Dynamic Instability of Laminated Composite Curved Panels in Hygrothermal Environment

Abstract

Composite materials are increasingly used in aerospace, naval and high performance civil engineering structures such as aerospace, submarines, automobiles. The structural components, subjected to in-plane harmonic loads may undergo parametric resonance or dynamic stability due to certain combinations of the applied in-plane forcing parameters and natural frequency of transverse vibration. The parametric instability itself requires investigation of vibration and buckling of structures. The present study deals with free vibration, buckling and parametric resonance behavior of laminated composite plates under in-plane periodic loading under varying temperatureand moisture. In this analysis, the effects of various parameters such as number of layers, aspect ratios, side-to thickness ratios, ply orientations, static load factors, lamination angle and the degree of orthotropic are studied. A simple laminated plate model based on the first order shear deformation theory (FSDT) is developed for the free vibration, buckling and parametric instability effects of composite plates subjected to hygrothermal loading. The principal instability regions are obtained using Bolotin’s approach employing finite element method (FEM). An eight-node isoparametric quadratic element is employed in the present analysis with five degree of freedom per node. The element is modified to accommodate the laminated composite plates under hygrothermal environment, considering the effects of transverse shear deformation and rotary inertia. The element stiffness matrix, geometric stiffness matrix due to residual stresses, element mass matrix, geometric stiffness matrix due to applied in-plane loads and nodal load vector of the element are derived using the principle of minimum potential energy. They are evaluated using the Gauss quadrature numerical integration technique. Reduced integration technique is applied to avoid the possible shear locking. A computer program based on FEM in MATLAB environment is developed to perform all necessary computations. The basic vibration and buckling experiments are performed on the industry driven woven fiber Glass/Epoxy specimens subjected to hygrothermal environment. The specimens were hygrothermally conditioned in a humidity cabinet where the conditions were maintained at temperatures of 300K-425K and relative humidity (RH) ranging from 0-1.0% for moisture concentrations. The numerical and experimental results show that there is reduction in natural frequencies and buckling loads with increasing temperature and moisture concentration for laminates both for simply supported and clamped boundary conditions. The dynamic instability study using FEM revealed that, due to the static component of load, the instability regions tend to shift to lower frequencies. The onset of instability occurs earlier and the width of dynamic instability regions increases with rise in temperature and moisture concentration for different parameters. With increase in lamination angle, the width of the instability region becomes smaller. The onset of instability occurs later for square plates than rectangular plates with wider instability region with increase of aspect ratio. The ply orientation significantly affects the onset of instability. It is observed that the excitation frequency increases with introduction of curvatures from flat panel to doubly curved panel in hygrothermal environments. Thus the instability behaviour of laminated composite panels is influenced by increase in number of layers, aspect ratio, side to thickness ratio, increase in static and dynamic load factor, geometry, material, ply lay-up and its orientation. This can be utilized to tailor the design of laminated composite panels in hygrothermal environment.