عنوان مقاله [English]
The unique electrical and mechanical properties of carbon nano-structures make them desired materials with a wide range of applicability. Hence, the study of their mechanical behavior is an important topic which has attracted a great deal of attention. Graphene, a two-dimensional carbon nano-structures with an one-atom thickness, exhibits so wonderful electrical, mechanical, thermal and optical properties which make it suitable to use in micro-/nano-electromechanical system such as nano-scale electromechanical resonators, vibrational behavior of which affects their functionality.The importance of incorporating the size effect on continuum mechanics to simulate the mechanical behavior of nano-structures is well-known. To this end, different higher-order continuum mechanics that contain additional material constants have been developed. However, the recognition of an efficient non-classical theory with respect to nano-materials is a challenging process. Modified couple stress theory, Eringens nonlocal elasticity, and Aifantiss theory are some of non-classical theories used because of existence one material length scale. Although Eringens nonlocal elasticity is the popular theory to simulate mechanical behavior of carbon nano-structures, its microstructure-dependent constitutive model does not allow the construction of the energy functional. Hence, this article is aimed at investigating the utilization of modified couple stress theory and Aifantiss theory to estimate natural frequencies of simply-supported single-layered graphene sheets. For this purpose, it is tried to estimate material length scale of theories by point-by-point matching between available first natural frequency of graphene sheets determined by molecular dynamic method and the obtained data. According to the results, modified couple stress theory cannot be used unless one substitutes square of length scale parameter with a minus value in stain energy equation or adds the gradient velocity to the kinetic energy equation by a new material length scale. The former leads to a non-positive definite strain energy function, and the later makes a gradient elasticity theory with two material length scales. Aifantiss theory is more applicable than modified couple stress theory in this situation. It is also seen that the higher order natural frequencies, estimated by different gradient elasticity theories and, material length scale of which is fixed by using molecular dynamic data, are different, although this difference decreases with an increase in side-length of square nano-plate. It can be concluded that chirality, side-length of nano-plate, and the used gradient elasticity theory affect the material length scales, as well.