”(£815 This is to certify that the thesis entitled Discrete and Continuous Models of Critical Damping for a Simply Supported Beam presented by Julia Ann Gray has been accepted towards fulfillment of the requirements for _Master's Mechanical Engineering degree in £54641?” @4/ Major professor Date A/N /2r1[7 Y/ 0-7639 IV‘ESI_J RETURNING MATERIALS: Place in book drop to LJBRARJES remove this checkout from .l-IICI-IIL. your record. FINES will be charged if book is returned after the date stamped below. DISCRETE AND CONTINUOUS MODELS OF CRITICAL DAMPING FOR A SIMPLY SUPPORTED BEAM BY Julia Ann Gray A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Mechanical Engineering 1981 ABSTRACT DISCRETE AND CONTINUOUS MODELS OF CRITICAL DAMPING FOR A SIMPLY SUPPORTED BEAM 3)’ Julia Ann Gray The concept of "critical damping" for both multidegree of freedom, discrete linear systems and distributed parameter systems, has been investigated by a number of researchers. Conditions under which a system is critically damped, which depend on certain combinations of the coefficient operators, have been obtained. Conditions under which a damping mechanism provides underdamping, overdamping, or mixed damping have also been described. In this thesis, specific results are applied to a particular continuous system, a simply supported beam. Results are obtained by considering both the analytic model of the beam and an approximate, discrete version of this model obtained by the finite element method. A variety of damping mechanisms are included in a number of example problems. TABLE OF CONTENTS LIST OF TABLES . ............................................. LIST OF FIGURES ............................................. I. INTRODUCTION ............................................ II. A DISCRETE MODEL OF CRITICAL DAMPING ................... 2.0 Introduction .................................. 2.1 Definitions ................................... 2.2 The Finite Element Model ...................... 2.3 A Simple Method for Calculating Ccr ........... 2.4 A Note On The Comparison Test ................. 2.5 Results ....................................... III. AN ANALYTIC MODEL OF CRITICAL DAMPING ................. 3.0 Introduction .................................. 3.1 The Undamped Simply Supported Beam ............ 3.2 The Damped Simply Supported Beam .............. 3.3 A Result on Discrete Damping .................. 3.4 Examples ...................................... IV. CONCLUSION ............................................. APPENDIX A - FINITE ELEMENT MATRICES AND THE UNDAMPED NASTRAN MODEL .................................. APPENDIX B - EIGENVALUES, AND MATRICES FROM CHAPTER II ...... APPENDIX C - FINITE ELEMENT DAMPING MATRICES FOR DISCRETE AND CONTINUOUS DAMPING ......................... APPENDIX D - EIGENVALUES FOR EXAMPLES 5-8 ................... LIST OF REFERENCES ..... . .................................... iii H #00qu 15 16 20 20 21 24 26 29 34 38 49 60 65 7O LIST OF TABLES Table Page 1 Comparison of Three Nastran Models with Theoretical Model ................................................ 17 2 Summary of Data for Examples 1-8 ..................... 20 3 Comparison of NASTRAN Model with Analytical Model .... 33 iv Figure 2 LIST OF FIGURES The Simply Supported Beam The Beam With Finite Element Grid Cross Section ...... A Typical Element of the Beam ........................ I. INTRODUCTION The damped, free vibrations of both multidegree of freedom discrete linear systems and continuous systems have received considerable atten- tion. The first matter to receive attention concerned the definitions of the various possible types of damping. For the simplest systems, the various types of damping, which result in systems that are under- damped (oscillatory), overdamped (nonoscillatory), or critically damped (also nonoscillatory), are easily defined in terms of the parameters of the system. Defining similar criteria for more complex systems has been an area of current interest. Another area of current interest concerns the qualitative nature of the motion of a system, under a given type of damping. It is desired to determine, without resorting to lengthy calculations, the type of damping a particular damping mechanism provides. Ideally, this problem is to be attacked by examining the parameters of the system, not by solving the equations of motion of the system, or, alternatively, re- sorting to experiment. The inverse of this problem has also been addressed. Given a system with a predetermined desired motion, it is necessary to identify a damping mechanism, or mechanisms, which will insure that the desired motion is attained. Again, this problem is to be attacked without resorting to lengthy calculations or experiment. In this paper, recent results on each of these topics will be applied