Abstract
In this paper, comparison of natural undamped frequencies of isotropic plates are investigated by using the dynamics stiffness element for isotropic plates. The DS Matrix for isotropic has been formulated by the application of classical plate theory. The generalized DS matrix has to solve by using WittrickWilliams algorithm. Results of different aspect ratio have been validated from the existing literature.
1. Introduction
Pure metals have limited applicability in engineering applications because of the requirement of the inconsistent property of materials. In engineering, materials may be required to be hard as well as ductile. To solve this requirement, there is no such material that exists in nature. The combination of one metal (in molten state) with another parent metal or nonmetals is required to solve the engineering problem [1]. Another advanced material which is formed by the combination of one or more than one material with different physical and chemical properties in solid states are called composite material The properties of the composite material are better than the individual parent materials [2]. In this article, we study the comparison of the natural frequency of isotropic plate using DSM and FEM. DSM provides very accurate results without depending on the number of elements in the nodal analysis and it is another effective method to the FEM [3]. Boscolo and Banerjee [4] studied about the free vibration analysis of homogeneous isotropic plates. In this paper, we present the comparison of nondimensional natural frequency of plate using DSM from the literature survey. We have used CPT on DSM formulation to derive the dynamic stiffness matrix. WittrickWilliams algorithm [5] has to solve the transcendental nature of element with find out the nondimensional frequencies of the structure. We show that the natural frequencies are more accurate than available literature and set a standard value for comparison purpose.
2. Theory
2.1. Model development for functionally graded plates
The isotropic plate system has taken in this paper consist of the flexible square plate has different boundary condition at all edges, which are not movable in the neutral surface. CPT and WittrickWilliams algorithm is described as the displacement field of plate surfaces are brief summaries below.
The Cartesian coordinate system ($x$, $y$, $z$), of the isotropic plate, is fixed as represented in Fig. 1 plate’s thickness is $h$. We assumed that the harmonic distributed force per unit area, ${f}_{1}\mathrm{cos}(\omega t$) in the $z$ direction. ${u}_{o}$($x$, $y$, $z$), ${v}_{o}$($x$, $y$, $z$), and ${w}_{o}$($x$, $y$, $z$) are the displacement component of an arbitrary point of the plane of the plate in $x$, $y$, and $z$ direction, respectively.
Fig. 1Cartesian coordinate of displacement field of a isotropic plate
2.2. Classical plate theory (CPT)
The displacement component of an isotropic plate is ${u}_{o}$($x$, $y$, $z$), ${v}_{o}$($x$, $y$, $z$), and ${w}_{o}$($x$, $y$, $z$) by using classical plate theory, shown in Fig. 1, are given by [4]:
In the above expression, the index ($\text{'}$) consider the displacement component of the plate geometric. The midsurface displacement, ${u}_{o}(x,y)$ and ${v}_{o}(x,y)$ of a thin homogenous isotropic place can be neglected. The only unknown in the transverse displacement ${w}_{o}$ is considered in Eq. (1).
Hamilton’s principle has to investigate the nondimensional natural frequency of isotopic plate using the fourth order differential equation of a plate with different boundary conditions:
The BCs $ar{e}^{1}$:
where ${D}_{eff}=\u03f5{h}^{3}/\left\{12\left(1{\upsilon}^{2}\right)\right\}$ is the flexural stiffness, $\u03f5$ is the property of the material called Young’s modulus, $h$ the plate thickness, and $\rho $ is the density of the material.
2.3. DSM formulation
For DSM formulation the first basic concept is to solve the fourth order differential Eq. (2) of the Isotropic plate. The levytype [6] boundary condition for exact solution is sought. An isotropic plate has given as simply supported (SS) on two sides and remaining sides can be fixed (F) or clamped (C) The basic formulation of the Eq. (2) which satisfied the boundary condition in following form:
where $\omega $ is the unknow frequency. The fourth order ordinary differential equation is to be obtained by substituting Eq. (4) into Eq. (2) as follows:
The solution of the above quadratic equation determined by applying a trail solution [4]. For solving the differential equation, the two solutions are possible which is to rely depend on the roots:
• Condition 1: ${\propto}_{m}^{2}\ge \omega \sqrt{\frac{{I}_{0}}{{D}_{eff}}}\Rightarrow $ all roots are real (${\propto}_{1m},{\propto}_{1m},{\propto}_{2m},{\propto}_{2m}$):
The solution is:
• Condition 2: ${\propto}_{m}^{2}<\omega \sqrt{\frac{{I}_{0}}{{D}_{eff}}}\Rightarrow $ different roots are (${\propto}_{1m},{\propto}_{1m},i{\propto}_{2m},{i\propto}_{2m}$):
The solution is:
Method to solve DS matrix for case 1 is explain below and similarly to solve in second case but is not show for brevity.
