Magneto-Elastic Analysis of an Annular FGM Plate Based on Classical Plate Theory Using GDQ Method

Using GDQ method, the radial and circumferential stresses in an annular FGM plate with a uniform thickness under a transverse axisymmetric load is investigated. It is assumed that a uniform radial magnetic field acts on the top surface of the plate. The modulus of elasticity E and the magnetic permeability coefficient  of the plate along its thickness are assumed to vary according to the volume distribution function. The Poisson’s ratio is considered to be constant. Based on the classical plate theory (CPT), equilibrium equations are deduced and the displacement fields are determined. The radial and circumferential stresses as well as transverse and radial displacements are obtained accordingly. The effect of volume fraction function power m on the maximum deflection in the absence and presence of the magnetic field is also investigated. Moreover, the effect of t/a and b/a ratios on displacements, stresses, induction magnetic field intensity and the resulting Lorentz force are also investigated. According to the results, for different points along the radial direction, the application of radial magnetic field to the top surface of the plate completely changes the state of stress in both tangential and radial directions, resulting in tensile and compressive stresses in these two directions. The results also indicate that in presence of magnetic field, the plate displacement and stress components are lowered considerably.


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according to a function.These materials can be used when good thermal and mechanical resistances are required simultaneously.For example, in a FGM plate made of ceramic and metal, the ceramic sector is a good insulator against heat and thermal shocks while the metallic sector acts as a good resistance against mechanical loads.The unique properties of these materials are due to their composite nature and their physical properties which gradually change across the thickness of the specimen.This in effect reduces the hysteresis stresses, as well as the stress concentration, with a significant improvement in mechanical properties of the overall material, especially at the ceramic and metallic junction.
In recent years, considerable research has been oriented toward FGM plates to better understand their behavior under various loading conditions.Bayat et al. (2014) investigated the magneto-thermo-mechanical response of a FGM annular rotating disc with variable thickness and observed that unlike the positive radial stresses developed in a mechanically loaded FGM disk, in a FGMM (functionally graded magneto-elastic material) disk, the radial stresses due to magneto -thermal load can be both tensile and compressive.Behravan Rad and Shariyat (2015) analyzed a porous circular FG plate with variable thickness subjected to nonaxisymmetric and non-uniform shear along with a normal traction and a magnetic actuation.The plate was supported on a non-uniform Kerr elastic foundation.They evaluated the effect of material, loading, boundary and elastic foundation on the resulting displacement, stress, Lorentz force, electromagnetic stress and magnetic perturbation quantities.Chi and Chung (2006) studied the mechanical behavior of FGM plates under transverse load using a numerical method.Ma and Wang (2003) studied nonlinear buckling and bending behavior of FGM circular plates under mechanical and thermal loads based on the classical nonlinear von Karman plate theory.They discussed the effects of material power distribution function and boundary conditions on the temperature distribution, nonlinear bending, critical buckling temperature and thermal post-buckling behavior of the plate in details.Najafizadeh and Heydari (2004) analyzed thermal buckling of FGM circular plates under various thermal loads based on higher order shear deformation theory (HSDT) and compared their results with those obtained using first order shear deformation theory (FSDT) and classical plate theory (CPT).They showed that HSDT theory predicts the behavior of FGM circular plates with higher precision compared to FSD and CP theories.Praveen and Reddy (1998) studied the thermo elastostatic and thermo-elastodynamic response of FGM plates subjected to varying pressure and temperature loading.They showed that the combination proportion of materials in FGM plates plays an important role on determining their response.Reddy et al. (1999) analyzed axisymmetric bending and stretching of FGM annular plates using the first-order shear deformation Mindlin plate theory and concluded that this theory gives good results on FGM annular plates whenever the Kirchhoff solution is not applicable.Reddy (2000) presented a theoretical solution and finite element model of FGM rectangular plates based on third order shear deformation theory (TSDT) and studied the effect of material distribution on deflections and stresses.Saidi et al. (2009), investigated the bending and buckling behavior of FGM circular plates, based on the third-order shear deformation theory and compared their results with those of first order shear deformation theory.Applying finite Hankel integral transforms, Wang and Dai (2004)  for magneto dynamic stress and perturbation response of an axial magnetic field vector in an orthotropic cylinder under thermal and mechanical shock loads.They deduced the response histories of dynamic stresses and the perturbation of the field vector.Yuda and Jing (2009) obtained the electrodynamic equations and electromagnetic force expressions of a currentconducting thin plate in an electromagnetic field, based on Maxwell equations.They investigated the nonlinear sub-harmonic resonance of the thin plate with two simply supports on opposite sides under a mechanical live load.In their analysis, an inconstant transverse magnetic field load was applied as well.
Ghorbanpour Arani et al. ( 2010) presented a semi-analytical solution for magnetothermo-elastic problem in a functionally graded (FG) hollow rotating disks with variable thickness under uniform magnetic and thermal fields.They showed that imposing a magnetic field on the disk significantly decreases tensile circumferential stresses.Using GDQ method, Lal and Saini (2015) analyzed the effect of two-dimensional non-homogeneity on transverse vibration of orthotropic rectangular plates of bidirectional thickness variation on the basis of Kirchhoff's plate theory.They investigated the effect of non-homogeneity parameters, density parameters, thickness parameter and the aspect ratio on natural frequencies, for the first three modes of vibration.In another paper which was published in the same year, they also investigated the effect of uniform tensile in-plane forces on the radially symmetric vibratory characteristics of functionally graded circular plates with linearly varying thickness along radial direction.The plate was resting on a Winkler foundation.
In this research, the effect of a radial magnetic field on a FGM annular plate is analyzed.The plate is subjected to a transverse mechanical load as well as a transverse load fz deduced from the radial magnetic field applied to the top surface of the plate.Due to complexity of the problem, the deduced differential equations are solved using DQM method.This is a numerical scheme which can be accurately applied to problems with or without a close form solution.Applying the classical plate theory, the effect of volume fraction function power m and the perturbation of magnetic field vector are studied on the induced displacement and stress components.Moreover, the effect of t/a and b/a ratios on the displacement and stress fields are studied.

