On the growth of nanoscale fatigue cracks

We identify nanoscale mechanisms of fatigue-crack growth in copper single crystals using molecular dynamics. By quantifying the nanoscale fatigue-crack growth rates, we can compare the growth rates for fatigue cracks on microstructural and macrostructural length scales. Computed crack growth rates in the nanometer range are shown to be very similar to those experimentally measured for small cracks (micron range), and at stress-intensity-factor ranges lower than the threshold for long cracks (millimeter range). Molecular dynamics simulations indicate that reversible plastic slip along the active crystallographic directions at the crack tip is responsible for advancing the crack during a fatigue cycle. In the case of single or double plastic slip localization at the crack tip, a typical Mode I fatigue crack deviates along a slip band, resulting in a mixed Mode I + Mode II crack-growth mechanism. For crystal orientations characterized by multiple slip systems concomitantly active at the crack tip, the crack advance mechanism is characterized by nanovoid nucleation in the high-density nucleation region ahead of the crack tip and linkage with the main crack leading to crack extension.

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