to a particular continuous system, a simply supported beam. The last of the above topics will be addressed by considering an 1 2 analytic model of the beam. The first two of the above problems will be attacked by considering an approximate, discrete version of this model obtained by the finite element method. First, however, a brief summary of other results on these topics will be presented. It is well known that the qualitative nature of the solution of one of the simplest vibratory systems, a single degree of freedom spring- mass-damper system (spring and damper in parallel) can be determined by examining the coefficients of the ordinary differential equation which describes its motion. This equation is mx + cx + kx = 0 (1.1) where m, c, and k are the positive values of the mass, damper, and spring, respectively, and x(t) is the "displacement" from static equilibrium of the system. Equation (1.1) can be rewritten as SE + 2ng< + mzx = o (1.2) where the natural frequently of the undamped system is m 2 /§-, and the damping ratio is c = c/Z/Em: The system is said to be overdamped if c > 1, critically damped if c = 1, and underdamped if c < 1. For more complicated single degree of freedom systems, the analysis is still straightforward and simple. The equation of motion for the more complex, multidegree of freedom, discrete, damped linear system is an analogous matrix differential equation M§(t) + Gin) + 1620;) = o (1.3) 3 where M and K are the positive definite, symmetric, mass and stiffness matrices respectively, and C is the positive semi-definite symmetric damping matrix. For this problem, the concepts of over, under, and critical damping are more subtle and require closer scrutiny than the single degree of freedom case. In fact, a number of results addressing the qualitative nature of damping of solutions of this equation have been published, each shedding some light in different areas. In previous work Duffin [1], in considering (1.3) defined an over- damped system in terms of a function of the quadratic ferms of the M, C, and K matrices. More recently, Nicholson [2] defined an underdamped system in terms of the eigenvalues of the mass, damping, and stiffness matrices. Maller [3], upon considering Nicholson's work, defined an underdamped system similar to that of Duffin and derived a sufficient con- dition for underdamping in terms of the definiteness of the coefficient matrices. Recently, additional results have been obtained by Beskos and Boley [4]. Techniques used in [4] are different from those found in matrix theory which were used in [1,2,3]. Essentially Beskos and Boley 1‘“ different damping k k=1 coefficients, find the locus of combinations of the {ck} leading to address the fbllowing question: Given {c critically damped motion. Their approach requires calculating critical damping surfaces in a parameter space (this surface corresponds to the concept of finding the "minimum value of damping required for non- oscillation"). By calculating critical damping surfaces in parameter space, one is led to the following observations: A combination of damping coefficient which correspond to a point in the parameter space "above" the critical damping surface yields a solution to (1.3) which 4 is overdamped (nonoscillatory); a combination of damping coefficients yields an umderdamped (oscillatory) solution of (1.3) if the point in parameter space corresponding to the combination lies "below" the critical surface. The conditions derived by Duffin, Nicholson, and Beskos and Boley require substantial calculations. In the cases of Duffin and Nicholson, the calculations are used for verification of their respective con- ditions; there are different reasons for the computational load found in [4]. It appears that the technique of Beskos and Boley works well when the damping is Rayleigh (C = aK + 3M) and the order of the system is small. This stems from the fact that the technique requires dif- ferentiations for which there are presently no efficient numerical schemes available. More complete results, which do not require substantial calcula- tions, have recently been obtained by Inman and Andry [S]. In this paper, the definition of critical damping is stated in terms of the coefficient matrices in a manner analogous to the single degree of freedom case. A critical damping matrix is defined in terms of the unique positive definite square roots of the matrices M and K. Having obtained Ccr’ a system is defined as overdamped, critically damped, or underdamped if the matrix (C - Ccr) is positive definite, zero, or negative definite, respectively. A fourth possibility, mixed damping, is defined if (C — Ccr) is indefinite. The computational load for these results is nominal and does not encounter any special problems due to high order. In the special case that the damping