BASIC FORMULATIONS
Consider a radial magnetic field vector H  as shown in Figure 1.The resulting Lorentz force (fz) and the perturbation of electric field vector e  act along z and  directions respectively.Now assume an annular circular plate with a uniform transverse load Po acting on its top surface (see Fig. 2) is exposed to this magnetic field.As a result, the total transverse load acting on the plate, along z direction, would be qz (qz=Po+ fz).This will induces a displacement field vector U  in the plate.
Latin American Journal of Solids and Structures 13 (2016) 2736-2762  Assuming the magnetic permeability ) (z  of the plate is equal to the magnetic permeabil- ity of its surrounding, neglecting the displacement electric currents, the Maxwell's electrodynamics equations for the plate may be written as (Wang and Dai, 2004); Latin American Journal of Solids and Structures 13 (2016) 2736-2762 where j  is the surface density vector of the electric current, e  is the perturbation of the electric field vector, h  is the perturbation of the magnetic field vector and to is the time.On using cylindrical coordinates ) , , ( z r  , application of the magnetic field vector To deduce the equilibrium equations, it is assumed that the plate properties are distributed along the thickness according to equations ( 8) -(10).
Here, m is the volume fraction function power, t is the plate thickness which is assumed to be uniform and 1  and 2  are the elastic moduli and magnetic permeability coefficients of phases 1 and 2 of the FG material, respectively (see In axisymmetric problems, deflection w, stresses and strains are independent of the circumferential direction and hence, their derivatives with respect to  are equal to zero.Con- sequently, in the present study, due to axisymmetric loading, deflection w becomes only a function of r and one may write; Additionally: where 0 u is the displacement of the middle surface of the FGM plate.Using Hooke's law, the radial and circumferential stresses are; Expressing the total potential energy as , then the strain energy U and the potential energy of the external forces f V are equal to; Latin American Journal of Solids and Structures 13 (2016) 2736-2762 In Eq. ( 15), r Q and z q (qz=Po+fz) are the shear force resultant and transverse load, i N , are the resultant forces and moments, and is the small transverse normal rotation about  axis, respectively.Using equation ( 15), the equilibrium equations may be written as follow: Simplifying Eqs. ( 16), one may write; On using Eq. ( 12), Eq. ( 13) may be written as;   where; Based on Eqs. ( 17a) and (20a), one may write; On using Eqs.( 17b) and (20b) and substituting , one can obtain: To solve the above equilibrium equations, four types of boundary conditions are considered as follow; 1. Clamped-Clamped (C-C) supports; Here, the outer and inner edges of the plate are clamped.In this case, the boundary conditions are; 2. Simply supported-Clamped (SS-C); Here, the outer edge of the plate is clamped while the inner edge is simply supported.In this case, the boundary conditions are; 3. Free-Clamped (F-C); Here, the outer edge of the plate is clamped while the inner edge is free.In this case, the boundary conditions are; 4. Simply supported-simply supported (SS-SS): Base on this type of boundary condition, the outer and inner edges of the plate are simply supported.In this case, the boundary conditions are; To solve the equilibrium equations ( 22) and ( 23), generalized differential quadrature (GDQ) is used, as will be described next.Structures 13 (2016) 2736-2762 3 GENERALIZED DIFFERENTIAL QUADRATURE (GDQ) GDQ is a numerical method that approximates the derivative of a function with respect to a variable as the sum of weighted linear function values at all domain points.This may be written as;

