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Mechanical failure of an ideal crystal is dictated either by an elasticinstability or a soft-mode instability. We show that the ideal strengthmeasurement of graphene based on nano-indentation experiments\cite{lee2008measurement, lee2013high}, however, indicates an anomaly: theinferred strain beneath the diamond indenter at the failure load is anomalouslylarge compared to the fracture strain predicted by soft-mode analysis oracoustic analysis. Here we present a systematic investigation - based onmulti-scale modeling combining the results of continuum, atomistic, and quantumcalculations; and analysis of experiments - that identifies the operativemechanism responsible for the anomalous difference between the fracturestrains. We suggest that a strain-shielding effect due tomechanically-activated covalent bond formation at graphene-indenter interfaceis responsible for this anomaly. Using Finite Elements Analysis (FEA) and MDsimulations of the nanoindentation experiments, we explicitly show that thebonded interaction at the graphene-indenter interface substantially disperses(shields) the strain beneath the indenter, preventing the intensification ofstrain. Our calculations indicate that the extent of strain shielding dependsupon the hydrogen coverage at the indenter surface; and at optimal hydrogencoverage, the strain-shielding effect can delay the onset of fracture to theexperimentally-observed indentation load and depth.

Strain shielding from mechanically-activated covalent bond formation in nanoindentation