matrix is diagonalized by the undamped modal matrix, all the conditions presented become necessary and sufficient; for the general case (C not diagonalized by the undamped modal matrix), suf- ficient conditions are presented which are easily verified. For these reasons, the results in [S] are more general than MGller's and reduce to Muller's in the special case. II. A DISCRETE MODEL OF CRITICAL DAMPING 2.0 Introduction In this chapter the first two problems outlined in the general introduction are considered for the special case of a simply supported beam. The beam is discretized using a finite element model and the definitions of the various types of damping in [S], for multidegree of freedom discrete systems, are directly applied to the discretized continuous system. The definitions of the various types of damping, including critical damping, are presented in the first section. A discussion of the physical implications of these definitions is also included. The con- sistent finite element model of the simply supported beam is presented in the second section. The coefficient matrices necessary to apply the definitions from section one are obtained from this model. The next two sections present derivations which make the definition of section one easier to apply. In the first, a simple method for calculating the critical damping matrix with respect to the original coordinate system is derived. In the second, it is shown that the test for the type of damping a given damping matrix provides can be carried out in the original coordinate system. Working in the original coordin- ate system eliminates the need to calculate square roots and inverses of matrices. Numerical accuracy is increased and computing costs are decreased. In the last section, the critical damping matrix is calculated and introduced into the equations of motion of the beam. These equations are solved directly using MSC/NASTRAN finite element code, and the resulting motion is shown to be indeed critically damped. Other example damping matrices are compared to the critical damping matrix and the results are presented. 2.1 Definitions Consider a multidegree of freedom, discrete, damped linear system described by the matrix differential equation M§+c§+ {i=0 . (2.1.1) where M and K are the positive definite, symmetric mass and stiffness matrices, respectively, and C is the positive semi-definite damping matrix. In previous work, Inman and Andry [S] rewrite (2.1.1) as 37+W+W=0 (2.1.2) -% -1 -i -i i where C: M CM , R = M KM , and M denotes the unique positive definite square root of the positive definite matrix.M. The critical damping matrix fer this system is defined to be c = 2M5 m_i)i= 2? (2.1.3) (21‘ in direct analogy to the single degree of freedom case. The motion of the system is defined to be critically damped if’e = Ccr’ to be over- damped if (B - Ccr) is positive definite, and to be underdamped if (C - Ccr) is negative definite. A fburth possibility, mixed damping, is defined if (2: - cc?) is indefinite. Defining the various types of damping in this way results in physically reasonable eigenvalues. As usual, the eigenvalues of (2.1.2) are taken to be the Zn roots of the polynomial equation IAZI + A8 + kl = 0 (2.1.4) where -l denotes the determinant. In particular, Inman and Andry show that (i) if (2.1.1) is critically damped then there are at most n dis- tinct negative real eigenvalues and no complex eigenvalues, (ii) if (2.1.1) is underdamped the eigenvalues are all complex and appear in complex conjugate pairs with negative real parts, which corresponds to all modes of the system oscillating in damped harmonic motion, and (iii) if (2.1.1) is overdamped, then the eigenvalues are all negative real numbers, and none of the modes oscillates. For the special case that C is diagonalized by the undamped modal matrix (2.1.1) exhibits mixed damping if and only if there is at least one real eigenvalue and at least one complex conjugate pair of eigenvalues. One of the modes of the damped system will oscillate and at least one will not. 2.2 The Finite Element Model The partial differential equation of motion for the damped, trans- verse, free vibrations of a simply supported beam is 3“v x t a 2v x,t _I_le . c 3? v(x,t) + g—I—Lr—la at = 0 (2.2.1) ax with boundary conditions v(0,t) = v(L,t) = 0 (2.2.2) V"(O,t) = V"(L,t) = 0 (2.2.3) where v(x,t) is the transverse deflection of the beam. This equation is valid for the case in which damping of constant magnitude c, is applied over the entire length of the beam, and the mass per unit length p, modulus of elasticity E, and moment of inertia I, are all assumed to be constant. This equation will be approximated by a five element, consistent finite element model [6]. The finite element model for the simply sup- ported steel beam being considered is shown in Figure l. 5m 1 ‘k-fihn'fi ” (1) (l (2) (3) ), (4) , (5) 4,___, .2m 1 2 4 5 6 _JL The simply supported beam The beam cross with finite element grid. section. Figure 1 The physical quantities assumed are E = 2.0 x 1011 PA 111 = 7.852 x 103 Elf-é I = 6.67 x 10-5 m“ A = .02m2 (cross sectional area) The node number for each node is located to the left and slightly below each grid point, and the element number is enclosed by parentheses on each element. Consider a typical element of the beam, (e), shown in Figure 2. It I u. 1“! uj 6i *3 in (e) j) «3) 9J. 0+____.’