7. Conclusion
In this paper, an analytical procedure for the in-phase and antiphase vibration modes of an annular DLGS including a circular defect and the surface effects was presented. Equations of motion were derived based on the nonlocal continuum theory. The surface effects are incorporated into the governing equations of motion using the GurtineMurdoch theory. Next, these equations were analytically solved using the translational addition theorem. The numerical results reveal that the surface effect parameters have a negligible effect on the variation of the fundamental natural frequency corresponding to the APM. The comparison study was presented to validate the accuracy and stability of the results. It is observed that natural frequencies of an eccentric CeC annular graphene sheet strongly depend upon the size and location of hole. In the CeF sheets, the hole's radius and the eccentricity do not significantly affect the first natural frequencies of in-phase and anti-phase modes. Also, it is concluded that regardless of the boundary conditions, the surface residual stress has more effects on the fundamental frequency of the sheets in comparison with the surface elastic modulus. This analytical approach can help other researchers to reach a better understanding about the geometrical defect in the DLGSs