  We investigated the effect of out-of-plane crumpling on the mechanical
response of graphene membranes. In our experiments, stress was applied to
graphene membranes using pressurized gas while the strain state was monitored
through two complementary techniques: interferometric profilometry and Raman
spectroscopy. By comparing the data obtained through these two techniques, we
determined the geometric hidden area which quantifies the crumpling strength.
While the devices with hidden area $\sim0~\%$ obeyed linear mechanics with
biaxial stiffness $428\pm10$ N/m, specimens with hidden area in the range
$0.5-1.0~\%$ were found to obey an anomalous Hooke's law with an exponent
$\sim0.1$.
