Research Article  Open Access
Free Vibration Analysis of the Unified Functionally Graded Shallow Shell with General Boundary Conditions
Abstract
The free vibration analysis of the functionally graded (FG) double curved shallow shell structures with general boundary conditions is investigated by an improved Fourier series method (IFSM). The material properties of FG structures are assumed to vary continuously in the thickness direction, according to the four graded parameters of the volume distribution function. Under the current framework, the displacement and rotation functions are set to a spectral form, including a double Fourier cosine series and two supplementary functions. These supplements can effectively eliminate the discontinuity and jumping phenomena of the displacement function along the edges. The formulation is based on the firstorder shear deformation theory (FSDT) and RayleighRitz technique. This method can be universally applied to the free vibration analysis of the shallow shell, because it only needs to change the relevant parameters instead of modifying the basic functions or adapting solution procedures. The proposed method shows excellent convergence and accuracy, which has been compared with the results of the existing literatures. Numerous new results for free vibration analysis of FG shallow shells with various boundary conditions, geometric parameter, material parameters, gradient parameters, and volume distribution functions are investigated, which may serve as the benchmark solution for future researches.
1. Introduction
The functionally gradient material (FGM) can be got by making the material properties of the structures change continuously in the thickness direction. This can make it possible to weaken or eliminate the internal defect and improve the bearing capacity of the structure and environmental adaptability. As the basic structure, shallow shell is widely used in the fields of aeronautics, astronautics, ship engineering, petrochemical container, and civil engineering. In the application of engineering practice, the main body of structures is made up of plate, cylindrical shell, spherical shell, hyperbolic paraboloidal shell, and so on. Therefore, it is of great significance to establish a unified analytical model to study the free vibration, buckling, and postbuckling characteristics of FG double curved shallow shells.
The double curved shallow shell structures have many special forms. The plate is one of the special forms by setting the curvature of two directions to infinity. As the most common form, there have been many results of the research on FG plate. Matsunaga [1] analyzed the free vibration and buckling stresses of simply supported FG plates according to a 2D higherorder deformation theory (HSDT). Nguyen et al. [2] established a FSDT plate model to study a simply supported square plate and a cylindrical bending sandwich plate clamped at both ends. Zhao et al. [3, 4] employed FSDT to investigate the free vibration and buckling analysis of FG plates on the basis of the elementfree kpRitz method. Shen et al. [5, 6] investigated the nonlinear bending of simply supported FG plates reinforced by singlewalled carbon nanotubes subjected to a transverse uniform or sinusoidal load in thermal environments based on a HSDT. Belabed et al. [7] obtained an efficient and simple higherorder shear and normal deformation theory for bending and free vibration analysis of simply supported FG plates. Neves et al. [8, 9] presented a quasi3D sinusoidal shear deformation theory and quasi3D HSDT for the static and free vibration and buckling analysis of FG plates by applying Carrera’s unified formulation and meshless technique. Tounsi et al. [10] presented a refined trigonometric shear deformation theory (RTSDT) for the thermoelastic bending analysis of FG sandwich plates. Benachour et al. [11] used fourvariable refined plate theory for free vibrations of FG clamped plates with arbitrary gradient based on Navier technique and Ritz method. Thai and Vo [12, 13] developed a new sinusoidal shear deformation theory for bending, buckling, and vibration of simply supported FG plates. Although a lot of researches about the vibration characteristics of FG plates have been done, there are many limitations in these studies, such as the various boundary conditions, varying gradient parameters, and volume distribution functions.
