The Journal of Engineering Research (TJER), Vol. 15, No. 1 (2018) 14-25 DOI: 10.24200/tjer.vol15iss1pp14-25 Vibrations of Circular Plates Resting on Elastic Foundation with Elastically Restrained Edge Against Translation L.B. Raoa,* and C.K. Raob a School of Mechanical and Building Sciences, VIT University, Chennai Campus, Vandalur-Kelambakkam Road, Chennai- 600127, Tamil Nadu, India. b Department of Mechanical Engineering, School of Engineering, Chaudari guda, Korremulla ‘X’ Roads, Near Narapalli, Ghatkesar (M), Hyderabad-500 088, India. Received 6 July 2016; Accepted 26 January 2017 Abstract: The present paper deals with exact solutions for the free vibration characteristics of thin circular plates resting on Winkler-type elastic foundation based on the classical plate theory elastically restrained against translation. Parametric investigations are carried out for estimating the influence of edge restraint against translation and stiffness of the elastic foundation on the natural frequencies of circular plates. The elastic edge restraint against translation and the presence of elastic foundation has been found to have a profound influence on vibration characteristics of the circular plate undergoing free transverse vibrations. Computations are carried out for natural frequencies of vibrations for varying values of translational stiffness ratio and stiffness parameter of Winkler-type foundation. Results are presented for twelve modes of vibration both in tabular and graphical form for use in the design. Extensive data is tabulated so that pertinent conclusions can be arrived at on the influence of translational edge restraint and the foundation stiffness ratio of the Winkler foundation on the natural frequencies of uniform isotropic circular plates. Keywords: Plate; Frequency; Elastic edge; Translational stiffness; Elastic foundation. اثناء احلركةاملرونة اهتزازات االلواح الدائرية املثبتة على قاعدة مرنة مع حافة مقيدة ب، تشيالبيال كامسوارا راو*،أ لوكافارابو بهاسكارا راو املثبتة على قاعدة قيقةلاللواح الدائرية الر ةاحلر اتخلصائ االهنزاز صحيحةالبحثية احللول ال : تتناول الدراسة لص امل معاملية حباث ا. مت إجراء لتقليديةا لواحمبوجب نظرية االاحلركة اثناءرونة امل مقيدة ذات حافة مرنة من نوع وينكلر و الدائرية. لأللواحصالبة القاعدة املرنة على الرتددات الطبيعية و احلركة اثناءيد احلافة يتأثري تق مدىلتقدير (بارامرتية) مع وجود القاعدة املرنة هلا تأثري كبري على خصائ اهتزاز ركةاحلعملية أثناءاىل أن احلافة مقيدة املرونة توصل البحث يعية لالهتزازات لقيم متفاوتة من تعرض الهتزازات عرضية حرة. ومت إجراء احلسابات للرتددات الطبيوح الدائري اليت لال اتاالهتزاز أمناطالنتائج الثين عشر منطا من و قدم البحثمعدل الصالبة املتعدية ومعامل صالبة القاعدة من نوع وينكلر. إىل منها حبيث ميكن التوصل شاملةيف شكل جداول و رسومات بيانية الستصدامها يف التصميم. كما مت جدولة بيانات نوع وينكلر على الرتددات الطبيعية للوحات من استنتاجات ذات صلة بتأثري تقييد احلافة املتنقلة ونسبة صالبة القاعدة الدائرية املتماثلة املوحدة. قاعدة مرنة ،نقلةصالبة مت ،حافة مرنة،الرتدد ،: لوح فتاحيةملا كلماتال * Corresponding author’s e-mail: bhaskarbabu_20@yahoo.com mailto:bhaskarbabu_20@yahoo.com L. B. Rao and C.K. Rao 15 Nomenclature h Thickness of a plate, mm a Radius of a plate, mm Poisson’s ratio E Young’s modulus, N/mm2 Density of a material, kg/mm3 ),( rW Transverse deflection of the plate, mm D Flexural rigidity of a plate, N.mm2 T K Translational spring stiffness, N/mm w K Stiffness of Winkler foundation, N/mm2/mm T Translational spring stiffness ratio Foundation stiffness ratio mn Natural frequency, rad/sec mn Eigenvalue without foundation, cycles/sec * mn Eigenvalue with Winkler foundation, cycles/sec nm, positive integers correspond- ing to the number of concentric circles and nodal diameters in each flexural mode. Vibrations of Circular Plates Resting on Elastic Foundation with Elastically Restrained Edge Against Translation 16 1. Introduction Circular plates resting on elastic foundation have a wide range of applications in the static and dynamic design of linear/nonlinear vibration absorbers, dynamic