  Thin membranes, such as monolayer graphene of monoatomic thickness, are bound
to exhibit lateral buckling under uniaxial tensile loading that impairs its
mechanical behaviour. In this work, we have developed an experimental device to
subject 2D materials to controlled equibiaxial strain on supported beams that
can be flexed up or down to subject the material to either compression or
tension, respectively. Using strain gauges in tandem with Raman spectroscopy
measurements, we monitor the G and 2D phonon properties of graphene under
biaxial strain and thus extract important information about the uptake of
stress under these conditions. The experimental shift over strain for the G and
2D Raman peaks were found to be in the range of 62.3 \pm 5 cm^-1/%, and 148.2
\pm 6 cm^-1/%, respectively, for monolayer but also bilayer graphenes. The
corresponding Gruneisen parameters for the G and 2D peaks were found to be
between 1.97 \pm 0.15 and 2.86 \pm 0.12, respectively. These values agree
reasonable well with those obtained from small-strain bubble-type experiments.
The results presented are also backed up by classical and ab initio molecular
dynamics simulations and excellent agreement of \Gamma-E2g shifts with strains
and the Gruneisen parameter was observed.