The FG cylindrical shell can be seen as the single curved FG shallow shell. For this kind of cylindrical shell structure, the researches on the vibration characteristics have been very mature. Sheng and Wang [14, 15] investigated thermal vibration, buckling, and dynamic stability analysis of a FG cylindrical shell based on rotary inertia, FSDT, and modal expansion method. Vel [16] presented an exact elasticity solution for the free and forced vibration of simply supported FG cylindrical shells by using either the Mori–Tanaka method or asymptotic expansion homogenization (AEH) method. Shah et al. [17] studied the vibration behaviors of FG cylindrical shells on the Winkler and Pasternak foundations. Matsunaga [18] examined free vibration and stability of FG simply supported circular cylindrical shells by applying twodimensional HSDT. Zahedinejad et al. [19] proposed a semianalytical 3D free vibration analysis of functionally graded curved cylindrical panels with two opposite edges simply supported and arbitrary boundary conditions at the other edges based on the differential quadrature method (DQM). Santos et al. [20] developed a semianalytical finite element model for the analysis of FG cylindrical shells by using the 3D linear elastic theory. Sepiani et al. [21] investigated vibration and buckling analysis of twolayered functionally graded cylindrical shell, considering the effects of transverse shear and rotary inertia based on FSDT and the classical shell theory (CST). Bagherizadeh et al. [22] studied the mechanical buckling of simply supported FG cylindrical shell on the Pasternak foundations based on HSDT. Beni et al. [23] analyzed the free vibration of sizedependent shear deformable simply supported FG cylindrical shell on the basis of FSDT and modified couple stress theory. From these results, it is not difficult to find that most of the boundary conditions of the cylindrical shell are simply supported boundary conditions or fixed boundary conditions. The effects of the elastic boundary conditions on the vibration are rarely mentioned.
As a kind of shallow shell structure with double curvature, some structural forms are often used in engineering practice, such as spherical shells, hyperbolic paraboloidal shell, and drum shell. It is not difficult to find that these structures can be obtained by changing the geometric parameters and the coordinates. Therefore, a unified analytical model is developed to study the vibration characteristics of varying FG shallow shell structures. Tornabene [24–26] focused on the free vibration and dynamic behavior of FG conical, cylindrical shells and annular plates with a fourparameter powerlaw distribution based on the 2D GDM and FSDT. Besides, Tornabene et al. [27] applied higherorder equivalent single layer theories to study free vibrations of freeform FG doubly curved shells. Neves et al. [28] applied radial basis functions collocation to examine the free vibration analysis of FG shells which included spherical as well as cylindrical shell panels with all edges clamped or simply supported based on HSDT. Pradyumna et al. [29] carried out free vibration analysis of FG curved panels using a higherorder finite element formulation. Matsunaga [30] analyzed natural frequencies and buckling stresses of FG shallow shells based on 2D HSDT. Three types of simply supported shallow shells with positive, zero, and negative Gaussian curvature were considered. Alijani et al. [31] investigated the nonlinear forced vibrations of simply supported FG doubly curved shallow shells based on Donnell’s nonlinear shallow shell theory and the Galerkin method. Su et al. [32] presented a unified solution for vibration analysis of FG cylindrical, conical shells and annular plates with general boundary conditions by applying the FSDT and RayleighRitz procedure. Qu et al. [33] described a unified formulation for free, steadystate, and transient vibration analyses of FG shells with arbitrary boundary conditions on the basis of the FSDT. In this paper, the free vibrations of functionally graded cylindrical, conical, and spherical shells with different combinations of free, sheardiaphragm, simply supported, clamped, and elasticsupported boundary conditions were discussed. Alijani et al. [34] employed a pseudoarclength continuation and collocation scheme to analyze thermal effects on nonlinear vibrations of simply supported FG doubly curved shells based on the HSDT. From the existing literatures, we can find that there have been a lot of discussions about the free vibration or buckling of the FG cylindrical shell, spherical shell, and conical shell. However, there are some limitations in the study of hyperbolic paraboloidal shell, which is a kind of the shallow shell structure. Moreover, the research on the multigradient parameters also has some limitations.
Stimulated by the restriction of the varying boundary conditions and establishment of unified model in the current literature researches, the free vibration analysis of the unified FG shallow shell with varying general boundary constraints is developed. Based on the FSDT, an improved Fourier series solution is extended, which is previously proposed by Li et al. [35, 36], to study the vibration of FG shallow shell with general boundary constraints. The five displacement functions are all written as a feasible superposition of the 2D trigonometric series expansion, ignoring the influence of boundary conditions, in spectral form, as a double Fourier cosine series and two supplementary functions. On the basis of the traditional Fourier series, these supplementary functions are added to eliminate the discontinuous or jumping phenomenon in the boundaries which are regarded as periodic functions and defined within the entire coordinates of FG shallow shells. All these unknown coefficients are defined in the generalized coordinates which can be solved by RayleighRitz procedure [37–40]. It is very easy to realize the change of different boundary conditions by changing the value of the five springs in the plate’s four edges [41–45]. This method can realize parametric study without modifying the main program. Besides, the results obtained by the present method show good convergence and accuracy by being compared with those results obtained by literatures. It should be pointed out that the varying thickness has a great effect on the natural frequencies of the plates and shells. Many references have already been devoted to the study of the free vibration of multilayered plates [46–49] and shells [50–55] by using advanced analytical models based on variable throughthethickness kinematic. Therefore, in this paper, the effects of various thicknesses on the natural frequencies of the FG shallow shells are manifested in various aspect ratios, such as lengththickness ratios and widththickness ratios. Besides, we also study the effect of the varying boundary constraints, four kinds of gradient parameters, two different volume distribution functions, and various geometric parameters on the free vibration of the FG shallow shells.