exciters, telephone receiver diaphragms, computer discs, printed circuit boards etc. (Leissa 1969). Due to the essential use of vibration data in the computation of stresses in such structures, reliable prediction of vibration data is of great importance. In view of its importance in engineering design, the problem of vibration of circular plates on elastic foundation has attracted the focus and attention of many researchers. Some of the recent studies have reinstated the classical approach efficiency in analyzing the vibrations of variety of structures. Circular plate problems allow for significant simplification in view of their symmetry, but still many difficulties arise when the plate boundary conditions become complex involving linear and rotational restraints. A recent review of literature shows that very few studies exist on the study of circular plates resting on elastic foundations. Wang and Wang (2003), who observed the switching between axisymmetric and asymmetric vibration modes, have recently investigated the effect of internal elastic translational supports. The vibration characteristics of plates resting on an elastic medium are different from those of the plates supported only on the boundary. Leissa (1993) discussed the vibration of a plate supported laterally by an elastic foundation. Leissa deduced that the effect of Winkler foundation merely increases the square of the plate natural frequency by a constant. Salari et al. (1987) speculated the same conclusion. Ascione and Grimaldi (1984) studied unilateral frictionless contact between a circular plate and a Winkler foundation using a variational formulation. Leissa (1969), who tabulated a frequency parameter for four vibration modes of a simply supported circular plate with varying rotational stiffness, presented one of the earliest formulations of this problem. Kang and Kim (1996) presented an extensive review of the modal properties of the elastically restrained beams and plates. Zheng and Zhou (1988) studied the large deflection of a circular plate resting on Winkler foundation. Ghosh (1997) studied the free and forced vibration of circular plates on Winkler foundation by an exact analytical method. (Chang and Wickert (2001); Kim et al. (2001) and Tseng and Wickert (1994) studied the dynamic characteristics of bolted flange connections involving circular plates displaying beating type of repeat frequencies and typical mode shapes of vibration. Bolted flange connections are practically the best examples for the elastically restrained boundary conditions of circular plates on partial or continuous Winkler type elastic foundation. The most general soil model used in practical applications is the Winkler (1867) model in which the elastic medium below a structure is represented by a system of identical but mutually independent elastic linear springs. Recent investigations have reiterated the efficiency of the classical approach (Soedel 1993) in analyzing the behavior of structures under vibrations. There are other works (Weisman 1970; Dempsey et al. 1984; Celep et al. 1988) dealing with the study of plates on a Winkler foundation. In general, those dealing with vibrating plates, shells and beams are concerned with the determination of eigenvalues and mode shapes (Leissa 1969). A good number of studies was conduced (Wang and Lin 1996; Kim and Kim 2001; Yayli et al. 2014) using the method of Fourier series for estimating the frequencies of beams with generally restrained end conditions including the effect of elastic soil foundation. The method includes the use of Stoke’s transformation in suitably modifying the complex boundary conditions. Very much similar to the dynamic stiffness matrix approach, the elements of the matrix involving infinite Fourier series are explicitly obtained in these studies. The determinant of this matrix for each case considered leads to the frequency equation and the same can be solved using well known numerical methods. The results obtained