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Half film and effective length theory of hybrid anisotropic materials

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https://www.eduzhai.net International Journal of Composite Materials 2017, 7(3): 103-114 DOI: 10.5923/j.cmaterials.20170703.03 Semi-Membrane and Effective Length Theory of Hybrid Anisotropic Materials S. W. Chung1,*, G. S. Ju2 1School of Architecture, University of Utah, Salt Lake City, USA 2Department of Architecture, Yeung Nam University, Tae Gu, Korea Abstract Among different shell theories, semi-membrane shell theory of isotropic materials is known to be developed nearly a century by Vlasov and it has been efficiently utilized to design and analysis. The aerospace vehicle structures and rocket fuel storage tanks designed efficiently however not much for composite materials, while most of recent structural materials are of the combination of anisotropic materials. This paper developed the theory for multiple combination of composite materials by means of asymptotic integration method starting from a three dimensional elastic element. The semi-membrane cylindrical shell theories are classified as that of very long effective length scale and we adopted here the longest possible longitudinal and short circumferential length scales, respectively, for the formulation. The edge effects due to the prescribed boundary condition penetrate differently depending on material orientation of each layer but all within the limit of length scale (ah)1/2 where Donnell-Vlasov bending theory is valid. Demonstrated that beyond the limit of edge effective zone, membrane or pseudo-membrane state dominates, it is traditionally named semi-membrane state. Governing equations of semi-membrane theory of cylindrical shell are formulated and the physical interpretation of the theory is described. The theory is very useful to analyze and design long cylindrical shells such as rocket fuel storage tanks, open top oil stage tanks and chimney structures. Keywords Semi-membrane theory, Pseudo-membrane theory, Donnel-Vlasov theory, Edge effect 1. Introduction Cylindrical shells are used for space shuttles, rocket fuel storage tanks, aircraft fuselages, above ground fuel storage tanks and deep water submarines. Depending on the environment where the shell structures are located, it will take positive or negative pressure, we must assure that the structure should be properly designed and never be collapsed. The advantages of cylindrical shells are not only its functional capacity and aerodynamic features but also a simple coordinate system for the mechanical analysis compared to spherical or conical shapes. For a shell of general shape we will first accept and use the classical assumption of Love then compare our theory with more elaborate theories of Reissner and Donnell-Vlasov as shown in the References [1-3]. It is known that Vlasov during the course of simplifying his cylindrical shell equation named Donnell-Vlasov theory he had noticed a proper equation for long longitudinal length, it is interesting that he might have invented the mathematics * Corresponding author: samuelchung00@gmail.com (S. W. Chung) Published online at https://www.eduzhai.net Copyright © 2017 Scientific & Academic Publishing. All Rights Reserved but never used the terminology “semi-membrane” at all. Who started to name the terminology and how Vlasov was honored are an interesting issue. When analyzing the cylindrical shells, it is common to utilize known physical parameters such as radius (a) thickness (h) and length (L). Donnell, Vlasov and many other scientists classified when develop the analytical model for circular cylindrical shell as following categories: Short and Intermediate effective length, very long effective Length. Once we built the analysis for the cylindrical shells we could easily convert to the other shape shells as shown in the References, [8, 17, 16, 19, 20]. However, the mathematics to describe its behavior and characteristics are complicated and it is more of challenge when the materials are anisotropic and combination of different anisotropic materials, hybrid anisotropic shell structures. Let us first start with three dimensional coordinate system of shell structures. The system is of longitudinal (X, z), circumferential (φ, θ) and radial (r) as shown in Figure 1 and 2. The original and non-dimensional coordinates as shown in the figures are used to allow an asymptotic integration process. According to the exact three-dimensional theory of elasticity, a shell element is considered as a volume element. All possible stresses and strains are assumed to exist and no simplifying assumptions are allowed in the formulation of the theory. We 104 S. W. Chung et al.: Semi-Membrane and Effective Length Theory of Hybrid Anisotropic Materials therefore allow for six stress components, six strain components and three displacements as indicated in the following equation: ij  Cijklkl i, j  1, 2, 3 k, l  1, 2 (1) where σij and εkl are stress and strain tensors respectively and Cijkl are elastic moduli. There are thus a total of fifteen unknowns to solve for in a three dimensional elasticity problem. On the other hand, the equilibrium equations and strain displacement equations can be obtained for a volume element and six generalized