2. Theoretical Formulations
2.1. Description of the Model
As shown in Figure 1, a FG double curved shallow shell model is established to analyze the vibration characteristics. A coordinate system in the midsurface of the double curved shallow shell is established. In this coordinate system, , , and represent the length, width, and thickness directions. Besides, and are signed as the length along and directions and is the thickness of the direction. and are the radii of the curvature along and directions, respectively. An artificial spring technique is used to gain the vibration characteristics of FG shallow shell structures. Five types of springs are used to simulate the boundary conditions of the model, which are linear springs and rotational springs , respectively. Different boundary conditions can be realized by setting different stiffness values of the springs. For instance, the fixed support can be easily gained when the spring coefficients on the four edges are infinity which is set to 1E15 in the numerical calculation.
Figure 2 shows the various studied FG shallow shells which are used to validate the accuracy and versatility of the proposed method. It should be pointed out that we can easily obtain a rectangular plate structure by setting the , as shown in Figure 2(a). The cylindrical shell can be gained when , , as the model in Figure 2(c). The spherical shell and hyperbolic paraboloidal shell can be obtained when and as shown in Figures 2(b) and 2(d).
(a) Plate
(b) Spherical shell
(c) Cylindrical shell
(d) Hyperbolic paraboloidal shell
2.2. Admissible Displacement Functions
In order to eliminate the discontinuous or jumping phenomenon in the boundaries of FG shallow shells with arbitrary elastic supports, we propose an IFMS to express the displacement and rotation functions [59–61] which can be regarded as periodic functions: in which , . The unknown 2D Fourier coefficients expansions are , , , , and (), respectively.
2.3. Kinematic Relations and Stress Resultants
The displacement of previously mentioned FG shallow shell model can be expressed according to the displacements and rotations of the middle surface [62, 63], which is based on the theory of the FSDT.where is the time variable. Then , , and represent the middle surface displacements in , , and direction, and the rotations of transverse normal for  and axes are denoted by and , respectively. According to the small deformation elasticity theory and linear strainsdisplacement relations, the relationship of strain–stress can be expressed aswhere , , and denote the symbols of the normal and shear strains and , , and express curvature and twist changes. Transverse shear strains are regarded as constant, ignoring the thickness change. These corresponding quantities can be written as
Based on the generalized Hooke’s law, the relationship between stress and strain can be obtained bywhere the normal stresses are and , shear stresses are , , and , and () represent the material coefficients which can be solved as follows:
Young’s modulus, density, and Poisson’s ratio are , , and which can be expressed as functions in the thickness direction.in which and represent the two different material constituents which show smooth and continuous mechanical behavior. Here, the volume fraction () will be represented by two general types of volume distribution function based on the four gradient parameters .where , , and determine the variation of the material in the thickness direction and is the gradient powerlaw exponent which can only take the positive. When the value of is infinite, the structure of is composed of the pure material, but the structure of is composed of the pure material. When the value of is 0, the material of structure is exactly the opposite. This relationship of volume fractions should be met as .
The constitutive equation of force and moment contacted with strain can be expressed aswhere the stress , shear force , torque , torsional moment , and transverse shear force resultants can be obtained by establishing the relationship with strains. Besides, the shear correction factor is which is generally set to 5/6. The extensional, extensionalbending, and bending stiffness coefficients are , , and which can be gained by the integration over the thickness:
Through the derivation of the formula, we can establish the relationship between internal force and kinematics of the provided FG shallow shell model.
