. .‘ \K ability»... . .9; . w... 3922.45 p.83... fluvial 3...“. .w :3 Ir": 4.". : .3 c. 1... u... “Emfl 1.1.... 5.. “funny... 3. I. liar: Jr.» awn». . 5,?! >- n... a v5 . . , 3.. 5?. . . a... .2 1296! 3 flaunt. . . é... 4.3.3.... » '13». .30”. a‘ .3 I n: n l 1“ ’v §Cx.§.x! 1 2L 4!: 5 .3“. .m. I. it. . t ,c. a» . v . h f .1. . 9 15:13.75: 4.1.2... .454}: 1.» . .2 SILL 5}. .15.. ~ fgzibll 2:515?! 3.5.7. .I. .6 r. .333, , . .cl-(il. . .. L: :35... i: . p.35 1... 1...), 1 . In 1,? (3a: lLlLlllllllll lullll‘llllllllllllllllllllll 3 1293 010 This is to certify that the dissertation entitled NONLINEAR RANDOM VIBRATION OF COMPOSITE LAMINATED PLATES presented by Mohamad Khaled Naja has been accepted towards fulfillment of the requirements for Doctor of Philosophy degreein Civil Engineering flfléwéeafl. Ronald Harichandran Major professor Date 11/18/93 MSU i: an Affirmative Action/Equal Opportunity Institution 0-12771 . LIBRARY M'Chigan State University PLACE IN RETURN 80X to MOI. thin checkoutfiom your record. To AVOID FINES return on or before date duo. 7 DATE DUE DATE DUE DATE DUE NONLINEAR RANDOM VIBRATION OF COMPOSITE LAMINATED PLATES By Mohamad Khaled Naja A- DISSERTATION Submitted to Michigan State University in partial fulfillment of the requirements for the degree of DOCTOR OF PHILOSOPHY Department of Civil and Environmental Engineering 1993 ABSTRACT NONLINEAR RANDOM VIBRATION OF COMPOSITE LAMINATED PLATES By Mohamad Khaled Naja Structural components in space vehicles, aircraft, automobiles, submarines, etc., that are made of filamentary composite laminae are usually subjected to stochastic loads. Com- posite laminae have strongly anisotropic properties and display significantly nonlinear elastic behavior when loaded in shear or along directions different from those of the fila- ments. While such components have been used for over a decade, their design has predom- inantly been performed using deterministic methods. In this study, the method of equivalent linearization is used in conjunction with the finite element method to perform nonlinear random vibration analysis of laminated composite plates. An approximate, but sufficiently accurate, sen'es representation of the nonlinear shear stress-strain law is used to facilitate the formulation. Classical laminate theory that accounts for the coupling be- tween extensional and bending responses is used, but higher-order shear deformation ef- fects are not considered in the course of this study. Four-noded elements with five degrees- of-freedom per node are used to discretize the plate. The displacement, strain and stress re- sponses are computed at different excitation load levels. The results indicates that the effect of nonlinearity on the responses for any given load level depends on the ply-arrangement, and as expected becomes more significant for higher loads. ACKNOWLEDGMENTS The author wishes to express his deep indebtedness to Dr. Ronald S. Harichandran for his guidance, encouragement, support, and valuable input, without which this work would not have been possible. Grateful acknowledgments are due to Professors R Wen of the Department of Civil and Environmental Engineering, N. J. Altiero of the Department of Materials Science and Mechanics and G. Ludden of the Mathematics Department, for serving on the author’s doctoral committee. Gratitude is also due to the Mathematics Department for their contin- uous financial supports during the period of this study. Thanks are also due to the director of the Minority Business Program Dr. E. Betts and his assistant Ms. L. Gross for their support and encouragement. In addition, the author would like to express his sincere appreciation and thanks to his parents in Lebanon, especially his mother Myassar for her prayers and encouragement, and the sacrifices that she has suffered, during the long course of this study. This work was funded by the State of Michigan Research Excellence Fund adminis- tered by the Composite Materials and Structures Center at Michigan State University. Finally, the author would like to thanks his students at Michigan State University and at Davenport College for their patience and understanding. iii TABLES OF CONTENTS 1. Introduction l 1.1 General ........................................................................................................................ 1 1.2 Objectives ................................................................................................................... 2 1.3 Literature Review ....................................................................................................... 2 1.3.1 Physical Nonlinearity ................................................................................ 2 1.3.2 Shear Deformations .................................................................................. 9 1.3.3 Stochastic Dynamic Analysis ................................................................. 10 2. Finite Element Formulationl9 2.1 General ...................................................................................................................... 19 2.2 Constitutive Equations .............................................................................................. 20 2.3 Finite Element Formulation using Classical Plate Theory ....................................... 25 2.3.1 In-plane formulation ............................................................................... 