Analysis of Geometrically Nonlinear Vibrations of Functionally Graded Shallow Shells of a Complex Shape

Geometrically nonlinear vibrations of functionally graded shallow shells of complex planform are studied. The paper deals with a power-law distribution of the volume fraction of ceramics and metal through the thickness. The analysis is performed with the use of the R-functions theory and variational Ritz method. Moreover, the Bubnov-Galerkin and the Runge-Kutta methods are employed. A novel approach of discretization of the equation of motion with respect to time is proposed. According to the developed approach, the eigenfunctions of the linear vibration problem and some auxiliary functions are appropriately matched to fit unknown functions of the input nonlinear problem. Application of the R-functions theory on every step has allowed the extension of the proposed approach to study shallow shells with an arbitrary shape and different kinds of boundary conditions. Numerical realization of the proposed method is performed only for one-mode approximation with respect to time. Simultaneously, the developed method is validated by investigating test problems for shallow shells with rectangular and elliptical planforms, and then applied to new kinds of dynamic problems for shallow shells having complex planforms.


INTRODUCTION
Structural elements modeled by shallow shells are widely used in various engineering fields, including, for instance: mechanical, aerospace, marine, military, and civil engineering.Such elements can have various planforms, boundary conditions (including mixed ones), and different types of curvature.In order to improve the strength of the modern design, a new class of composite materials, i.e., functionally graded materials (FGM), has been recently applied.In spite of the observation that Latin American Journal of Solids and Structures 14 (2017) 1648-1668 FGM are inhomogeneous, they have essential advantage, i.e., material properties undergo smooth and continuous variations in the thickness direction.This is why the stress concentration present in laminated structures can be eliminated.However, an analysis of functionally graded (FG) shells is more complicated than of homogeneous material structures, since partial differential equations (PDEs) with variable coefficients govern the shallow shells made of FGM, and the strain-stress fields are coupled.As it is known, getting a validated solution to the mentioned PDEs creates a very difficult problem even in the case of shells having relatively simple planforms.In addition, the problem becomes more complicated if FG shallow shells perform vibrations at large amplitudes.
In this paper we present a numerical-analytical method based on the R-functions theory in more detail.Formulation of the problem is performed using theories of the shallow shells, i.e., classical (CTS) and geometrically refined nonlinear theory of the first order (FSDT).Distinctive feature of the proposed approach is the original method of reducing the input nonlinear system of PDEs to a nonlinear system of ordinary differential equations.In order to realize this approach appropriately, it is needed to perform a free vibration analysis of FG shells with high accuracy, since eigenfunc-tions of the linear vibration problem are used on each step of the proposed algorithm.The proposed method is validated by investigating test problems for shallow shells of rectangular and elliptical planforms, and then applied to new vibration problems for shallow shells of complex planforms.

