loading cycle. Furthermore, by linking the type of selective oxide formed at the crack tip to the degree of deformation in the crack tip zone, a correlation has been made between the increase in the slip line density in the crack tip zone during the preceding high frequency loading and the increase of the crack resistance to environment degradation effects during the subsequent low frequency loading.