[en] Graphene is a two-dimensional crystal in which sp²-hybridized carbon atoms have valence bonds with three neighbors. Theoretically, other two-dimensional carbon structures were predicted, in which each carbon atom has valence bonds with three neighbors. In this paper, the molecular dynamics method is used to analyze the mechanical properties and structural transformations of such materials under uniaxial and biaxial stretching. The dependences of the tensile membrane forces on the applied tensile strain are constructed, the limiting values of the membrane forces and strains are determined. The three structures studied differ in their density, and it could be expected that the strength of the structures should decrease with decreasing density. However, it turned out that such a correlation did not manifest itself in all cases: a less dense structure may turn out to be stronger due to the fact that all interatomic bonds in it turn out to be loaded more uniformly. The results can be useful in analyzing the potentialities of application of sp²-carbon membranes in various technologies.