  The discovery of graphene, a single monolayer of graphite, has closed the
discussion on stability of 2D crystals. Although thermal fluctuations of such
crystals tend to destroy the long-range order in the system, the crystal can be
stabilized by strong anharmonicity effects. This competition is the central
issue of the crumpling transition, i.e., a transition between flat and crumpled
phases. We show that anharmonicity-controlled fluctuations of a graphene
membrane around equilibrium flat phase lead to unusual elastic properties. In
particular, we demonstrate that stretching $\xi$ of a flake of graphene is a
nonlinear function of the applied tension at small tension:
${\xi\propto\sigma^{\eta/(2-\eta)}}$ and ${\xi\propto\sigma^{\eta/(8-\eta)}}$
for clean and strongly disordered graphene, respectively. Conventional linear
Hooke's law, ${\xi\propto\sigma}$ is realized at sufficiently large tensions:
${\sigma\gg\sigma_*},$ where $\sigma_*$ depends both on temperature and on the
disorder strength.