for various elastically restrained beam cases in these studies tallied well with those available in the literature establishing the efficiency of this method. In view of the necessity of using complex combinations of rotational and translational springs at the circular plate boundary to suitably simulate the practical non-classical boundary connections being adopted in a wide range of industrial applications (Bhaskara Rao et al. 2009; Bhaskara Rao et al. 2010; Lokavarapu and Chellapilla 2013; Bhaskara Rao et al. 2015; Lokavarapu et al. 2015; Rao et al. 2016), the use of exact method of solution becomes imperative L. B. Rao and C.K. Rao 17 and hence the same is adopted in this paper. Even though the method adopted here is classical, the particular case of vibration of elastically restrained circular plate resting on elastic foundation considered here is not addressed within the available literature. Utilizing the classical plate theory, this paper deals with an exact method of solution for the analysis of thin circular plate free transverse vibrations that is elastically restrained against translation and resting on Winkler-type elastic foundation. For estimating the influence of edge restraint against the elastic foundation translation and stiffness on circular plates natural frequencies, parametric investigations are carried out by varying the values of elastic edge restraint stiffness against the elastic foundation translation and stiffness. The results obtained on natural frequencies of vibration clearly show that the vibration characteristics of the circular plate undergoing free transverse vibrations are found to be profoundly influenced by these variations. Computations are carried out for natural frequencies of vibrations for varying values of translational stiffness ratio and stiffness parameter of Winkler-type foundation. The results that are presented for twelve modes of vibration both in tabular and graphical forms are believed to be quite useful for designers in this area. 2. Mathematical Formulation of the System The considered elastic thin circular plate is supported on a Winkler foundation as shown in Fig. 1. In the classical plate theory (Leissa 1969), the following fourth order differential equation describes free flexural vibrations of a thin circular uniform plate: 0 2 t/)t,,r(w 2 h)t,,r(w 4 .D (1) where ) 2 1(12/ 3 EhD is the flexural rigidity of a plate and ,E,,h,a are the plate’s radius, thickness, density, Young’s modulus and Poisson’s ratio, respectively. The homogeneous equation for Kirchhoff’s plate on one parameter elastic foundation is given by the following equation: )t,,r(wwK)t,,r(w 4 .D (2) 2 t/)t,,r(w 2 h =0 Displacement in (2) can be presented as a combination of spatial and time dependent components as follows: Let ti erWtrw ),(),,( (3) Now substitute (3) in (2) 0),r(W). 2 hwK(),r(W 4 .D (4) The solution of the equation takes the following form ncos. a rmn nImnC a rmn nJ mnA),r(mnW , n > 0 (5) where mnA and mnC are constants, nJ is Bessel function of the first kind of first order and nI is modified Bessel function of the first kind of first order, indexes m and n are positive integers and correspond to the number of concentric circles and nodal diameters in each flexural mode. Considering an elastically supported plate as shown in Fig. 1, boundary conditions can be formulated at ar , in terms of translational stiffness ( TK ) as follows: 0),a(rM (6) ,aWTK,arV (7) where the Kelvin-Kirchhoff and bending moment are defined as follows: Figure 1. A thin circular plate with translational elastic edge restraint and supported on elastic foundation. Vibrations of Circular Plates Resting on Elastic Foundation with Elastically Restrained Edge Against Translation 18 2 ),a(W 2 2 r 1 r ),a(W r 1 2 r ),a(W 2 .D),a(rM (8) ),a(W 2 r 1 r ),a(W 2 r 1 r 1 )1(),a(W 2 r .D),a(rV (9) By applying Eqs. (6) and (8), we obtain the following equation 0 2 ),a(W 2 2 r 1 r ),a(W r 1 2 r ),a(W 2 (10) From Eqs. (5) and (10), we derive