VIBRATION ANALYSIS OF INFLATABLE STRUCTURE

Abstract

In the field of Engineering and Architecture, Membrane structures play a vital role in many ways. Examples include textile covers and roofs, aircraft and space structures, parachutes, automobile airbags, sails, windmills, human tissues and long span structures. They are typically built with very light materials which are optimally used. These structures are characterized because they are only subjected to in-plane axial forces. A membrane is essentially a thin shell with no flexural stiffness. Consequently a membrane cannot resist any compression at all. However, membrane theory accounts for tension and compression stresses, and the need for a computational procedure that takes into account tension stresses only is needed. In membrane- theory only the in-plane stress resultants are taken into account. The position of points on the two-dimensional surface in the Euclidean space gives the deformation state for a membrane. A numerical solution for membranes may be found using the finite element method. In this the modal analysis for the predicating the behavior of inflatable membrane structure of general shape with a thickness in millimeter using the various smart material which optimally within structural member subjected to pre-stressed rather than bending or moments. A numerical solution for membranes may also be found using the finite element method. In this, flat thin membranes is chosen and analyzes its behavioral effect using different properties for the different smart material and compare the frequency, Eigen values and generalized mass, corresponding to mode of frequency. This analysis makes more effective in future to selects the smart material in the space technology. Vibration analysis of arbitrary shape of membrane is also done using a finite element package, ABAQUS. The analysis shows good agreement between finite element a'nd analytical solutions. Inflated structures undergo severe environmental changes as the structure pass from orbital day to orbital eclipse. Such environmental changes subject the inflated structure to pressure fluctuations and consequent shockwaves that vibrates the structures. Therefore, space structures must be robustly controlled from a vibration standpoint because signal transmission to and from the earth mandates tight tolerances. The control action in space structures is exerted by a small number of actuators. Hence, it is desirable that the dvnamir characteristics of the space structures be known to a high degree of accuracy in order to provide high control authority. The - space structure consists of assembly of many structural components. Inflatable beam, torus and parabolic structures are the main components of a typical space inflatable structure. For example, an inflatable beam and toroidal structure (torus) are often used to provide structural support.. Similarly, inflatable parabolic structure acts as reflector in satellite antenna and as concentrator in solar concentrator applications. One possible way to reduce the vibration would be to control the vibration of its main support structure -- inflated struts/ beams, torus or parabolic reflector surface itself. Motivated by these facts, vibration analysis of inflated beams, torus and parabolic reflector has been carried out. The focus of the present study is to develop an understanding of the inflatable structures technology for future space applications. The analysis of inflated structures are not straight forward because these structures are made of very thin and highly flexible material and have negligible inherent stiffness. These structures gain strength and proper shape due to inflation pressure. Due to this pressure, the structure is in a condition of pre-stress. Dynamic response of inflated structure undergoing small amplitude vibration can be obtained by a linear dynamic analysis. However, a non-linear static analysis is required to find out the initial stress state and consequently the stiffness of the matrix. Since, the initial deformations due to the applied inflation pressure on the structure are large, nonlinear strain displacement equations are used. Large deformation static analysis is carried out in which the stiffness matrix is c updated at each load step. The final updated stiffness matrix is used in the dynamic analysis with the assumption that the structure is undergoing small amplitude vibration. Due to the vibration of the structure, the inflation fluid gets compressed and rarefied. Therefore, a standing pressure wave is created in the fluid medium accompanied with a small change in pressure and density. The pressure variation in the fluid due to the vibration of the structure can be obtained by solving the linear acoustic equation by making the usual assumption of an inviscid ideal fluid. The interaction between this pressure wave and the structural vibration affects the vibration of the structure. To construct a model describing the dynamics of the coupled system, it is necessary to incorporate the shell dynamics and the coupling due to acoustic-structure interactions. A commercial finite element package, ABAQUS is used to model the inflated structures. d