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The present work demonstrates a micromechanical technique to investigate the Low Cycle Fatigue (LCF) behavior of Ni microbeams under fully reversed bending loadings. The technique extends the range of measured fatigue lives from the previously reported technique for High and Very High Cycle Fatigue (HCF/VHCF) characterization in the same microbeams. The results highlight significant differences in the slope of stress and strain-life behavior and crack propagation rates that differ from an average of 10 -12 m/cycle in HCF/VHCF to an average of 10 -8 m/cycle in LCF.These results, in addition to postmortem fractography work, suggest that the mechanisms follow the conventional mechanisms of crack tip stress intensification in the LCF regime. This is in stark contrast to the void controlled mechanisms that were previously identified in the HCF/VHCF regime. These results demonstrate that the transition in governing mechanisms from void controlled to conventional mechanisms is highly influenced by the size effects present in the microbeams.

Fatigue crack formation and early growth is significantly influenced by microstructural attributes such as grain size and morphology. Although the crystallographic orientation is a primary indicator for fatigue cracking, the neighbourhood conformed by the first and second neighbour grains strongly affect the fatigue cracking driving force. Hence, two identical grains may result in different fatigue responses due to their interactions with their microstructural ensemble, which determines the fatigue variability. Naturally, macroscopic samples with millions of grains and thousands of competing microstructural neighbourhoods can effectively resemble a representative volume element in which fatigue failure may seem deterministic. However, when considering systems in which fatigue failure is controlled by hundreds or less of grains, fatigue failure is stochastic in nature and the samples are not a representative but a statistical volume. This work studies fatigue crack nucleation in micron-scale Ni beams that contain a few hundred grains. This work presents 3D crystal plasticity finite element models to compute stochastic distribution of fatigue indicator parameters that serve as proxies for crack nucleation in statistical volume elements. The integration of experiments with models provides a method to understand the irreversible deformation at the grain level that leads to fatigue cracking. Our results explain the role of grain morphology of crack nucleation distribution