2.4. Energy Expressions
As mentioned earlier, the main work of our study is the free vibration characteristics of FG shallow shell with elastic supports. Thus, we study the FG shallow shell vibrations based on RayleighRitz energy method [64–67] which can obtain more accurate results. The Lagrangian energy function for the shallow shell can be written as
The total kinetic energy of the FG shallow shell is written aswhere , , and are expressed as
is the strain energy stored inside the FG shallow shell:
By combining (4), (9), and (14) together, we can divide the strain energy formula into three parts, which can be rewritten as . In this expression, , , and are stretching energy, bending energy, and bending–stretching coupling energy which can be expressed as follows:
For the arbitrary elasticsupported case, the potential energy with the five kinds of restrained springs [68–74], which is the simulation of boundary conditions, should be written as
2.5. Equation Solving
Partial derivative of unknown coefficients by Lagrange equation is zero based on the RayleighRitz technology [75–78]. Therefore, a matrix form can be obtained by substituting (1a)–(1e) into (11)–(16), which can be written as where the stiffness and mass matrixes for the FGM shallow shell are denoted by and . The unknown Fourier coefficients can be expressed as a vector in the form of
So far, the natural frequencies and coefficients can be easily got by solving the eigenvalues and eigenvectors of a standard matrix in (18). Substitute into (1a)–(1e) and get the specific displacement expressions of the FG shallow shell. Then, the corresponding mode shape can be immediately obtained.
3. Numerical Results and Discussion
In this part, a number of examples for the different FG shallow shells with varying elastic boundary conditions will be discussed. In the numerical calculation, the fast convergence of this method is proved by setting different truncation value of the displacement function expressions. By comparing with the available results extracted from the existing literatures, it proves the accuracy of the proposed method and model in this paper. It should be noted that the frequency parameters used in this paper are . In order to facilitate the description, this paper uses alphabetic strings to represent the boundary constraints (BC). For example, SCFE represents a kind of boundary constraints by setting the spring values of the four edges to the numbers shown in Table 1. Specifically, is simply supported, is clamped edge, is free edge, and is elastic support. Besides, the material properties involved in this paper are presented here, and we will not give special explanation anymore in the following example: Al: GPa; ; kg/m^{3} Al_{2}O_{3}: GPa; ; kg/m^{3} ZrO_{2}: GPa; ; kg/m^{3}.

3.1. Convergence Study
As previously mentioned in (1a)–(1e), we can get the convergence criterion by the truncation value of and in the displacement function expressions in the numerical calculation. Therefore, the values of and are the direct representation of the convergence for the proposed method. In Table 2, we can get the convergence of first eight frequency parameters for the various shallow shells structures with SSSS boundary constraints whose functionally graded material is Al/Al_{2}O_{3}. Their geometrical dimensions are , , and m.

The method presented in this paper shows fast convergence which can be found in Table 1. For example, the biggest difference for the worst case of the plate which is made by the contrast of 8 × 8 and 18 × 18 is less than 0.02%. It is not difficult to find that the biggest difference for the contrast of 8 × 8 and 18 × 18 of the cylindrical shell is also less than 0.02%. Besides, for the spherical shell and hyperbolic paraboloidal shell, the biggest difference between these two truncated values is 0.017% and 0.019%. Through these results, we can find that the method shows good uniform convergence for different structures which shows the accuracy of the unified model. In addition, it is necessary to point out that, in order to make the results obtained in the following numerical examples more accurate, we use the invariant truncation values of .
3.2. Validation and Some New Results
In this section, the accuracy of the proposed method is verified by being compared with the literature results. In addition, this paper also gives some new results, which can provide a benchmark for the future research. The first five frequency parameters for Al/Al_{2}O_{3} square plate with SSSS boundary constraints and varying lengththickness ratios and gradient powerlaw exponent parameter are given in Table 3. The geometric parameters are m and the material parameters of alumina and aluminum have been given previously. The results of the present method are compared with those of Jin et al. [56] which show good agreement. From Table 3, we can easily find that the frequency parameter of the square plate is decreasing gradually with the increase of and . In Table 4, the first frequency parameters for Al/ZrO2 plate with various boundary constraints are given. The influence of different lengthwidth ratio , thicknesswidth ratio , and gradient powerlaw exponent parameter on the plate frequency is also discussed. The results of the free vibration under SSSS boundary conditions are compared with those of Kiani et al. [57], which show good agreement. It is worth noting that these results in Table 4 are consistent with the results obtained in Table 3.