+X 1 Figure 2. A Typical Element of the Beam 10 Each node has two degrees of freedom associated with it; one vertical translation u and one rotation 6. Four unknowns must be determined for each element. A cubic displacement function, whose four coefficients will be determined in terms of the four unknown displacements, will be assumed of the form (e) _ 2 3 v (x,t) - ao(t) + a1(t) x + a2(t) x + a3(t) x (2.2.4) Substituting the known boundary conditions for each element, and making the small angle assumption, tan 6 = 6, gives v(e)(0,t) ao(t) = ui(t) v(e)(l,t) ao(t) + a1(t) + a2(t) + a3(t) = uj(t) v(e)'(0,t) a1(t) = -Bi(t) v(e)'(1,t) a,(t) + 2a.(t) + 3a.(t) = ej(t) where the superscript (e) denotes a typical element and ( )' denotes differentiation of ( ) with respect to the spatial coordinate x. Solving for the unknown coefficients ai(t), collecting terms, and rearranging gives v‘e)(x,t) = Nfe)(x)ui(t) + N§°)(x)ei(t) + N§e)(x)uj(t) + N£°)(x)ej(t) or in vector notation v(e) (x,t) = 55(6) (x) 37(6) (t) where N1(x) = l - 3x2 + 2x3 w(8)(t) ="ui(t)' N2(x) = -x(1 - 2x + x2) 61(t) N3(x) a 3x2 - 2x3 uj(t) Nu(x) = x(x - x2) he). (t)J 11 The interpolating polynomials chosen are by definition Hermitian polynomials since both the function and its first derivative are specified at nodes i and j. The vector N(e)(x) is composed of the shape functions for the element. As an approximation to equation (2.2.1), the kinetic and potential energy of the beam and the Rayleigh dissipa- tion function will be calculated from the finite element model. These quantities will then be introduced into Lagrange's equations of motion and the approximate equations of motion will be obtained. The kinetic energy of a typical element, dropping the superscript (e) for convenience, is T =-% J m GT 0 d vol vol 1 I O %é-J WT NT NI; dx I NIH t ,...., B hu-l H (D V S 1 where [m](e) = mA I NT (x) N (x) dx is the element mass matrix 0 156 -22 54 13 (e) =.Efl_ - [m] 420 22 4 -13 -3 54 -13 156 22 13 -3 22 4 The Rayleigh dissipation function for a typical element is 1 . O F = %] c(x) IT fiToc) N(x) i3 dx 0 C 1 . =%cflj flu)Mndx$ 0 12 where [c](e) is the element damping matrix [c](e) c [1 Firm N(x) dx 0 156 -22 S4 13 ~22 4 -13 -3 S4 -13 156 22 13 -3 22 4 .2. 420 The potential energy of a typical element is EI 1 32 “TL (3??de 2 2 +, + 2 + But %;;-= 3x [NT(x) w(t)] = BT(x) w(t) where B(x) =-%;y (N(x)) Substituting gives 1 V = El-WT J BT‘B dx'w 2 o .1. 2T [1.] (e) z 1 where [k](e) = EI I BT‘B dx is the element stiffness matrix 0 [k](e)=EI -3 2 -6 3 -3 1 3 2 O)!» oar-I Using the usual direct stiffness method of assembly [7] one obtains the mass matrix ”0’ the viscous damping matrix Co’ and the stiffness matrix K0, for the entire beam. Hence, the kinetic and potential energy and the dissipation function for the entire beam is l +T 3 1 +T + 1 4T 4 T =-§ Mo w, V =-§ K0 w, F - E-w Co w +T where w = [111 61 u2 ... 116 66] Substituting into Lagrange's equations of motion. %— (ET— -§T +£+§—Y_‘=O, i=1, 2, .00, 12 t at} ”"i at} 3“'1 ‘ i i gives M0 ; 4' CO I: 4' K0 I: = 0 This is a system of 12 equations in 12 unknowns. A usual occur- rence in constructing a finite element model is that the system does not satisfy the vertical translation boundary conditions. It also con- tains spurious equations which arise by differentiating with respect to known constant quantities. The overall analysis is not affected by eliminating the spurious equations and satisfying the boundary condi- tions. In particular, notice u1 a 1111 = 0. Substituting this fact into the system will eliminate the first and eleventh columns of Mo, Co, and K0. Also notice the first and eleventh equations were obtained by differentiating with respect to the known quantities u and “11’ and 1 hence must be eliminated from the final system. The final equations of motion fer the simply supported, steel beam, with the boundary conditions satisfied, are a + M; + Cw + K: = O For the final mass, stiffness, and damping matrices see Appendix A. 14 2.3 A Simple Method fer Calculating Ccr Consider again the matrix differential equation of motion for a continuous system obtained from a finite element model of the system MS? + c3? + )6? = 0 (2.3.1) 1 Proceeding in the same manner as Inman and Andry in [5],let i M 2 