Buckling and Free Vibration of Doubly Curved Sandwich Shells with Viscoelastic Core and Functionally Graded Material Constraining Layer

Abstract

Vibration is one of the primary causes of failure and underperformance in a wide range of structures, machineries and machine components belonging to aerospace, defence and mechanical engineering-based industries. Vibration can affect these structures in different ways ranging from mild inaccuracy in operating performance to catastrophic failure depending on its severity. Periodic loads are also dangerous for the structures prone to bend or buckle under their normal operating conditions. Therefore, detailed understanding about the vibration and buckling characteristics of such structures is very important to prevent any danger or failure in their operations. Based on these observations, the current research presented in this dissertation is primarily focused on the buckling and free vibration of doubly curved sandwich shells with viscoelastic core and functionally graded material constraining layer. Plates and shells are the primary building blocks of almost all types of major structural components in the mechanical and civil engineering domain. The primary motive to conduct research on the sandwich doubly curved shell panels is their excellent structural characteristics and their versatility to be modified by changing the layer properties of the sandwich based on the application. The sandwich shell addressed in the present research comprises of three layers. The base layer is made up of isotropic elastic material core of soft viscoelastic material and the constraining top layer is made up of metal-ceramic functionally graded material (FGM). Due to the excellent characteristics of viscoelastic materials and FGMs respectively in vibration suppression and providing additional strength and stability to the structure, the preset sandwich structure can be a better choice compared to their conventional counterparts. The mathematical modeling of the present sandwich shell panel is done using an eight noded isoparametric finite sandwich shell element based on the first order shear deformation theory (FOSDT). The governing equations of motion have been derived using finite element method in conjunction with the Hamilton’s energy principle. This dissertation is primarily subdivided into four technical sections which are dedicated to the investigation of buckling and vibration behavior of the current sandwich shell panel under different support and loading conditions as well as environments of engineering importance. A detailed parametric study has been conducted in each section to show the influence of critical system parameters on the buckling and free vibration behavior of the sandwich shell panel. Free vibration and buckling of viscoelastic-FGM doubly curved sandwich shell panels under different boundary conditions along with a detailed parametric study is conducted. The critical observations reveals that all sides clamped (CCCC) boundary condition imparts additional strength and stiffness to the structure compared to other boundary conditions. This eventually results in the increased natural frequencies and critical buckling loads of the structure. At the same time, modal loss factors of the structure have been observed to be decreased. Also, the elliptical paraboloid shell geometry is observed to have increased natural frequencies in many instances compared to the other shell geometries reported. Influence of high temperature environment on the free vibration and buckling behavior of viscoelastic-FGM doubly curved sandwich shell panels is investigated. As evident that increased temperatures are prone to negatively influence the strength and stiffness of many metallic structures, the same has been also observed in the current findings. However, due to the unique characteristics of the different layers of present sandwich shell panels, the influence of high temperature doesn’t seem to be that significant as it could be with the conventional metallic structures. Still, a considerable reduction in the modal natural frequencies as well as critical buckling loads of the structure has been observed majorly. The investigations of free vibration and buckling behavior of viscoelastic-FGM doubly curved skewed sandwich shell panels reveal that the skewness of the panel plays an important role in its structural behavior. Based on the observations, an increase in the skew angle results in the increased natural frequencies as well as critical buckling loads of the structures with negligible influence on the modal loss factor of the system in majority of the findings. Investigations of the buckling and free vibration characteristics of viscoelastic-FGM doubly curved sandwich shell panels resting on two parameter elastic foundations suggests that both the Winkler’s as well as the shear foundation parameters when increased are responsible in increasing the modal natural frequencies and the critical buckling loads of the structures, whereas, slightly decreases the system modal loss factors on the other hand. However, the shear foundation parameter is observed to be more dominant than the Winkle’s parameters in influencing the structural behavior of the present structure.