Cylindrical shell, as a special form of shallow shell with double curvature, is well known and widely used in engineering practice. The free vibration characteristics of FG cylindrical shells are studied based on the unified doubly curved shallow shell model established in this paper. Table 5 gives the first nine frequencies of cylindrical shell with FFCF boundary constraints and varying gradient parameter of powerlaw exponent . There are two kinds of functionally graded materials whose properties are GPa, GPa, , kg/m^{3}, kg/m^{3}. The geometric parameters of cylindrical shell are m, m, m, m. The first frequency parameter for Al/ZrO_{2} cylindrical shell with SSSS boundary constraints is given in Table 6. At the same time, the effects of various geometric parameters, including lengththickness ratio and radiuslength ratio , and powerlaw exponent are also discussed. The accuracy of the results is verified by being compared with the related references [26, 57, 58]. As shown in Tables 5 and 6, the frequency of the cylindrical shell is reduced gradually when the radius , the thickness , or the gradient powerlaw exponent increase. In Table 7, we give the firstorder frequency parameter of free vibration of Al/Al_{2}O_{3} cylindrical shells with different boundary conditions and varying geometric parameters of and . In the studied cylindrical shell model, the radius is m. In this table, the effects of two general types of volume distribution function ( and ) on the frequency of cylindrical shells are also given. It should be pointed out that the frequencies of the cylindrical shell under and have some small changes when the shell has the same boundary conditions and geometric parameters.



The first frequency parameter of Al/ZrO_{2} spherical shell with SSSS boundary constraints is given in Table 8. The effects of various geometric parameters, including lengththickness ratio and radiuslength ratio , and powerlaw exponent are also discussed in this table. It shows good agreement when the results obtained by the proposed method are compared with those results obtained by Kiani [57]. We can find that, with the radius increasing, the frequency of the spherical shell decreases when other parameters remain unchanged. Some new results are given in Table 9. In this table, we give the firstorder frequency parameter of the free vibration of Al/Al_{2}O_{3} spherical shell with varying boundary conditions and geometric parameters . Geometry parameters of this spherical shell are radius m and m, respectively. Under the two general types of volume distribution function of and , the firstorder frequency of free vibration of spherical shell is given in Table 9. The frequencies of the spherical shell under and have some small changes which has the same laws shown in Table 7. For the free vibration of this FG spherical shell, it is significant to study the change of natural frequency under different boundary constraints.


As mentioned earlier, the hyperbolic shell is a special form which can be obtained by setting the radius of curvature as . It should be noted here that the plate and the hyperbolic paraboloidal shell are all about the symmetrical distribution of the middle surface. So, for the two kinds of structural distribution of and , the frequencies for free vibration of the plate and the hyperbolic paraboloidal shell have no difference. Therefore, the following research does not distinguish under the two general types of volume distribution function. Table 10 gives the firstorder frequency of the Al/ZrO_{2} hyperbolic paraboloidal shell with E^{1}E^{2}E^{1}E^{2} boundary constraints. The geometry parameter is set as m. With the increase of gradient powerlaw exponent parameter , the frequency decreases gradually. Furthermore, the increase of radius and thickness leads to the decrease of frequency. In Table 11, the first three frequency parameters of Al/Al_{2}O_{3} hyperbolic paraboloidal shell with different boundary constraints and varying geometric parameters are given. In this table, the radius remains constant and is set to m. It is easy to see from the table that the boundary conditions have a great influence on the frequency for the free vibration of the FG hyperbolic paraboloidal shell. There is an interesting phenomenon that the increase of thicknesswidth ratio has little effect on the natural frequency. But the lengthwidth ratio has a great influence on the frequency; that is, the frequency increases with the increase of .


3.3. Parametric Study
In this section, the free vibration characteristics of FG shallow shells are studied by using the four kinds of special structures with varying boundary constraints and gradient powerlaw exponent parameter . In order to compare conveniently, this section uses Al/Al_{2}O_{3} as the function gradient material, and its volume distribution functions are all .
