Characterization of the different stages of damage evolution and crack growth in pure nickel during ultrasonic fatigue

In the very high cycle fatigue (VHCF) range fatigue life of non-defect afflicted materials is foremost dominated by crack initiation rather than crack growth. However, this is primarily true for applications where the existence of microcracks/-notches at the surface (e.g. due to manufacturing processes) can be ruled out. In order to assess fatigue life in the VHCF regime with regard to the true nature of early or late failure, the transition line between crack initiation processes, such as transient behavior and microcrack formation, and discontinuous and continuous crack growth have to be characterized as a whole. Using the example of commercially pure nickel Ni 201, fatigue life in the VHCF regime and the various stages of damage evolution up to total failure during ultrasonic fatigue testing are discussed on the basis of two different microstructures, an as-received and a coarse-grained condition. The scatter of fatigue results at stress amplitudes as low as 150 MPa ranges from 2 × 107 cycles up to 2 × 109 for a run-out sample of the as-received condition. The run-out sample exhibited slipband and microcrack formation at the surface. As a consequence, such microcracks have to be evaluated with regard to their propagation capabilities according to grain orientation and the barrier function of grain boundaries. Moreover, the huge scatter of fatigue results tested at similar stress amplitudes has to be investigated with regard to the stages of crack initiation and crack growth. In order to meet the demand for a distinction between the different stages of damage evolution, ultrasonic fatigue testing is accompanied by the analysis of the feedback signal and its higher harmonics, representing the nonlinear material behavior due to both microstructural changes as well as crack growth effects. The chances and limits of this method for an in-situ characterization of the overall damage process at ultrasonic test frequencies are critically discussed.