Abstract

Analytical particular solutions of the polyharmonic multiquadrics are derived for both the Reissner and Mindlin thick-plate models in a unified formulation. In the derivation, the three coupled second-order partial differential equations are converted into a product operator of biharmonic and Helmholtz operators using the Hörmander operator decomposition technique. Then a method is introduced to eliminate the Helmholtz operator, which enables the utilization of the polyharmonic multiquadrics. Then, the analytical particular solutions of displacements, shear forces, and bending or twisting moments corresponding to the polyharmonic multiquadrics are all explicitly derived. Numerical examples are carried out to validate these particular solutions. The results obtained by the present method are more accurate than those by the traditional multiquadrics and splines.

1. Introduction

Boundary-type numerical methods have been emerged as a popular research field because only boundary discretizations are required when they are applied to solve homogeneous problems. These methods include the boundary element method (BEM) [1], method of fundamental solutions (MFS) [2, 3], and Trefftz methods (TM) [4, 5]. When they are applied for solving an inhomogeneous partial differential equation, the dual reciprocity method (DRM) can be used [6, 7].

In the early development of DRM, the ad hoc radial function, , was exclusively used. In order to improve the accuracy of the computation, Golberg and Chen [8] and Karur and Ramachandran [9] applied the theory of radial basis functions (RBFs) to the DRM. Among these RBFs, Hardy’s multiquadrics (MQ) [10] and Duchon’s augmented polyharmonic spline (APS) [11] are the most popular ones. For example, Golberg [12, 13], Chen [14], and Karur and Ramachandran [9] demonstrated the superiority of the APS over the ad hoc radial function. Then, Golberg et al. [15] further improved the accuracy of the approximated particular solution by utilizing the MQ. Recently, Tsai [16] generalized the MQ to the polyharmonic multiquadrics (PMQ) and showed further accuracy improvement of the PMQ over the traditional MQ. In this study, the accuracy improvement of the PMQ over the MQ and APS is demonstrated for the Reissner and Mindlin thick-plate models.

When the MQ is adopted in a DRM procedure, the applicability depends on the availability of the analytical particular solution of the basis function associated with the partial differential operator of a given problem. Golberg et al. [15] derived the analytical particular solutions for the Laplace equation. Samaan and Rashed [17, 18] and Tsai and Hsu [19] found the analytical particular solutions, respectively, for the two- and three-dimensional elasticity problems, which can be converted to a biharmonic equation using the Galerkin-Papkovich vector [20]. Basically, the applicability of MQ is limited to the harmonic and biharmonic operators [21] before Tsai [16] derived the analytical particular solutions of MQ associated with the polyharmonic operators, which are denoted as the PMQ in the following.

When applying the DRM for solving the Mindlin thick-plate model [22], Wen et al. [23] and Tsai and Wu [24] found the analytical particular solutions for the ad hoc RBF and the APS, respectively. In addition, the analytical particular solutions for the Chebyshev polynomials [25] and the APS [26] were derived for the Reissner thick-plate model [27, 28]. Basically, a product operator of biharmonic and Helmholtz operators resulted when applying the Hörmander operator decomposition technique [29] to the coupled PDEs of the Reissner and Mindlin thick-plate models as demonstrated in the previous studies [24–26]. This seems to infer that the accuracy improvement of the PMQ over the APS is not possible for the thick-plate models.

In this study, a method is introduced to eliminate the Helmholtz operator, which makes the application of the PMQ for the thick-plate models possible. Numerical experiments are carried out to demonstrate the superior accuracy of the PMQ over the traditional MQ and the first few orders of the APS.

This paper is organized as follows: the unified Reissner/Mindlin plate model is introduced in Section 2. Then, a review of the PMQ is given in Section 3. And the particular solutions of the PMQ associated with the Reissner/Mindlin plate model are derived in Section 4. Section 5 gives the formulation of MFS-DRM. Finally, some numerical experiments are carried out to validate the particular solutions in Section 6 and the conclusions are drawn in Section 7.

2. Reissner/Mindlin Plate Model

In the following, indices are in the range and indices are in the range . As described in Figure 1, we consider a plate of uniform thickness with its middle plane being a domain with boundary in the plane and thickness coordinate . The plate is subjected to a normal loading with intensity . Then, the equations of equilibrium are given by where is the transverse shear force and is the bending or twisting moment.

Figure 1 

Configuration of a thick plate.

In the Reissner [27, 28] and Mindlin [22] thick-plate theory, represents an average slope angle and is the lateral deflection of the plate in the middle surface. According to the theories, the constitutive equations are given by with for the Mindlin plate model and for the Reissner plate model. Combining (1)~(2) results in the following governing equation: withwhere and are Young’s modulus and Poisson ratio, respectively.

Equation (5) is a coupled system of three second-order PDEs for the three unknown functions , and, therefore, three boundary conditions are required as where are given boundary data.