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where n is the number of grid points in r direction.In this work, for interval b ≤ r ≤ a, Chebyshev polynomials are used to identify the grid points.These point are defined by Eq. ( 25).
Latin American Journal of Solids and Structures 13 (2016) 2736-2762 According to Eqs. ( 32) to (44), equilibrium Eqs. ( 32) and ( 33) may be written in a matrix form as; Here, n and nn are the total number of grid points in r and z directions respectvely.In Eqs.(45), the determinant of matrix [K] is equal to zero.Application of the previously described boundary conditions to Eq. ( 45) results in values of wi (i = 1, n) and ui (i = 1, n).Using these values, one can calculate the radial and tangential stresses in each case..

NUMERICAL RESULTS AND DISCUSSION
To investigate the effect of plate geometric parameters on the induced displacements and stresses, it is assumed that the annular FGM plate is experiencing a uniform mechanical load .The bottom and top surfaces of the plate are made from pure metal and pure ceramic, respectively.The material properties used to extract the results are given in Table 1.Also, the dimensionless parameters used in the upcoming figures are defined as; Latin American Journal of Solids and Structures 13 (2016) 2736-2762 Figures 3 shows the variation in midplane deflection in terms of r a , based on four different types of boundary conditions given in section 2. In all cases, the transverse load z q is the sum of mechanical and magnetic loads.As expected, the (C-C) condition experiences the least deflection among all cases.In all cases, a pure ceramic plate with m = 0, experiences the least deflection compared to other values of m.This is due to an increase in flexural rigidity of the plate which is caused by a decrease in m.
To investigate the effect of parameter m on radial deflection, a typical (C-C) plate with similar loads and dimensions was selected.According to the results shown in Fig. 4, any increase in power ratio m has a similar effect on midplane radial displacement.According to these results, for a homogeneous plate with m = 0, the radial displacement of the midplane is equal to zero.This is due to symmetry in mechanical properties of the plate which makes 11 B equal to zero.In addition, it is realized that zero radial deflection occurs at a point where transverse deflection w is maximum ( 0.66 r a  ).Additionally, the nature of variation in both stress components appears to be highly dependent on the type of boundary conditions imposed on the plate.The results indicate that (SS-C) boundary conditions produce the least radial stress, while a plate with (F-C) boundary conditions experiences the highest tangential stress in the midplane.vector substantially affects the location and values of both stresses at the inner and outer edges of the plate, as well as other locations.Additionally, any change from positive to negative stresses in both directions seems to be highly affected by the presence of magnetic field.According to these figures, the (SS-SS) boundary conditions appear to exhibit less complex distribution for both Lorentz force and the induction magnetic field intensity along the plate radius.Additionally, reducing the plate dimensions will lower the magnitude of displacement components, and hence relevant stresses are reduced, as shown in Figs.20 and 21.This is due to plate stiffening as a result of lower dimensions and lower plate's inner radius.It is worth to mention that based on the values given in Figs.12-13 and 27-28, for all values of H r , zero radial and tangential stresses occur both in radial and transverse directions.