y _1 and premultiply (2.3.1) by M 2 to give - -190 -1 -0 - -1 Mimzy+MiCMiy+Mimiy=0 (2.3.2) Let S be the orthogonal, undamped modal matrix (STS = I), let y = SE, and premultiply (2.3.2) by ST to obtain - _1 " _ _1 ’ _1 _1 STim2$2+STMiCMZSE+STM1Wis-2:0 (2.3.3) 1 Notice ST i MM 5 S = I and ST -% KM.2 S = A, a diagonal matrix with the eigenvalues of the undamped system along the diagonal. Let o = M'i S. T T }T ‘ . Then ¢ 8 S M But(M 3F 2 M 5, since M is symmetric. Substituting into (2.3.3) gives T oM¢E+¢T T (:3? + a K43 = 0 (2.3.4) Notice m is orthogonal with weightll(¢TM¢ = I) and ¢TK¢ = A. Hence, from vibration theory [8], ¢ is the undamped modal matrix normalized with respect to the mass matrix M. For critical damping, M.i CM'% = Ccr in (2.3.3). But, from matrix theory [9], ST Ccr S = 2A3. Thus (2.3.3) becomes 3; g; z 4- 2A 2 + A? = 0 (2.3.5) Comparing the coefficient of 2 in (2.3.4) and (2.3.5) one concludes 15 that for critical damping T 1 4 C o = 2A cr 01‘ 3 T -1 i -1 Ccr 2(¢ ) A O (2.3.6) -1 T T -1 . . . Note, however, that o = t M, and (4 ) = M0. Subst1tut1ng into (2.3.6) gives c = zuonitTM (2.3.7) CT Thus, the critical damping matrix in the original coordinates is obtained by simply perfbrming the indicated matrix multiplications. 2.4 A Note On The Comparison Test Given an arbitrary damping matrix C, with respect to the original coordinates, the test for the type of damping provided by C can be carried out in the original coordinates. By definition, C - Ccr is positive definite if and only if *T 8 - c ) 3 > o 2 4 1 x ( - cr x ( . . ) + . for x an arb1trary nonzero vector. Denoting the critical damping matrix A A - 1 in ori inal ' = -$ -1 ' ' g coord1nates as ccr’ we have Ccr M Ccr M . Subst1tut1ng .'.-~ I '. ~ -i 91 . into (2.4.1) w1th C a M CM; gives SETau‘i CM'1 - M‘i écr n‘i)§ > o 01' ET ‘3(c - ccr) u'i I > 0 (2.4.2) Letting ; = n‘5 I in (2.4.2) gives 16 'y’T(c - cm)? > 0 (2.4.3) . + . . . + . But, Since x is an arbitrary nonzero vector, so 15 y. Hence 8 = Ccr 15 positive definite if and only if C - Ccr is positive definite. A similar argument also works for the negative definite and indefinite cases. 2.5 Results The critical damping matrix for the simply supported steel beam under consideration is calculated using (2.3.7). Both the eigenvalues and the eigenvectors, normalized with respect to the mass matrix, of the undamped system are needed to calculate the matrices in (2.3.7). They are obtained by using MSC/NASTRAN finite element code. The NASTRAN model (see Appendix A) of the beam consists of S CBAR elements. As a model check, the consistent mass and stiffness matrices generated by NASTRAN are printed out. These matrices are exactly the same as the mass and stiffness matrices calculated by hand. As a further model check, the eigenvalues generated by NASTRAN, using the Given's method, are compared to the eigenvalues obtained by solving the separated partial differential equation of motion of the beam, see Table 1. Good agreement in the first fbur modes indicates that the NASTRAN model is working properly. Table 1 also shows that better numerical accuracy in the higher modes can be obtained by modeling the beam with a larger number of elements. An examination of the eigenvectors generated by NASTRAN shows the expected sinusoidal behavior. As a final check, all calculations were perfbrmed using IBM double precision scientific subroutines, with nearly identical “501115- 1 The matrix A is formed by replacing the diagonal of the 10 x 10 identity matrix with the square root of the eigenvalues obtained from seem.Hq(x) (3.3.2) 1 and the linear differential operator L2 to be L2H] = % LN] (3.3.3) at where L a El BEF' as in equation (3.2.1). Also define the operator L3 to be L3[¢] = (4L2 - L§)[¢] (3.3.4) Inman and Andry have shown in [12] that the type of damping provided by the damping mechanism described by (3.3.1) depends on the definiteness of the operator L3. Recall, an operator L is said to be positive definite if and only if, fer any comparison function u, the 27 following holds, I I uL[u]dD Z 0 (3.3.5) D and equality holds only for u a 0. A function is a comparison function if it is sufficiently differentiable over the domain D and it satisfies all the boundary conditions of the eigenvalue problem. One set of comparison functions for the simply supported beam being considered is the eigen- functions of the undamped system, {sin 1F}::1 . Utilizing the above definitons, the definiteness of the operator L3 is seen to depend on the sign of the quantity dn’ defined by L L d = 4 J vn(x) L2[vn(x)]dx - J I‘ll m n o I 2 x5. (xlej (20¢i (x)cj (x)v;(x) 0 i=1 j=1 1 L m L = 4 J vn(X) Lzlvn(X) dX - 11 J C; (X) xE (X) v; (X) dX (3.3-6) 0 is o i where V“(X) is the nth eigenfunction of the undamped system. It can be shown that the nth mode is underdamped if dn > 0,. critically damped if dn = 0 and overdamped if dn < 0. Notice the similarity of these results to the results obtained fer the discrete, multidegree of freedom systems discussed in section 2.1. These results can be used to determine either the placement of damping or the magnitude of damping necessary for the system to be critically damped when one or the other of the parameters is in some way con- 'strained. 