▎ 摘　　要

Phenomenological theories of deformation in brittle solids generally envisage fracture under tension as the breaking of atomic bonds perpendicular to the fracture plane, thereby ignoring the role of bond rotation. Using density functional theory calculations, we found that during the early stage of the uniaxial tension of graphene allotropes with high Poisson's ratios, bond rotation effectively lessens bond stretch and increases the fracture strain. Specifically, in the deformation of an allotrope, Gr10, with a Poisson's ratio of 0.8, bond rotation results in an S-shaped stress-strain curve, akin to those of elastomers. Moreover, the tensile strength (sigma(th)) and the Young's modulus (E) of Gr10 exceed the theoretical cohesive strength limit sigma(th) approximate to E/10, reaching sigma(th) approximate to E/1.7. However, a universal relationship between bond lengths and charge density distribution along bond paths was found to be suitable for all carbon-carbon covalent bonds. Consequently, all carbon-carbon bonds obey a common shape of bond-force vs. bond-strain curve, with bond strength, S, and bond stiffness, K, following S approximate to K/9; hence, sigma(th) approximate to E/10 remains valid for the low-Poisson's-ratio graphene allotropes whose deformation is dominated by bond stretch. Overall, we suggest the trade-off between bond stretching and bond rotation can be utilized to enhance the fracture strain of two-dimensional carbon structures. (C) 2018 Published by Elsevier Ltd.