By combining continuum elasticity theory and tight-binding atomistic
simulations, we work out the constitutive nonlinear stress-strain relation for
graphene stretching elasticity and we calculate all the corresponding nonlinear
elastic moduli. Present results represent a robust picture on elastic behavior
of one-atom thick carbon sheets and provide the proper interpretation of recent
experiments. In particular, we discuss the physical meaning of the effective
nonlinear elastic modulus there introduced and we predict its value in good
agreement with available data. Finally, a hyperelastic softening behavior is
observed and discussed, so determining the failure properties of graphene.Comment: 4 page