28 Consider the special case of damping of constant magnitude ci placed on the beam such that the intervals over which the damping is distributed are centered about the maximum and minimum values of the eigenfunctions. For this case, the open intervals B1 are of the form E.- Z—iL—l-5L,Zi‘l+s L i=l,...,n 1 2n 2n with 2nd 5 1. Evaluating the first integral in (3.3.6) with vn (x) a sin E%§-and L2 as defined in (3. 3. 3) gives L 4 4f sin “—L" (131) 3., (sin ms)ax= ZE—I—L(— n) . (3.3.7) 0 3x L Evaluating the second integral gives 21-1 m L c? (x) m C? ( 2n + ”L zMI-g—z—x. (x) de' —. . .... p 21 1 n: i=1 ( 2 - 6)L n m C2 o 0 i 1 . 21 - 1 . 21-1 121 L 37-[6 —'za;|(51n 2nu( 2n + 6) - s1n Znn ( 2n - 6)] m L C? a if -2——l,mp [2nn8 + sin 2nn8 ] . (3.3.8) For the nth mode of the beam to be critically damped or over- damped dn 5 0. Substituting (3.3.7,8) into (3.3.6) gives 2E1 V(fln)u "TL“’ g c2 [Znnd + sin Znnd] S 0 01' I. Ill 32. nn 2 If the magnitude of damping is the same over each interval 51’ then for critical damping‘we have 29 - 111 2 ‘ 081 (1r) (ccr)n - 2(L ) IanS 4- sin Znné (3'3°10) Notice, if 2nd a 1, that is, the damping is of constant magnitude, continuously distributed along the entire beam, there results "n 2 (ccr)n = 2 (E-) “051 (3.3.11) This is the same value as was obtained by considering the partial differential equation for this type of damping. 3.4 Examples Two sets of example problems, consisting of four related examples each, are considered. In the first set, the amount of damping necessary to critically damp the first mode of vibration is calculated. In each example, the damping is distributed over a single interval whose center coincides with the geometric center of the beam. The width of this interval is varied from .04m to 5m; a width of .04m being a physical model of damping applied at a single point, and a width of Sm repre- senting damping applied over the entire beam.) The intermediate cases considered include damping distributed over the entire center element and center three elements. See Table 2 for the amounts of damping necessary to critically damp the first mode using these configurations of dampers. In the second set of example problems, the amount of damping necessary to critically damp the fifth mode of vibration is calculated. In each example, the damping is distributed over five intervals, each of whose centers coincides with the geometric center of an element. The widths of each of these intervals is varied from .04m to 1m; the first case representing damping applied at five discrete points, and 30 the last case representing damping applied over the entire beam. The intermediate cases considered include damping applied over widths of .2m and .6m. See Table 2 for the amounts of damping necessary to critically damp the fifth mode using these configurations of dampers. Table 2 Summary of Data fbr Examples 1-8 Example Damping NT/m Complex Eigenvalues Frequency Number Magnitude 57333. (real) (imag) (Hertz) 1 127853.760 -6.30172 3114.9086 18.28827 2 58084.188 -72.7073S t 90.34698 14.37917 3 38035.899 -109.5956 t 35.55132 5.658167 4 36138.722 -115.0625 1 1.682878 .2678384 5 3196344.000 6 l452119.700 Examples 5 through 8 7 950897.460 see Appendix 8 903468.040 for this information The original finite element damping model (see Chapter 2) can be modified to represent the various cases of discrete damping described above by recognizing a characteristic each of these cases has in com- mon. In each case, the damping is applied over one or more regions, each of whose center coincides with the center of an element. Hence an element damping matrix fer this general type of damping is calculated, and the final finite element damping matrix is obtained using the same procedures employed in the original model. The general element damping matrix for discrete damping is obtained in a manner analogous to the original model. Assume the damping is dis- tributed over the intervals Bi 8 (ai, bi)’ where 0 5 a1 < b1 5 1, and i - number of elements which experience damping. Then the Rayleigh 31 dissipation function for a typical element experiencing damping is 1 O O F=%J c(x)xE (x)-wTNTN-\zdx o i .. b- ' a é- wT[c J 1 NT(x) N(x) dx '15 £11 = 2%- :T [c](e) .1: where [c](e) is the element damping matrix b. + [c] (e) = c I 1 w(x) N(x) dx a. 1 156011 ai) 22(bi ai) 54(bi ai) 13(bi ai) = 4:0 4(bi ai) 13(bi ai) 3(bi ai) - 2- 2 156(bi ,ai) 22 (b1 ai) s _ SYM «bi-aim The Rayleigh dissipation function for elements which do not experience damping is identically zero, and hence the element damping matrix for this type of damping is the 4 x 4 zero matrix. The final finite element damping matrix for each case is obtained by proceeding as in the original model, using the appropriate elements damping matrices. See Appendix C for the damping matrices fer each example described above. The discretized matrix differential equations of motion (2.1.1) is solved directly on the computer using NASTRAN with the various damping matrices in place. The eigenvalues and frequency of the first mode of the first four examples are listed in Table 2. Notice, as the width of the region over which the damping is distributed is increased, 32 the magnitude of the real part of the eigenvalue also increases. This indicates that the motion of the first mode decays faster as the width of the region over which the damping is applied is increased. Also notice, as the width of the region over which the damping is applied is increased, the imaginary part of the eigenvalues and the frequency decreases. Critical damping is attained only in the last example, while the first three examples exhibit underdamped motion. This could be due to the finite element model itself, which consistently over- estimates these quantities. Indeed, the imaginary part of the eigen- value for the last example can be shown to be zero, by examining the decoupled ordinary differential equations which govern the motion of this example. The eigenvalues and frequencies of all ten modes of the last four examples can be found in Appendix D. The first four modes should be critically damped; Again, as the width of the regions over which the damping is applied is increased, the theoretically predicted behavior is more closely achieved. Indeed, the fifth example shows only the first mode overdamped, the sixth example shows only the first two modes overdamped, the seventh shows the first three modes overdamped, and the last example shows the first four modes overdamped, with the remainder underdamped. Again, this behavior could be due to the finite element model. This model results in accurate eigenvalues for only the first feur modes of the last example. This can be seen by examining Table 3 where the theoretically predicted eigenvalues are compared to the eigenvalues obtained from the finite element model. Mode No. 10 Table 3 Comparison of NASTRAN Model with Analytical Model THEORETICAL .Eigenvalues (Real) (Imago) -2.3021644 -5750.9155 ~37.058599 -5716.1591 -l92.86618 -SS60.351S _ -666.28015 -5086.9375 -2864.0435 -2889.1742 -2876.6088 15961.0677 —2876.6088 29698.1044 -2876.6088 213557.925 -2876.6088 t17730.18l -2876.6088 122281.889 33 NASTRAN Eigenvalues (Real) (Ima8-) -2.302657 -S750.801 -37.18228 -S715.922 -196.0541 -5557.051 -702.3069 ~5050.801 -2876.552 -1385.286 -2876.554 +138S.286 -2876.SS4 t3600.778 -2876.554 :6062.379 -2876.554 19085.953 -2876.556 112500.37 -2876.558 114345.40 IV. CONCLUSION The damped transverse free vibrations of a simply supported beam have been considered. The question of critical damping for this beam has been addressed in two different ways. Both a continuous model and a discretized approximation to this model have provided infermation. The continuous model of the beam was discretized using a five element, consistent finite element model. A critical damping matrix for this model was calculated, and subsequently shown to critically damp the first ten modes of vibration simultaneously. Various other damping matrices were also shown to provide the predicted type of damping. It should be emphasized that the critical damping matrix which has been calculated is very heavily model dependent. A different finite element model will result in a different, although possibly similar (in structure), critical damping matrix. Alternatives to the finite element model presented in this paper include a model consisting of a larger number of elements and a lumped parameter finite element model. Note that the latter of these alternatives would not be appropriate for this analysis. The mass matrix in a lumped parameter model is singular, not positive definite. Thus the theory on which these calculations are based would not apply. It has already been demonstrated that a model consisting of a larger number of elements provides more accurate results. A critical damping matrix developed for such a model would critically damp a larger number of modes. Naturally, the physical effort involved in 34 35 computing such a critical damping matrix would also increase. However, the theory upon which these calculations are based does not increase in complexity as the order of the system is increased. Thus, the number of modes which are critically damped is limited only by the amount