MATHEMATICAL FORMULATION
Consider a functionally graded shallow shell with uniform thickness h, made of a mixture of ceramics and metal.It is assumed that the shell has an arbitrary planform.The volume fraction of ceramic c V and metal phases m V are related by formula The volume fraction of ceramics c V can be expressed as In the above formula, the index k   0 k   denotes the volume fraction exponent, z is the distance between the current point and the shell midsurface.In the case when the power index k is equal to zero, one obtains a homogeneous material (ceramics), but if k approaches infinity, the shell is purely metallic.
It should be noted that FG materials are widely used in high-temperature environments and their mechanical characteristics can be different, depending on temperature changes.Therefore, this temperature dependence must be taken into account to obtain more accurate solution.We use the relations given in [Reddy (2009); Shen (2009)] in the following form   where 0 , P 1 , P  1 , P 2 , P 3 P are coefficients determined for each specific material.A table of values of these coefficients for some materials is presented in [Reddy (2009); Shen (2009); Alijani et al. (2011a)].
As it is known [Reddy et al. (1999); Reddy (2009); Shen (2009); Tornabene andViola (2007, 2009); Tornabene et al. (2011); Alijani et al. (2011a,b); Bich et al. (2012)], in FGM structures the material properties along the thickness are proportional to the volume fraction of the constituent materials, i.e., we have where According to the nonlinear first-order shear deformation theory of shallow shells (FSDT), displacement components 1 2 3 , , u u u at a point ( , , ) x y z are expressed as functions of the shell middle surface displacements , u v , and w in the Ox, Oy , and Oz directions, and the independent rotations , x y   of the transverse normal to the middle surface about the Oy and Ox axes, respectively as [Reddy (2009); Reddy et al. (1999)]: x y ; ; T      at an arbitrary point of the shallow shell follow where and are the principal curvatures of the shell along the coordinates x and y, respectively.
x y w w x y where indicator  is the tracing constant taking values 1 or 0 for the FSDT and CST, respectively. where Transverse shear force resultants , , where 2 s K denotes the shear correction factor (in this paper it is selected to be 5 / 6).Further, let us consider materials with Poisson's ratio independent of the temperature, being the same for ceramic and metal, i.e., Mass density  is also estimated by integration along the shell thickness, to yield Latin American Journal of Solids and Structures 14 (2017) 1648-1668 . 1 3 SOLUTION METHOD

U u x y t v x y t w x y t x y t x y t U u x y t v x y t w x y t x y t x y t
where  is a vibration frequency.Applying Hamilton's principle, we get a variational equation in the following form where U and T are strain and potential energy, respectively, and , , whereas and are defined by formulas ( 7), ( 9), and Minimization of functional ( 19) is performed using the Ritz method, and the necessary sequence of coordinate functions is built with the help of the R-functions theory [Rvachev (1982); Kurpa (2009b)].

Nonlinear Problem
For simplicity, let us describe the algorithm for solution to the nonlinear vibration problem in the frame of the classical theory (CTS).Note that inertia terms are ignored in motion equations while  Solids and Structures 14 (2017) 1648-1668 solving the nonlinear problem.Let us formulate the given problem with respect to shell displacements: where linear differential operators The expressions of the nonlinear terms standing on the right-hand side of the system ( 21)-( 23) are defined as follows ,  Solids and Structures 14 (2017) 1648-1668 where:     are defined by formulas (20), and , .
The vector   N  is defined by formula (8).
Note that the type of linear operators is simplified in the case of plates due to the condition , which implies .
Let us take the unknown functions ( , , ), w x y t ( , , ), u x y t ( , , ) v x y t in the form of an expansion in terms of eigenfunctions , , of the linear problem with coefficients depending on time, i.e. we have , , .
The functions u v should satisfy the following system of differential equations The system of equations ( 26) can be solved with the RFM for any planform and various kinds of boundary conditions.It is possible to show that the variational formulation of this problem is reduced to finding the minimum of the following functional: where ( , 1,2) i j  are defined by the following expressions , .
The functions 1 F and 2 F are: where l and m are directional cosines of the normal n to the border.
Observe that in case of boundary conditions with clamped edge, we have and consequently, a contour integral in formula ( 27) equals zero.The system of basic functions for functional ( 27) is built with the help of the R-functions theory.Substituting expressions ( 24)-( 25) for the functions into equations of motion ( 21)-( 23) and applying the Bubnov-Galerkin procedure, we obtain the following system of nonlinear ordinary differential equations for the unknown functions ( ), The formulas defining the coefficients follow: , where Latin American Journal of Solids and Structures 14 (2017) 1648-1668 , , , .
Solution to the system of equations ( 31) can be found with the use of various approximation methods.Here, numerical implementation has been done with one mode.Thus, instead of the system of equations ( 31), we find the solution to one second-order ODE of the form where the coefficients are calculated using formulas ( 32)-( 33

NUMERICAL RESULTS
In order to validate the results obtained by means of the proposed approach, a few of test problems are investigated first.

