Radially Symmetric Steady State Thermal and Mechanical Stresses of a Poro FGM Hollow Sphere
A general solution for the one-dimensional steady-state thermal and mechanical stresses in a hollow thick sphere made of porous functionally graded material (FGPM) is presented. The temperature distribution is assumed to be a function of radius, with general thermal and mechanical boundary conditions on the inside and outside surfaces of the sphere. The material properties, except Poisson's ratio, are assumed to vary along the radius r according to a power law function. The analytical solution of the heat conduction equation and the Navier equation lead to the temperature profile, radial displacement, radial stress, and hoop stress as a function of radial direction.
Functionally graded materials (FGMs) are made of a mixture with arbitrary composition of two different materials, and the volume fraction of each material changes continuously and gradually. The FGMs concept is applicable to many industrial fields such as chemical plants, electronics, and biomaterials . Thick hollow sphere analysis made of FGM under mechanical and thermal loads and in asymmetric and two-dimensional () state was conducted investigating navier equations and using legendre polynomials . The analytical solution for the stresses in spheres and cylinders made of functionally graded materials are given by Lutz and Zimmerman [3, 4]. They considered thick spheres and cylinders under radial thermal loads, where radially graded materials with linear composition of the constituent materials were considered. Obata and Noda  studied one-dimensional steady thermal stresses in a functionally graded circular hollow cylinder and a hollow sphere using the perturbation method. The transient thermal stresses in a plate made of FGM are presented by the same authors . By introducing the theory of laminated composites, Ootao and Tanigawa derived the three-dimensional transient thermal stresses of a nonhomogeneous hollow sphere with a rotating heat source . Jabbari et al. presented the analytical solution of one and two-dimensional steady state thermoelastic problems of the FGM cylinder . Two-dimensional nonaxisymmetric transient mechanical and thermal stresses in a thick hollow cylinder is presented by Jabbari et al. . Shao and Wang presented three-dimensional solution to obtain stress fields in an FGM cylindrical plate with a finite length which is mechanical and thermal load tests and showed the results an a graph . The transient thermal stress problem in a hollow sphere with homogeneous and isotropic properties is analytically solved by Cheung et al. . Their assumed thermal boundary conditions are varied along the direction and the problem is solved with the potential function method. Takeuti and Tanigawa have employed the potential function method to obtain the analytical solution of a homogeneous spherical vessel with rotating heat source . They considered the general form of thermal boundary conditions and analyzed the problem in transient condition with the potential functions method.
Porous spheres of nanometer to micrometer dimensions are being pursued with great interest because of several possible technical applications in catalysis, drug delivery systems, separation techniques, photonics, as well as piezoelectric and other dielectric devices [13, 14]. The study of the thermomechanical response of fluid saturated porous materials is important for several branches of engineering [15–19]. Some of the important cases are the disposal of high-level radioactive waste, the extraction of oil or geothermal energy, the storage of hot fluids, and road subgrade or furnace foundations, which are usually subjected to cyclic changes of temperature. Wang and Papamichos [20, 21] presented analytical solutions for the temperature, pore pressure and stresses around a cylindrical wellbore and a spherical cavity subjected to a constant temperature change and a constant fluid flow rate by coupling the conductive heat transfer with the pore-fluid flow. Using the so-called heat source function method. Kurashige  analyzed the heat and fluid flow problem of a spherical cavity subjected to a constant heat flux in an impermeable medium.
Inspite of conducted studies on spherical and cylindrical vessel made of FGM to obtain mechanical displacements and mechanical and thermal stresses, there is not any study on composition of poro and FGM materials. This study investigates the effect of mechanical and thermal stresses on one-dimensional steady state in the poro FGM hollow sphere that fluid trapped in the poro medium which is located in undrained conditions.
Consider a thick hollow sphere of inside radius and outside radius made of poro FGM. The sphere material is graded through the radial -direction. Thus, the material properties are functions of . Let be the displacement component along the radial direction. The strain-displacement relations are The stress-strain relations one-dimensional in the poro FGM hollow sphere with fluid trapped in the poro pure are where where and are the stress and strain tensors, is the temperature distribution determined from the heat conduction equation, is the coefficient of thermal expansion of effective stress, and , , , and are elastic constants, Biots coefficient of effective stress, poro pressure, and kronecker delta.
Equations in a porous material are For fluid in undrained condition, where , , , , , , , and are variation of fluid content, volumetric strain, Biots modulus, undrained bulk modulus, drained bulk modulus, bulk modulus of fluid, compressibility coefficient, and porosity.
The equilibrium equation in the radial direction, disregarding the body force and the inertia term, is Substituting (5) into (2) leads to where The sphere’s material is assumed to be described with a power law function of the radial direction as where is the power law index of the material.
Using relations (1)–(9), the Navier equation in term of the displacement is where
3. Heat Conduction Problem
The heat conduction equation in the steady-state condition for the one-dimensional problem in spherical coordinates and the thermal boundary conditions for a poro FGM hollow sphere are given, respectively, as where is the thermal conduction coefficient and are either thermal conduction coefficient , or convection coefficient , depending on the type of thermal boundary conditions. The terms and are known constants on the inside and outside radii. It is assumed that the thermal conduction coefficient is a power function of as Using (13), the heat conduction equation becomes Integrating (12) twice yields Using the boundary conditions (16) and (17) to determine the constants and yields
4. Solution of the Navier Equation
The Navier equation for the radial displacement was given in (10). Equation (10) is the nonhomogeneous Euler differential equation with the general and particular solutions. The general solution, , is obtained by assuming Substituting (18) into the homogeneous form of (10) yields where (19) has two real roots and asThus, the general solution is The particular solution is assumed to have the form
Substituting (22) in (10) yields Equating the coefficients of the identical powers yields The complete solution for is the sum of the general and particular solutions as Thus, Substituting (27) into (1) and (2), the strains and stresses are obtained as where To determine the constants and , the boundary conditions for stresses may be used. Consider the mechanical boundary conditions on the inside and outside radii as
5. Numerical Results and Discussion
The analytical solution obtained in the previous section may be checked for a one of example. Then, consider a thick hollow sphere of inner radius = 1 m, and the outer radius = 1.2 m of poro FGM material with properties are given in Table 1. The boundary conditions for temperature are taken as and . The hollow sphere may be assumed to be under internal pressure of 50 MPa and zero external pressure ( = −50 MPa and = 0 MPa). For different values of , temperature profile, radial displacement, radial stresses, and hoop stresses along the radial direction are plotted in Figures 1–4. Figure 1 shows that as the power law increases, the temperature decreases. Figure 2 shows that for higher values of , radial displacement decreases. Figure 3 represents the radial stress along the radial direction and decreases as the power law index increases. The circumferential stress versus the radial direction is shown in Figure 4. It is seen that for , the circumferential stress decreases along the radial direction. When , the situation is reversed, and the circumferential stress increases along the radial direction. The curve associated with shows that the variation of circumferential stress along the radial direction is minor and is almost uniform across the radius. Effective porosity and compressibility on mechanical radial displacement in Figures 5 and 6, radial stress in Figures 7 and 8 and hoop stress in Figures 9 and 10 for are shown. As noted from Figures 5–10, distribution that with increasing porosity increases radial displacement, and all mechanical stresses decrease. Also, with increasing compressibility, radial displacement decreases, and all mechanical stresses increases.
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