Firstly, the effect of gradient powerlaw exponent parameter on the vibration characteristics of the Al/Al_{2}O_{3} shallow shells is studied. Figure 3 shows the firstorder natural frequency parameters for the free vibration of plate, cylindrical shell, spherical shell, and hyperbolic paraboloidal shell with varying powerlaw exponent and six kinds of boundary constraints. In this case, the geometrical dimensions are , , and m. It can be seen from Figure 3 that, under different boundary conditions, the effects of the function gradient powerlaw exponent parameter on the vibration characteristics of the shells cannot give a definite conclusion easily. The effects of different boundary conditions on the vibration characteristics of shell structures may be very different. For example, for the FG shallow shell with the boundary constraints of CE^{2}CE^{2}, SCSC, and E^{1}E^{1}E^{1}E^{1}, the firstorder frequency decreases with the increase of . In particular, the frequency decreases very quickly, when is between 0.1 and 100. However, for the four kinds of shallow shells with varying boundary constraints of E^{2}E^{2}E^{2}E^{2}, E^{3}E^{3}E^{3}E^{3}, and SE^{3}SE^{3}, the frequency increases when increases.
(a) Plate
(b) Cylindrical shell
(c) Spherical shell
(d) Hyperbolic paraboloidal shell
The effects of material parameters , , and on the vibration characteristics of the Al/Al_{2}O_{3} shallow shells with E^{3}E^{3}E^{3}E^{3} are discussed. Figure 4 shows the frequency parameters of the shallow shells with the varying gradient powerlaw exponent parameters and four types of material gradient parameters. Four kinds of gradient parameters types are Type 1: , ; Type 2: , , ; Type 3: , , ; and Type 4: , , . From Figure 4, it can be found that the material gradient parameters have great effect on the vibration characteristics of the FG shallow shells. The changes of frequencies for the shallow shells with Type 1, Type 2, and Type 3 are the same, with the frequency increasing quickly when increases from zero to 5 and then decreasing with continuing to increase. But the frequency of the shallow shells with Type 4 is quite different with them. Especially when increases to 5, the frequency increases and then decreases slowly. Therefore, it is very important to study the vibration characteristics of FG shells with the varying material gradient parameters. This is because the change of material gradient parameters will completely change the mechanical properties of FG structure in the thickness direction. It is the reason why FGM has more advantages than traditional materials.
(a) Plate
(b) Cylindrical shell
(c) Spherical shell
(d) Hyperbolic paraboloidal shell
According to the above results, the 3D view model shapes of the Al/ZrO_{2} shallow shells with CCCC boundary constraints and volume distribution functions are given in Figure 5. The geometric parameters of the plate, cylindrical shell, and spherical shell in these figures are all m, m, but the lengthwidth ratio is varied which has been given in Figure 5. These 3D view models are adopted to understand the free vibration characteristics of FG shallow shells.
4. Conclusions
In this paper, an improved Fourier series solution is proposed to solve the free vibration analysis of the unified FG double curvature shallow shells with general boundary constraints. The theoretical analysis is based on the FSDT and the RayleighRitz technique. The energy equation is solved by five displacement functions which are expressed as a series of 2D Fourier series. By introducing the product form of sine and cosine function, we can effectively eliminate the discontinuous or jumping phenomenon of the displacement function on the boundaries. Artificial virtual spring technique is used to simulate the boundary conditions; that is, five groups of springs are arranged on the four edges. They are linear springs and rotational springs , respectively. Different boundary conditions can be realized by setting different stiffness values of the springs.
Through the analysis and discussion of the results, we can draw the following conclusions:(1)The proposed method shows good convergence and the accuracy is verified by being compared with the existing literatures.(2)The frequencies of the FG shall shells are changed a lot when the boundary conditions change. The results show that the spring stiffness is directly proportional to the frequency of the shallow shells.(3)The geometric parameters also have a great influence on the free vibration of the FGM shallow shells. The frequency parameter of the square plate decreases gradually with the increase of lengththickness ratio while the frequency of the cylindrical shell reduces gradually when the radius and the thickness increase.(4)The frequencies of the shallow shells are inversely proportional to the gradient index. The volume distribution functions only affect the cylindrical shells and spherical shells and have no effect on plates and hyperbolic paraboloidal shells.
It is of great value to study the free vibration characteristics for practical engineering applications. For example, the parametric studies of this paper provide the technical support and theoretical basis for the vibration reduction, noise reduction, and so on. In the following research, we aim to study the forced vibration analysis of the unified functionally graded shallow shell with general boundary conditions. And the work finished in this paper has laid a foundation for further study.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Grant nos. 51679056 and 51705537) and Natural Science Foundation of Heilongjiang Province of China (E2016024).
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