Results Validation
Latin American Journal of Solids and Structures 14 (2017) 1648-1668 at .
To solve this problem, the following solution structure [Kurpa (2009)] is employed , .
, 1,..., 5 are undefined components represented as a truncated expansion of the complete system of functions: In this paper, the power polynomials are chosen for the functions of such a system.In order to verify the accuracy of the presented results, convergence of the numerical solution is examined.As a result of the computational experiment, it has been found that convergence of the results (at least to the third decimal) occurs for the ninth-degree polynomials and for the tenth-degree polynomial 3  , and the results presented below were obtained for such a number of coordinate functions.
A comparison of the natural frequencies for various shallowness ratios and different thicknesses is carried out for (plate), (spherical shell), (cylindrical shell), and (hyperbolic paraboloid).
The comparison of the obtained results for the side-to-thickness ratio / 10 a h is shown in Ta- ble 1 and for / 5 a h in Table 2.
The comparison shows that the results obtained using the refined first-order theory (RFM, FSDT) are almost the same as those reported in reference [Chorfi and Houmat (2010)].A deviation from the results obtained by means of the theory of the higher order (HSDT) [Matsunaga (2008)] does not exceed 4%.Deviation results obtained by using the classical theory (RFM, CST) with the results of [Alijani et al. (2011a)] do not exceed 2%.In general, it should be noted that the classical theory, in most of cases, overestimates the fundamental frequencies compared with the refined theory.

Free Vibrations of Functionally Graded Shells of Complex Planforms
In order to present novel results and illustrate the versatility and efficiency of the proposed method, two free vibration problems are considered.Let us investigate a shallow shell with complex planform (see Fig. 1).The fixed geometrical parameters are as follows: The properties of the FG mixture are the same as those presented in paper [Chorfi and Houmat (2010)], i.e., (FG1) ; (FG2) .( 35) Suppose that the shell is clamped.Then, the solution structure may be taken in the following form (36) where is the equation of the border of the shell planform.In order to realize the solution structure (36), one should construct the equation of the border .Using the R-operations [Rvachev (1982)], the equation is built in the form , where is a vertical band bounded by straight lines , and Solids and Structures 14 (2017) 1648-1668 is a horizontal band bounded by straight lines .Finally, is a part of the plane within the ellipse.Due to the doubly symmetric nature of the shell, numerical implementation is performed only for one-quarter of the investigated domain.Thus, the sequences of polynomials are chosen as follows Now, in order to investigate convergence of the natural frequencies, the computer experiments are carried out.It has been found that the third decimal is stabilized while maintaining the degree of approximating polynomial (11,11,14,11,11) which corresponds to the following number of coordinate functions for : 21, 21, 36, 21, 21, respectively.In this case, the form shown in Fig. 1 is very close to elliptical.Results for this type of the shell have been compared to the similar form of the shell studied in reference [Chorfi and Houmat (2010)].It is obvious that the results obtained for simple (ellipse, black and blue colors of curves) and complicated (dashed lines) forms coincide reasonably well.This fact allows us to validate our (numerically obtained) results.The same figure shows the The values of the natural frequency parameter for spherical, cylindrical, hyperbolic paraboloid shells and plates are presented in Table 3.
It follows from Table 3 those values of the natural frequencies decrease for all types of the shell curvature and material properties of the mixtures, provided that the volume fraction exponent increases.Frequencies 'asymptotically' approach the frequencies of the metal shell or of the plate.It should be noted that for all values of   0 ,1 0 , k  the spherical shells have the maximum value of the fundamental frequencies, while plates have the smallest value of the fundamental frequencies.In order to carry out nonlinear analysis let us analyze the dependence between the amplitude and the ratio of nonlinear frequency to linear frequency.The backbone curves for clamped spherical shells made of FG1 material are shown in Fig. 3 A comparison of the backbone curves for cylindrical and spherical clamped panels for two kinds of materials is presented in Fig. 7  ).
Figure 8 shows the effect of the index k on the ratio of nonlinear to linear frequency.It can be seen that for materials FG1 this ratio increases to k=1, and then it decreases to k=6.However, if k>6 then the ratio again increases.The same behavior is characteristic for the cylindrical shell made of material FG1 (see Table 5).But if the panels are made from materials FG2, then the ratio L N   / increases to k =6, and then it slowly begins to increase (see Tables 4, 5).
The carried out analysis of the backbone curves for the shells under consideration shows that these curves are sufficiently close to each other.However, the values of the nonlinear frequencies depend substantially on the volume fraction index k.Effects of volume fraction index k on the nonlinear frequencies of the clamped spherical and cylindrical shells are shown in Figure 9 and 10, respectively.The proposed approach has allowed for a detailed analysis of the clamped cylindrical and spherical panels with the planforms shown in Fig. 1 being made of two kinds of FGM.It should be mentioned that owing to employment of the R-functions theory, we can easily pass from one geometric form to another and hence to investigate different boundary conditions of the shell using the same developed software.Our research shows that it is important that the desired solution to linear auxiliary tasks is presented in an analytical form.This is a significant factor that should be taken into account while using the presented approach to solve nonlinear problems.