the following equation )( 4 2 2 )( 4 2 2 2 2 2 2 mnn mn mn mn mn mnn mn mn mn mn mn I n ST J n PQ C (11) where mn P = )mn(1nJ)mn(1nJ ; )mn(2nJ)mn(2nJmnQ ; )mn(1nI)mn(1nImnS ; )mn(2nI)mn(2nImnT ; From Eqs. (7) and (9), we get the following ),a(W.TK ),a(W 2 r 1 r ),a(W 2 r 1 r 1 )1(),a(W 2 r .D (12) From Eqs. (5) and (12), we derived the following equation )mn(nIT 3 mn 8 mn 4 3 mn 2 n)3(8 mnS 2 mn 2 n)2(44 3 mn mnT2 mnU )mn(nJT 3 mn 8 mn 4 3 2 n)3(8 mnP 2 mn 2 n)2(44 3 mn mnQ2 mnR mnC (13) where, D Ka T T 3 )()( 11 mnnmnnmn JJP ; )mn(2nJ)mn(2nJmnQ ; )()( 33 mnnmnnmn JJR ; )()( 11 mnnmnnmn IIS ; )mn(2nI)mn(2nImnT ; )()( 33 mnnmnnmn IIU ; If T K then this case becomes a simply supported boundary condition as shown in Fig. 2. The frequency equation can be calculated From Eqs. (11) and (13), which allows determining eigenvalues mn . The mode shape parameters mnC can be determined corres- ponding to these eigenvalues by using either Eq. (11) or Eq. (13). The amplitude of each vibration mode in Eq. (5) is set by the normalization constant mnA determined from the following condition. nqmpmnpq a mn MrdrdrWrW ),().,( 2 0 0 (14) where, mn M is a mass of the plate, 1 nqmp if qnpm , and nqmp = 0 if m ≠ p or n ≠ .q Figure 2. A simply supported thin circular plate resting on elastic foundation. L. B. Rao and C.K. Rao 19 The normalization constant mn A can be derived using Eqs. (5) and (14) as given below: 1 2 0 a 0 rdrd 2 ncos. a rmn nI.mnC a rmn nJ . 2 a 1 mnA (15) In Eq (4), the natural frequency is defined as mn h D 2 a 2 mn (16) It is clear from Eq. (16) that the natural frequency parameter mn is dependent on the plate radius ‘a’. From Eq. (16) we can express D 2 mn 4 ha4 mn (17) 244* mnmn (18) where D aK w 4 2 (19) 4 1 24* mnmn (20) 3. Solution Using Matlab programming, computer software with symbolic capabilities, solves the above set of equations. The program determines eigenvalues ( *mn ), for a given range of boundary conditions. The boundary’s linear translational non-dimensional restraint parameter can be defined as follows: D aK T T 3 (21) D aK w 4 2 (22) The following represent the input parameters to the program; (i) Translational stiffness ratio ( T ); (ii) Foundation ratio ( ); (iii) Poisson’s ratio ( ); (iv) Upper bound for eigenvalues ( N ); (v) Suggested accuracy for eigenvalues ( d ); (vi) Number of mode shape parameters ( n );. The program finds eigenvalues *mn by using Matlab root finding function. 4. Results and Discussion The code developed is used to determine eigenvalues of any set or range of translational and foundation constraints. This code is also implemented for various plate materials by adjusting Poisson’s ratio. Such a wide range of results is not available in the literature. The eigenvalues for the plate edge, which is elastically restrained against translation and fully resting on the elastic foundation, at various values of the translational stiffness ratios, are computed and the results are given in Table 1. The effects of the translational stiffness ratios are plotted in Fig. 3. As seen from Fig. 3, eigenvalues increase with an increment in the translational stiffness ratio, and the plates become unstable in the region when the translational stiffness ratio exceeds a certain value. Twelve vibration modes are presented in Fig. 3. The smoothened stepped variation is observed in Fig. 3. The stepped region increases with increase in translational stiffness ratio and vibration modes. The location of the stepped region with respect to T changed gradually from the range of 0.01526 † [9.9997] ‡ – 5587.5316 [10] to 16.62296 [14.6739] – 611824.96917 [16.75055]. Here †.represents translational stiffness ratio and ‡ represents Eigen values throughout the text. Here the value in the bracket represents eigenvalue. The simply supported boundary conditions (Fig. 2) could be accounted for by setting ( T K ) shown in Fig. 1. The frequency in this case is 2.23175 and this is in good agreement with the results published by Wang (2005). Another result, considered for comparison, is from Rao and Rao (2009) on a study of the case of vibrations of elastically restrained circular plates supported on partial Winkler foundation. When the Table 1. Eigenvalues for different Translational stiffness ratio for =100 & =0.33. log10 𝑇 𝜆00 𝜆10 𝜆20 𝜆01 𝜆11 𝜆21 𝜆02 𝜆12 𝜆22 𝜆03 𝜆13 𝜆23 -3 10 10.0205 10.3517 10.10358 10.79568 12.46774 10.29701 11.43857 13.52984 10.63672 12.2505 14.67332 -2 10 10.02051 10.35171 10.10359 10.79568 12.46775 10.29702 11.43857 13.52985 10.63673 12.25051 14.67332 -1 10.00005 10.02061 10.3518 10.10368 10.79576 12.46779 10.29711 11.43864 13.52989 10.63682 12.25056 14.67335 0 10.00047 10.02162 10.35263 10.10462 10.79649 12.46827 10.29803 11.43927 13.53026 10.63769 12.25109 14.67365 1 10.003 10.03287 10.36125 10.11468 10.804 12.47302 10.30745 11.44568 13.53403 10.64649 12.25645 14.67667 2 10.00568 10.1199 10.46887 10.2319 10.89095 12.52351 10.42003 11.5759 13.57333 10.74585 12.31326 14.70772 3 10.00614 10.203 11.00421 10.49407 11.61576 13.1793 10.91134 12.27823 14.08459 11.41839 12.99719 15.0966 4 10.00619 10.21322 11.14468 10.53794 11.94459 14.15323 11.03916 12.88272 15.33948 11.70347 13.90786 16.53545 5 10.00619 10.21423 11.15725 10.54212 11.97273 14.2438 11.05093 12.93527 15.47979 11.72934 13.99516 16.74092 6 10.00619 10.21433 11.15849 10.54254 11.97546 14.25215 11.05209 12.94027 15.49231 11.73186 14.00329 16.75866 12 10.00619 10.21434 11.15862 10.54258 11.97576 14.25307 11.05222 12.94082 15.49369 11.73214 14.00418 16.7606 2 0 V ib ration s of C ircu lar P lates R estin g on E lastic F ou n d ation w ith E lastically R estrain ed E d g e A g ain st T ran slation Table 2. Eigenvalues for different Foundation stiffness ratio for T =100 & =0.33. Table 3. Eigenvalues for different Translation and Foundation stiffness ratios for =0.33. log10 𝜉 & log10 𝑇 𝜆00 𝜆10 𝜆20 𝜆01 𝜆11 𝜆21 𝜆02 𝜆12 𝜆22 𝜆03 𝜆13 𝜆23 -3 0.2115 3.0115 6.2054 4.52915 7.73687 10.9091 5.93655 9.18564 12.3826 7.27469 10.57846 13.80849 -2 0.37648 3.01187 6.20544 4.52925 7.73689 10.90911 5.93659 9.18566 12.38261 7.27471 10.57846 13.8085 -1 0.67602 3.0156 6.20584 4.53032 7.73709 10.90918 5.93708 9.18578 12.38266 7.27499 10.57855 13.80854 0 1.30374 3.067 6.21074 4.54336 7.73963 10.91008 5.94302 9.18733 12.38328 7.27834 10.57959 13.80899 1 3.25317 3.90325 6.34932 4.87899 7.81237 10.93614 6.10341 9.23131 12.40119 7.36838 10.60869 13.82199 2 10.00568 10.1199 10.46887 10.2319 10.89095 12.52351 10.42003 11.5152 13.57333 10.74585 12.31326 14.70772 3 31.62297 31.62939 31.65958 31.63958 31.68744 31.78104 31.65573 31.72292 31.85232 31.67796 31.76831 31.94926 Log10 𝜆00 𝜆10 𝜆20 𝜆01 𝜆11 𝜆21 𝜆02 𝜆12 𝜆22 𝜆03 𝜆13 𝜆23 -3 2.18341 4.70075 6.69703 5.56683 7.98678 10.99197 6.50356 9.33292 12.43922 7.59879 10.67534 13.84974 -2 2.18341 4.70075 6.69703 5.56683 7.98678 10.99197 6.50356 9.33292 12.43922 7.59879 10.67534 13.84973 -1 2.18365 4.70077 6.69704 5.56684 7.98679 10.99197 6.50357 9.33292 12.43922 7.59879 10.67535 13.84974 0 2.20704 4.70315 6.69786 5.56827 7.98727 10.99216 6.50447 9.33323 12.43935 7.59936 10.67555 13.84983 1 3.3284 4.92488 6.77875 5.7064 8.03541 11.01074 6.5926 9.36352 12.45218 7.65514 10.69583 13.84914 2 10.00568 10.1199 10.46887 10.2319 10.89095 12.52351 10.42003 11.5159 13.57333 10.74585 12.31326 14.70772 3 31.62296 31.62664 31.63867 31.63037 31.6549 31.73756 31.63691 31.68259 31.81038 31.6491 31.72496 31.90972 L . B . R ao an d C .K . R ao 2 1 Vibrations of Circular Plates Resting on Elastic Foundation with Elastically Restrained Edge Against Translation 22 support position is in full span, which means that when 1b , the case becomes a circular plate having full foundation support with elastically restrained edge against translation. For this case, the frequency is 2.1834 and that is in good agreement with the frequency of 2.18341 obtained from the present study. The eigenvalues at various values of the foundation stiffness ratios for 33.0&100 T are computed and the results are given in Table 2. The