Abstract As the materials used in various structures become more advanced, especially in the aviation and marine industries, due to the inefficiency of the linear elastic theory, the demand for the use of the linear viscoelastic theory has been increasing. The increasing use of polymer materials and advanced composites in special structures and the incompatibility of the mechanical behavior of these materials with the theories of elasticity and viscosity pro‎mp‎ted researchers to further investigate the viscoelastic properties of these materials for the precise design and analysis of such structures. In viscoelastic materials, the response to applied loads is a combination of the responses of elastic and viscous materials. In other words, the response of these materials depends on the existing stresses as well as the history of applied stresses. Time dependence is one of the inherent properties in describing the behavior of viscoelastic materials. In the current research, the time function and the limit it creates for the stability of moderately thick fully viscoelastic and viscoelastic composite plate are investigated by defining a new function in terms of time. For this purpose, using the finite strip method and based on the first-order shear deformation theory and the effective modulus method, the stiffness and geometric stiffness matrices are obtained for each strip in the Laplace-Carson domain, then by mounting the matrices and applying the boundary conditions, the matrices of stiffness and geometric stiffness for the entire plate are determined in the Laplace-Carson domain. Finally, the buckling critical load of a perfectly elastic sheet is obtained by solving an eigenvalue problem at zero moment. Having the critical buckling load at zero moment, it is calculated by applying different percentages of the critical load to the viscoelastic plate and solving the eigenvalue problem for that unknown parameter of the introduced time function. The calculated time function can be a destabilizing factor in the viscoelastic plate. This issue creates a limit for the critical buckling load of the viscoelastic sheet. In this research, for the first time, time dependent instability for viscoelastic composite plate under static loading is investigated in detail. Also, the effect of changes in thickness, dimensions, boundary conditions, and one-way, two-way and shear loading on the time function of the fully viscoelastic plate is investigated by citing numerous examples. It is shown that the change in these geometric parameters has little effect on the time function of viscoelastic plate.

سعيد صرامي، نسرين جعفري

مجتبي ازهري، مهدي زندي