  Mechanical failure of an ideal crystal is dictated either by an elastic
instability or a soft-mode instability. We show that the ideal strength
measurement of graphene based on nano-indentation experiments
\cite{lee2008measurement, lee2013high}, however, indicates an anomaly: the
inferred strain beneath the diamond indenter at the failure load is anomalously
large compared to the fracture strain predicted by soft-mode analysis or
acoustic analysis. Here we present a systematic investigation - based on
multi-scale modeling combining the results of continuum, atomistic, and quantum
calculations; and analysis of experiments - that identifies the operative
mechanism responsible for the anomalous difference between the fracture
strains. We suggest that a strain-shielding effect due to
mechanically-activated covalent bond formation at graphene-indenter interface
is responsible for this anomaly. Using Finite Elements Analysis (FEA) and MD
simulations of the nanoindentation experiments, we explicitly show that the
bonded interaction at the graphene-indenter interface substantially disperses
(shields) the strain beneath the indenter, preventing the intensification of
strain. Our calculations indicate that the extent of strain shielding depends
upon the hydrogen coverage at the indenter surface; and at optimal hydrogen
coverage, the strain-shielding effect can delay the onset of fracture to the
experimentally-observed indentation load and depth.