3. Polyharmonic Multiquadrics

Before deriving the particular solutions of PMQ associated with the Reissner/Mindlin plate model, we give a short review on the PMQ previously derived by the method of undetermined coefficients together with the Laurent series in [16]. Considering the th order PMQ , it is governed by the following hierarchical relation: Here, (10) is introduced so that the PMQ are hierarchically unique and is a shape parameter to tune the numerical accuracy [10].

Then, by using the method of undetermined coefficients, we can find the solution of (8)~(10) as with for any positive integer . And, the unknown in (11) can be solved by using the uniqueness condition (10) as with In (14), the double factorial is defined as The PMQ defined in (11) can be expanded into the Maclaurin series as Observing (16), we can conclude that the proposed PMQ are infinitely differentiable since they only consist of even-coefficient polynomials [16, 30]. Then, substituting (16) into (8) and (9) and performing some mathematical manipulations can result in which can be used for solving the unknown coefficients and in (11) and (16). In practice, we simply enforce equal to in (17) which results in linear equations for solving the unknown coefficients and of the PMQ. After and are obtained, (13) can be used to have . Then, the PMQ can be computed simply by (11). For the cases when is very small or equal to zero, one can alternatively use the Maclaurin series (16).

For the purpose of a later derivation of the particular solutions associated with the Reissner/Mindlin plate model, we need to find the formulas when the radial differential operator is applied onto (11) and (16). This can be done for arbitrary nonnegative integer since the proposed PMQ are infinitely differentiable. Then, the required formulas for (11) and (16) can be derived, respectively, as with In deriving (18), we have used And, in deriving (20) and (21), we have used the product rule of differentiation as follows: with and being two arbitrary radial functions. Finally, additional formulas are required to complete (20) and (21), respectively, as follows: with

The proposed formulas can be implemented into a floating-point subroutine for evaluating with arbitrary nonnegative integers and . Therefore, the subroutine can be invoked when implementing numerical methods. In addition, the source codes can be obtained from the author via e-mails.

4. Analytical Particular Solution of Plate Model

Analytical particular solutions are required when the DRM is applied for solving a Reissner/Mindlin plate problem under an arbitrary loading. In this section, the analytical particular solutions of the PMQ associated with the Reissner/Mindlin plate model will be derived, which are governed by where

Formally, the particular solutions , governed by (27), should be obtainable by using the Hörmander operator decomposition technology [29], which begins with the definition of the adjoint operator as where is the determinant of and is the three-by-three identity matrix. Here, both and can be obtained by linear algebraic manipulations, respectively, defined asThen, the particular solutions can be found by assuming where is an unknown function to be determined. In (32), the required operator can be obtained as

After substituting (32) into (27) and using (29) and (30), the resulted equation indicates that the following particular solution is required: However, the analytical particular solution of the above equation is not known due to the existence of the Helmholtz operator.

In this study, we find that the Helmholtz operator can be eliminated by rewriting (33) as where the operator is defined by Then, (29) and (35) can be combined to have The above equations have suggested us to modify the Hörmander operator decomposition technology by assuming where is an unknown function to be determined. Substituting (38) into (27) and using (37) result in By using the PMQ definitions (8) and (9), we can have the particular solution of (39) as Substituting (40) into (38) and using (36) can result in the desired analytical particular solutions of displacements as where we have used In addition, the required formulas of and have been provided in the previous section. Then, the corresponding shear forces and bending/twisting moments can be obtained by using the definitions Equations (41), (42), (44), and (45) are sufficient to obtain the analytical particular solutions of shear forces and bending/twisting moments, respectively, as follows: where the additional terms and have been given in the previous section.

This completes the derivation of the analytical particular solutions of the PMQ associated with the Reissner/Mindlin plate model.

5. MFS-DRM Formulation

Now, we are in a position to review the application of the MFS-DRM procedure [24, 26] for solving the well-posed thick-plate problem governed by (5) and (7). First of all, we need the principle of superposition as where the particular solution satisfies without specifying any boundary condition. In addition, the homogeneous solution satisfies with modified boundary conditions where and .

In order to apply the DRM for solving the particular solution , the loading intensity needs to be first approximated by the PMQ as follows: where is the distance between and the th DRM field point, , as depicted in Figure 2. In order to solve the unknown coefficient , (52) should be collocated on the DRM field points as for and is the distance between the th and th DRM field points. Then, the corresponding particular solution can be approximated by with the particular solutions given by (41) and (42). Also, the particular solutions of shear force and bending or twisting moment can be approximated, respectively, as In (55) and (56), the particular solutions and are given in (46) and (47), respectively.

Figure 2 

Field points of the DRM.

After the particular solutions are approximated by the DRM, the modified boundary conditions (51) become well defined. Therefore, it is ready to use the MFS for solving the homogeneous solution. Formally, the homogeneous solution can be approximated by where are the fundamental solutions defined byand are source points outside the plate domain as described in Figure 3. In (57), the unknowns can be solved by collocating the modified boundary conditions (51) on boundary points. More details of the MFS and the explicit fundamental solutions can be found in [24, 26]. After both the particular and homogeneous solutions are solved, the principle of superposition (48) can be applied for obtaining the desired solution.