effects of the foundation stiffness ratio on eigenvalues are plotted in Fig. 4. As seen from Fig. 4, the eigenvalue increases with increase in the foundation stiffness ratio, and the plate becomes stiffer and stronger as the value of foundation stiffness becomes greater than 102. As seen from the Tables 1 and 2, the influence of foundation stiffness ratio on eigenvalue is relatively greater than that of the translation stiffness ratio in increasing the overall natural frequencies of the plate support system. As seen from Fig. 4, for all the modes considered here, up to a value of 10 the eigenvalues stay constant and beyond this value all the curves tend to converge to a constant eigenvalue as the foundation stiffness ratio increases up to 103. The convergence starts from 1.07897 [2.0325779] and continues up to a constant value of 9.63274 [13.84796]. The eigenvalues at various values of the translational stiffness ratios and foundation stiffness ratios are computed and the results are given in Table 3. The effects of the translation and foundation stiffness ratios on eigenvalues are clearly observed in Fig. 5, eigenvalues increases with an increment in both the translational and foundation stiffness ratios. As observed from the Table 1 and 3, the influence of foundation stiffness ratio on eigenvalue is more predominant than that of translation stiffness ratio alone. From the results presented in Tables 1 and 3, we can see that the influence of foundation stiffness ratio on eigenvalues is more predominant than that of translation stiffness ratio. From the results given in Tables 1 to 3, one can easily find that the eigenvalues become lower for lower values of foundation and translation stiffness ratios. As seen from Fig. 5, all the curves are stable up to a certain region beyond which the curves tend to converge for increasing values of translation and foundation stiffness ratios. Figure 3. Effect of translational stiffness ratio T on eigenvalues, mn . L. B. Rao and C.K. Rao 23 Figure 4. Effect of foundation stiffness ratio, on eigenvalues, .mn Figure 5. Effect of translational, T and foundation, stiffness ratios on eigenvalues, .mn Table 4. Eigenvalues for different Poisson ratios. ν 1000T 10,100 T 1000,10 T 10,1 T 50,50 T 0 31.62925 4.92456 10.03027 3.62553 7.28924 0.1 31.62929 4.92466 10.03111 3.64904 7.2896 0.2 31.62934 4.92476 10.0319 3.6708 7.28993 0.3 31.62938 4.92485 10.03265 3.69097 7.29025 0.4 31.62942 4.92494 10.03336 3.70974 7.29056 0 5 10 15 20 25 30 35 -4 -3 -2 -1 0 1 2 3 4 E ig e n v a lu e s , λ m n Foundation parameter,Log10 ξ λ00 λ10 λ20 λ01 λ11 λ21 λ02 λ12 λ22 λ03 λ13 λ23 Vibrations of Circular Plates Resting on Elastic Foundation with Elastically Restrained Edge Against Translation 24 Figure 6. Effect of Poisson ratio, on eigenvalues, .mn 5. Conclusion This work deals with a method of computation of eigenvalues of flexural vibrations of a circular plate with translational edge supported and resting on Winkler foundation using a specifically written MATLAB code. The computed numerical results are presented in a tabular format to enable estimating the accuracy of approximate methods being used by other researchers for solving such problems. Two- dimensional plots of eigenvalues are drawn for a wide range of translational and foundation stiffness ratios facilitating their use in design. From the numerical and graphical results presented in this paper, it can be easily observed that the eigenvalues remain constant only for a limited range of constraints specific to each vibration mode and then steeply increase with the increasing values of foundation stiffness ultimately converging towards a constant value. It is also observed that the influence of foundation stiffness ratio on eigenvalues is more predominant than that of translational stiffness ratio. Conflict of Interest The authors declare no conflicts of interest. Funding No funding was received for this research. 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