181 DYNAMIC RESPONSE OF A GRID-STIFFENED COMPOSITE CYLINDRICAL SHELL REINFORCED WITH CARBON NANOTUBES TO A RADIAL IMPULSE LOAD A. Davar, R. Azarafza, * M. S. Fayez, S. Fallahi, and J. E. Jam Keywords: forced vibration, carbon nanotube, grid-stifened shell, dynamic response, frst-order shear deformation theory Equilibrium equations of a rib grid-stifened composite cylindrical shell reinforced with carbon nanotubes (CNTs) are derived based on the frst-order shear deformation theory considering the efect of shear deformation and moment of inertia. The distribution of CNTs across the shell thickness is assumed uniform, and the elastic modulus of the CNT-reinforced polymer is calculated using the rule of mixtures. In order to determine the equivalent stifness of the grid-stifened composite cylindrical shell, the smeared stifness method is used. Equilibrium equations for free and forced vibration of the rib grid-stifened composite cylindrical shell are solved using the Galerkin method, and the efects of grid ribs on the dynamic response of the shell are investigated. The results found indicate that the use of circumferential ribs in the structure can increase the frequency, change the fundamental mode shape, and reduce the radial displacement by ~ 12% (especially in higher modes). In addition, the results demonstrate that a 5-degree increase in the angle of helical ribs can decrease the radial displacement linearly by 5%. Eventually, the corresponding outcomes reveal that the rib thickness and presence of CNTs may signifcantly increase the natural frequencies and decrease the radial displacement of such shells. Introduction Grid-stifened composites shells are among the structures widely used in aviation, aerospace, and automobile industry. These structures possess a considerable strength, high specifc stifness, good corrosion resistance, and high Mechanics of Composite Materials, Vol. 57, No. 2, May, 2021 (Russian Original Vol. 57, No. 2, March-April, 2021) Faculty of Materials & Manufacturing Technologies, Malek Ashtar University of Technology, Lavizan, Tehran, Iran * Corresponding author; tel.: +98 21 2293-2256; fax: +98 21 2294 5141; e-mail: azarkntu@yahoo.com 0191-5665/21/5702-0181 © 2021 Springer Science+Business Media, LLC Russian translation published in Mekhanika Kompozitnykh Materialov, Vol. 57, No. 2, pp. 261-290, March-April, 2021. Original article submitted May 18, 2020; revision submitted November 30, 2020. DOI 10.1007/s11029-021-09944-3