  Understanding failure in nanomaterials is critical for the design of reliable
structural materials and small-scale devices that have components or
microstructural elements at the nanometer length scale. No consensus exists on
the effect of flaws on fracture in bulk nanostructured materials or in
nanostructures. Proposed theories include nanoscale flaw tolerance and
maintaining macroscopic fracture relationships at the nanoscale with virtually
no experimental support. We explore fracture mechanisms in nanomaterials via
nanomechanical experiments on nanostructures with pre-fabricated surface flaws
in combination with molecular dynamics simulations. Nanocrystalline Pt
cylinders with diameters of 120 nm with intentionally introduced surface
notches were created using a template-assisted electroplating method and tested
in uniaxial tension in in-situ SEM. Experiments demonstrate that 8 out of 12
samples failed at the notches and that tensile failure strengths were 1.8 GPa
regardless of whether failure occurred at or away from the flaw. These findings
suggest that failure location was sensitive to the presence of flaws, while
strength was flaw-insensitive. Molecular dynamics simulations support these
observations and show that incipient plastic deformation commences via
nucleation and motion of dislocations in concert with grain boundary sliding.
We postulate that such local plasticity reduces stress concentration ahead of
the flaw to levels comparable with the strengths of intrinsic microstructural
features like grain boundary triple junctions, a phenomenon unique to
nano-scale solids that contain an internal microstructural energy landscape.
This mechanism causes failure to occur at the weakest link, be it an internal
inhomogeneity or a surface feature with a high local stress.