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The locations of zero stress components across the plate thickness appear to be on a surface which is slightly above the z =0 plane.

CONCLUSIONS
In this work, the radial and transverse displacements, as well as the radial and circumferential stress components in an annular FGM plate subjected to a combination of transverse load Po and Lorentz force fz, were calculated based on four types of boundary conditions.The elastic modulus of the plate as well as the magnetic permeability coefficient across the plate thickness were assumed to vary according to the volume distribution function, while the Poisson's ratio was taken to be constant.Classical plate theory was applied to analyze the problem.The deduced equilibrium equations were solved using generalized differential quadrature method.According to the results, the effect of additional load fz induced by the radial magnetic field H  seems to be substantial on plate radial and transverse deflections.Additionally, in presence of a transverse load Po, application of a radial magnetic field to the plate top surface, completely changes the state of tangential and radial stresses along its radius, resulting in positive and negative stresses in these two directions.The effect of additional transverse load fz induced by the magnetic field vector H  on both stress components across the plate thickness seems to be more on bottom surface (pure metal).Moreover, the magnitude of the Lorentz force and the induced magnetic field intensity z h across the plate thickness appear to highly depend on the volume fraction function power m.Introduction of a

Figure 2 :
Figure 2: Geometry, loading and coordinate system of the annular plate.

Figure 3 :
Figure 3: The effect of parameter m on transverse deflection of the plate for three different support conditions.

Figure 4 :Figure 5 :Figure 6 :
Figure 4: The effect of parameter m on the midplane radial displacements, (C-C) supports.

Figure 7 :Figure 8 :Figure 9 :
Figure 7: The effect of parameter m on the mid-plane radial stresses based on (SS-C) supports.

Figure 10 :
Figure 10: The effect of radial magnetic field variation H  on plate transverse deflection w, (C-C) supports.

Figure 11 :
Figure 11: The effect of radial magnetic field variation H  on plate radial deflection, (C-C) supports.

Figure 12 :Figure 13 :
Figure 12: The effect of radial magnetic field variation H  on plate radial stress, (C-C) supports.

Figure 14 :Figure 15 :Figure 16 :
Figure 14: The effect of variation in power m on (a) Lorentz force fz and (b) induction magnetic field intensity for (C-C) supports.

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Figure 17: The effect of variation in power m on (a) Lorentz force fz and (b) induction magnetic field intensity for (F-C) supports.

Figure 18 :
Figure 18: The effect of variations in plate dimensions on transverse deflection, (C-C) supports.

Figure 19 :
Figure 19: The effect of variation in plate dimensions on radial displacement, (C-C) supports.

Figure 20 :Figure 21 :
Figure 20: The effect of variation in plate dimensions on radial stress distribution, (C-C) supports.

Figure 22 :Figure 23 :
Figure 22: The effect of plate dimensions on Lorentz force distribution, (C-C) supports.

Figure 24 Figure 24 :
Figure24shows the transverse displacement of different points across the plate thickness

Figure 25 :Figure 26 :
Figure 25: The effect of power m on distribution of the induction magnetic field intensity across the plate thickness, (C-C) supports.

Figure 27 :Figure 28 :
Figure 27: The effect of radial magnetic field on radial stress component accros the plate thickness, (C-C) supports.

Table 1 ,
Ghorbanpour Arani et al. 2010).Moreover, the Poisson's ratio  of the plate is considered to be constant.

Table 1 :
Mechanical properties of the individual constituents used for the annular FGM plate (Ghorbanpour Arani et al. 2010).Latin American Journal of Solids and Structures 13 (2016) 2736-2762