In (Eqs. (6) and (4)), the known displacement, the rotation ${\phi}_{y}$, moment ${M}_{xx}$ and shear force ${V}_{x}$ explain in the following form Eq. (3):
$+{B}_{m}{\propto}_{1m}\mathrm{cosh}\left({\propto}_{1m}x\right){C}_{m}{\propto}_{2m}\mathrm{sinh}\left({\propto}_{2m}x\right)+{D}_{m}{\propto}_{2m}\mathrm{cosh}\left({\propto}_{2m}x\right))\mathrm{sin}{(\propto}_{m}y),$
${+B}_{m}({{\propto}_{1m}}^{3}(2\upsilon ){\propto}_{m}^{2}{\propto}_{1m}\mathrm{cosh}\left({\propto}_{1m}x\right)$
$+{C}_{m}{({\propto}_{2m}}^{3}(2\upsilon ){\propto}_{m}^{2}{\propto}_{2m}\mathrm{sinh}\left({\propto}_{2m}x\right)$
${+D}_{m}{({\propto}_{2m}}^{3}(2\upsilon ){\propto}_{m}^{2}{\propto}_{2m}\left)\mathrm{cosh}\left({\propto}_{2m}x\right)\right)\mathrm{sin}{(\propto}_{m}y)$
$+{C}_{m}({{\propto}_{2m}}^{2}+\upsilon {\propto}_{m}^{2})\left)\mathrm{cos}\left({\propto}_{2m}x\right){D}_{m}\right({{\propto}_{2m}}^{2}\upsilon {\propto}_{m}^{2}\left)\mathrm{sin}\left({\propto}_{2m}x\right)\right)\mathrm{sin}{(\propto}_{m}y).$
The displacements boundary condition for the plate are:
$x=b,{W}_{m}={W}_{2},{\varphi}_{ym}={\varphi}_{y2}.$
Similarly, the boundary condition for the forces are:
$x=b,{V}_{xm}={V}_{2},{M}_{xxm}={M}_{2}.$
From Fig. 2, we are implementing the different BCs, i.e. putting Eq. (12) in to Eqs. (6) and (9), the following expression is forming:
i.e.:
where:
${C}_{i}=\mathrm{cos}{(\propto}_{im}b),{S}_{i}=\mathrm{sin}{(\propto}_{im}b),i=\mathrm{1,2}.$
The BCs for forces, i.e. putting in Eq. (13) into Eqs. (10) and (11), the following matrix has formed:
i.e.:
where:
where $i=$ 1, 2.
From Eqs. (15) and (18) the DS matrix $K$ for the isotropic plate can be obtained and BE leave out the constant factor $C$ to get:
where:
From the general expression Eq. (21) to form DS matrix which is similar to six variable terms ${s}_{vv}$, ${s}_{vm}$, ${s}_{mm}$, ${f}_{vv}$, ${f}_{vm}$, ${f}_{mm}$ this variable terms describe the effect on shear and moment reason of displacement put on the “same” (s) nodal line, and the “far” nodal line. Thus, the dynamic stiffness matrix ($K$) expressed in following way:
2.4. Algorithm used for the DSM element
The global DS matrix for natural frequency of plate is to obtain by using WittrickWilliams algorithm [5]. Due to nonlinear behavior of dynamic stiffness element, to drive frequency determinant is excessive difficult. Wittrick and Williams algorithm [5] must be used to solving this problem and ensures that there are no frequencies missed out of the structure:
3. Numerical results
The numerical results of the DSM formulation have been inserted in a MATLAB and to find the frequencies and mode shape of the structure. This part is to analysis the natural frequencies of the isotropic plate by using DSM with CPT based on Levitype BCs to the available literature Leissa [7] values.
The natural frequencies of simple supported (SS) isotropic plates are shown in Table 1, and Table 2. The four different edge boundary condition are applying as notification SSFSS C.
Table 1Nondimensional natural frequencies (ϖ=ωa2 ρch/Dc for SSFSSF plate
SFSF  $a/b$  $\nu =$ 0  $\nu =$ 0.3  $\nu =$ 0.5  
$mn$  0.4  Present  Ref [10]  Present  Ref [10]  Present  Ref [10] 
1 1  9.8696  9.8696  9.7600  9·7600  9.4506  9·4506  
2 1  39.4784  39.4784  39.2386  39.2387  38.3771  38.3771  
1 1  1  9.8696  9.8696  9.6313  9.6314  9.0792  9.0793 
2 1  39.4784  39.4787  38.9449  38.9450  37.5191  37.5192  
3 1  88.8264  88.8264  87.9866  87.9867  85.4898  85.4899  
1 1  2.5  9.8696  9.8696  9.4841  9.4841  8.7042  8.7042 
2 1  39.4784  39.4787  38.3629  38.3629  35.8798  35.8799  
3 1  88.8264  88.8264  86.9684  86.9684  82.5093  82.5093  
4 1  157.9136  157.9137  155.3211  155.3211  148.725  148.7256 
Table 2Nondimensional natural frequencies (ϖ=ωa2 ρch/Dc for SSSSSSSS isotropic square plate
SSSS  
$mn$  Present  Ref [8]  Ref [9]  Ref [10] 
1 1  19.7392  19.7392  19.739  19.739 
1 2  49.3480  49.3490  49.348  49.348 
2 1  49.3480  49.3490  49.348  49.348 
2 2  78.9568  79.4007  78.957  78.957 
1 3  98.6960  100.1729  99.304  100.17 
3 1  98.6960  100.1868  99.304  – 
3 2  128.3048  130.3895  –  – 
4. Conclusions
Present work is used to understand the vibration analysis of isotropic plate was studied using classical plate theory under different levy type boundary conditions. This present study to solve the natural frequencies of plate by formulating the dynamic stiffness matrix together with CPT. WittrickWilliam algorithm to take into account the nonlinear behaviour of the DSM element. Formulation is used to solve the dynamics stiffness matrix with applying boundary condition has been import in MATLAB and extract the nondimensional frequencies of plate beside a high degree of desired accuracy. Thus, the result has been investigated to the acceptable comparison of natural frequencies of isotropic plate.
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