of physical effort one is willing to expend in the computa- tion of the critical damping matrix. The critical damping matrix which has been developed was used primarily for comparison. A given damping matrix was compared to the critical damping matrix to determine the type of damping it would pro- vide. Although this is a very immediate and important use for this matrix, a more satisfying use would be the direct application of this matrix to the system. This, however, requires that the critical damping matrix be interpreted physically, i.e., what magnitude of damping, placed where on the beam would result in this damping matrix. This question is difficult to answer because the critical damping matrix is fully populated. A typical finite element damping matrix possesses a very orderly "block diagonal" structure. A closer look at the critical damping matrix reveals an approximate block diagonal structure. One possible cause of the nonzero "off-block" terms could be that the colunns of the orthogonal modal matrix used to calculate the critical damping matrix are not quite orthogonal. This is a matter which requires further investigation. The question of critical damping fer the beam was addressed in a second way, utilizing a continuous model of the beam. An expression for critical damping of a given mode was developed for a particular configuration of discrete regions on the been over which the damping was applied. The model was discretized using the same finite element 36 model as in the first approach, with a damping matrix modified to describe discrete damping. Several examples were considered. The results from these examples were mixed. The model worked well for the cases in which the damping was distributed over the entire beam, or a large fraction of the entire beam. However, the model worked less well for the cases in which the damping was more truly discrete. This is not surprising, since the original, consistent finite element model was designed to model continously distributed parameter system. Perhaps a different finite element model, such as a lumped model, would give improved results in the discrete cases. The above calculations were carried out for a particular configur- ation of discrete damping. This configuration was chosen for a mmber of reasons. It was symmetric about the center of the beam, it simplified the analysis, and it simplified the finite element model. .However, at this time, it is impossible to say if this particular configuration was physically the best choice for this problem. Determining the best possible configuration of discrete damping fer this problem is another matter which requires further study. 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II I.o....ooaoI.pI mo.uI.a.a...~I -. o .on..co...o ~o.w.~.¢.a. o no.6o..~aa.oI no.u.ano~.. u. a. a oo.uo.~.oa.. ~o.wa.o=...a .e.uo-aoa.nI no.u.noo...~I ~. ~ ca.umomnn..n. ~onu....c~.~ -IIIIIImmumw.~.on.. ..I no.u~.amm..~I .. . .au.u....au ....upu. .c.:.. ...... .cua-a I .mm ox..x.a .uxwaaua. w:.¢»:ug.a 2a..u....u .ao. » z ¢ 2 a a a u a . m > r a o . u u u . . c o u m mar—2433. was. ......o‘san 5.2.... .3an Im~ . sacs . I.-oiwm zen—«a: .99. .- ..as a .: ».a..azo 94..¢wm.w LIST OF REFERENCES 10. 11. 12. LIST OF REFERENCES Duffin, R. J., "A Minimax Theory for Overdamped Networks," Journal of Rational Mechanics and Analysis, Vol. 4, 1955, pp 221-233. Nicholson, D. N., "Eigenvalue Bounds for Damped Linear Systems," Mechanics Research Communications, Vol. 5, No. 3, 1978, pp. 147-152. Muller, P. C., "Oscillatory Damped Linear Systems," Mechanics Research Communications, Vol. 6, No. 2, 1979, pp. 81-85. Beskos, D. E. and Boley, B. A., "Critical Damping in Linear Discrete Dynamic Systems," Technical Report, Grant / Contract #N00014- 75-C-1042. Inman, D. J., and Andry, A. N., "Some Results on the Nature of Eigenvalues of Discrete Damped Linear Systems," Journal of Applied Mechanics, Dec. 1980. Cook, R. 0., "Concepts and Applications of Finite Element Analysis." John Wiley and Sons, Inc., New York, 1974, pp 237-239. Segerlind, L. J., "Applied Finite Element Analysis," John Wiley and Sons, Inc., New York, 1976, pp 105-109. Meirovitch, L., "Analytic Methods in Vibrations," the Macmillan Co., N.Y., 1967, pg. 80 Bellman, R., "Introduction to Matrix Analysis," 2nd Ed., McGraw- Hill, New York, 1970, p. 95. Strang, 6., "Linear Algebra and Its Applications," Academic Press, New York, 1976, p.241. Meirovitch, L., "Analytic Methods in Vibrations,” The Macmillan Co., N.Y., 1967, Chs. S., 9. - Inman, D. J., and Andry, A. N., "The Nature of Temporal Solutionsth of Damped Distributed Systems with Classical Normal Modes," 18 Annual Meeting of the Society of Engineering Science, Providence,I Rhode Island, Sept. 1981. 71 7520 s E RI M" B" U! "II-I! final “”6 V"o Mums U"o ! 1293 !|!!|!|!!!|!ll!||!!!|!!!