Fatigue Crack Initiation and Growth in Nanocrystalline Ni at Multiple Length‐Scales

The presented study focuses on the analysis of fatigue crack growth rate (FCGR) in focused ion beam‐notched microcantilevers prepared from nanocrystalline (NC) Ni as a model material. The results are directly compared to the previously obtained data on NC Ni with mesoscale and macroscale test specimens. The fatigue crack growth threshold Δ K _th range was measured to be in the range of 1.5–2.3 MPa·m ^−1/2 in microscale samples. This value compares well with macroscopic samples showing Δ K _th ≈ 2 MPa·m ^−1/2 measured for R = 0.1 and R = 0.7. Due to the microcantilever width and thickness of ∼5 µm, such microscale specimens do not exhibit any crack closure effects, and the fatigue crack growth initiation threshold is identical to the effective threshold. However, the effect of previous loading history was shown to have a noticeable effect on the crack growth rate after growth initiation, associated with the change of crack tip geometry and formation of surface extrusions/intrusions at the crack tip. It was also observed that the methodology for the Δ K ‐driven FCGR testing in cantilevers with microscopic dimension is limited to small Δ K (<2 MPa·m ^1/2 ), as it does not sufficiently describe the fatigue process at higher Δ K due to considerable sample plasticity.