elasticity equations can be used. A total of fifteen equations can thus be formulated and it is basically possible to set up a solution for a three-dimensional elasticity problem. It is however very complicated to obtain a unique solution which satisfies both the above fifteen equations and the associated boundary conditions. This led to the development of various theories for structures of engineering interest. A detailed description of classical shell theory can be found in various references [1] through [13]. The cylindrical shell theory that we are concerned is among the three classifications of above is very long effective length shell which is well compared to Donnell-Vlasov theory. As shown in Figure 4, the edge effective zone due to the prescribed edge boundary condition represented by curved pattern is limited within close distance from the end and after the zone the deformation is nearly linear with respect to the longitudinal axis, which is very close to membrane analysis. It is more distinct for a cylindrical shell of one end fixed or hinged boundary condition and the other free open. That is semi-membrane status, we will now mathematically formulate the governing equations. According to Calladine, Vinson and Chung, References [18, 21, 22] respectively, we pick and choose the longitudinal (L) and circumferential (ℓ) length scales as follows: (2) While we keep the circumferential length scale as same as the inner radius (a) of the shell, the longitudinal length scale is the longest we can physically describe. Insert the equation (2) into stress displacement and equilibrium equations and use a small thin shell parameter λ = h/a, where h is the total thickness and a is the inner radius of the shell, we will obtain the following equations. Stress-displacement relations (3) Equilibrium equations International Journal of Composite Materials 2017, 7(3): 103-114 105 (4) By using the asymptotic expansion parameter λ is very small, as shown in the References (12) and (13) we can obtain the desired approximate systems of shell theory. (5) The first three equations of above integrated and obtained as follows: (6) Where Vr, Vz and Vθ are the components of the initial displacement at y=0 or r=a surface. By substituting the equation (6) into next three equations we can obtain the following stress strain equations: 106 S. W. Chung et al.: Semi-Membrane and Effective Length Theory of Hybrid Anisotropic Materials (7) The last three equations of the equations (5), (6) and (7) can be examined and interpreted as the characteristics of the following phenomena. Transverse Strains; the first three equations of (5) are zero and all displacements shown in (6) independent of thickness coordinate, r, which means it is only membrane state. In-plane Circumferential and Shear Strains; the longitudinal strain is represented by the combination of longitudinal and circumferential stresses (tz and tθ). In-plane Shear Stresses; trz does not appear in the first approximation theory. We now complete the second approximation theory of the asymptotic expansion and integration procedure, which can be shown as follows: (8) By integrating the first three equations of the above, we will obtain the following equations: (9) International Journal of Composite Materials 2017, 7(3): 103-114 107 Inserting the displacements obtained in the equation (9) into the middle three equations of (8) to obtain the following equations: (10) By combining the equation for Vz,x in the equation (7) and the last two equations of (9), we can obtain the in-plane stress-strain relations as follows: (11) where (12) 108 S. W. Chung et al.: Semi-Membrane and Effective Length Theory of Hybrid Anisotropic Materials Substituting tz, tθ and tθz into the first approximation equations of (5), we will obtain the following transverse stresses: (13) The boundary conditions at the inner and the outer surface of the shell can be specified as follows: (14) While the pressure term will be dimensionless as: (15) Note that p* is axi-symmetric and only allowed to vary in longitudinal direction. (16) After mathematical manipulation and inserting boundary conditions, we can obtain the following final equations: (17) where (18) Finally the above equations are the governing equations of the semi-membrane theory of hybrid anisotropic materials. Note that the higher derivatives are those associated with the circumferential coordinate φ. This is due to the shorter circumferential length scale compare to the longer longitudinal length scale. The equations are further simplified if we consider the physical nature of phenomena, which are axi-symmetric load and axi-symmetric deformations and stress variations. International Journal of Composite Materials 2017, 7(3): 103-114 109 Figure 1. Coordinates of the Cylindrical Shell Figure 2. Normal and Shear Stresses 110 S. W. Chung et al.: Semi-Membrane and Effective Length Theory of Hybrid Anisotropic Materials Figure 3. A Laminated Cylindrical Shell, Material Orientation γ International Journal of Composite Materials 2017, 7(3): 103-114 111 Figure 4. Cylindrical Shell under Internal Pressure 112 S. W. Chung et al.: Semi-Membrane and Effective Length Theory of Hybrid Anisotropic Materials Figure 5. Edge Effective Zone of Bending and Pure Membrane Theories

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