CONCLUSIONS
This paper proposes a method of investigation of geometrically nonlinear free vibrations of functionally graded shallow shells of a complex planform.The method is based on the theory of the Rfunctions, Ritz variational method, Bubnov-Galerkin procedure, and Runge-Kutta method.The tests conducted for shells of square and elliptical planforms have proved the reliability and effec-Latin American Journal of Solids and Structures 14 (2017) 1648-1668 tiveness of the presented method.Graphical and numerical results are obtained for shells having complicated shapes.In the future, the developed method is planned to be implemented into a multimode approximation.
the power law (1), are defined as follows The study of equations of the form (34) involved a lot of scientists[Mahmoud Bayat et al (2012,2013); ImanPakar et al. (2014Pakar et al. ( ,2012)); MahdiBayat et al. (2014), and  others].In this paper we have carry out numerical investigations by employing the classical Runge-Kutta method.

Figure 1 :
Figure 1: Form and planform of the shells under consideration.

Figure 2 :
Figure 2: Effect of volume fraction index k on the natural frequencies of the spherical and cylindrical panels.

Fig
Fig. 2 shows the dependence of the natural frequencies c c L E a / 4 2 1    
while for those made of material FG2 are presented in Fig.4.Geometrical parameters 1

Figure 9 :
Figure 9: Effect of the volume fraction index k on nonlinear frequencies c c N N E a / 2    

Figure 10 :
Figure 10: Effect of the volume fraction index k on nonlinear frequencies c c N N E a / 2    

Table 2 :
Comparison of the fundamental frequency parameters of the square FG shallow shells with simply supported movable edges ( , )

Latin American Journal of Solids and Structures 14 (2017) 1648-1668 effect
of the volume fraction exponent k regarding the values of the natural frequency for spherical shells with radii of curvature equal toThe results have been obtained for two types of materials.As in the case of elliptical shells, the frequency values are substantially greater for the FG2 mixture than for the mixture of FG1.

Table 3 :
Effect of volume fraction index k on the natural frequencies

Table 4 :
Effect of volume fraction index k on the ratio made of materials FG1 and FG2 provided that the remaining geometric parameters are the same as in the previous case.Effects of various values of volume fractions k on the ratio of nonlinear frequency to linear frequency and the amplitude for the spherical shell are shown in Table4.Effects of various values of volume fractions k on ratio of nonlinear frequency to linear frequency and amplitude for the cylindrical shell with radii of curvature equal to

Table 5 .
The related backbone curves for spherical and cylindrical shell, made of FG1 material are presented in Fig.5.andFig.6,respectively.

Table 5 :
Effect of volume fraction index k on the ratio