25 2.3.2 Out-of-plane formulation ........................................................................ 28 2.3.3 Formulation of the linear elemental stiffness matrix ............................. 31 2.3.4 Formulation of the elemental consistent mass matrix ............................ 34 2.3.5 Numerical integration ............................................................................ 35 3. Equivalent Linearization 36 3.1 Introduction ............................................................................................................... 36 3.2 Formulation ............................................................................................................... 37 3.3 Derivation of the Nonlinear Elemental Stiffness Matrices ....................................... 40 3.4 Random Vibration Analysis ...................................................................................... 48 3.5 Computation of Strains and Stresses ........................................................................ 49 3.6 Laminate Strength Analysis ...................................................................................... 51 4. Optimizing Computational Effort 53 4.1 Introduction ............................................................................................................... 53 4.2 FORTRAN Implementation ..................................................................................... 53 4.3 Optimization Techniques .......................................................................................... 56 4.3.1 Optimizing a and B ................................................................................. 57 4.3.2 Covariance optimization ......................................................................... 59 4.3.3 The indexing scheme .............................................................................. 61 4.3.4 Elimination of zero-valued computations ............................................... 66 4.3.5 Unrolling of DO-loops ............................................................................ 69 5. Numerical Results 72 5.1 General ...................................................................................................................... 72 5.2 In-Plane Loading ....................................................................................................... 72 5.2.1 Extensional loading ................................................................................. 72 iv 5.2.2 Shear loading .......................................................................................... 86 5.3 Out-Of-Plane Loading .............................................................................................. 95 6. Conclusions and Recommendations ...... - __ _ - ..... 109 6.1 Conclusions ............................................................................................................. 109 6.2 Recommendations for future work ......................................................................... 110 LIST OF TABLES TABLE 4.1. Number of terms in each group ............................................................................. 64 TABLE 4.2. Comparison of total times. .................................................................................... 71 TABLE 5.1. First five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level so = 12000 lb2 sec ...................................... 74 TABLE 5.2. Number of iterations required for convergence ..................................................... 74 TABLE 5.3. First five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level so = 120001b2sec ..................................... 78 TABLE 5.4. Number of iterations required for convergence ..................................................... 79 TABLE 5.5. First five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level so = 12000 in2 sec. ..................................... 83 TABLE 5.6. Number of iterations required for convergence ..................................................... 83 TABLE 5.7. First five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level So = 4000 lb2 sec. ....................................... 87 TABLE 5.8. Number of iterations required for convergence ..................................................... 87 TABLE 5.9. First five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level So = 300001b2 sec ...................................... 91 TABLE 5.10. Number of iterations required for convergence ..................................................... 92 TABLE 5.11. First five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level so = 801D2 sec ............................................ 96 TABLE 5.12. Number of iterations required for convergence ..................................................... 96 TABLE 5.13. First five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level So = 801b2 sec ......................................... 100 TABLE 5.14. Number of iterations required for convergence ................................................... 100 TABLE 5.15. First five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level So = 801D2 sec .......................................... 104 TABLE 5.16. Number of iterations required for convergence ................................................... 104 TABLE 5.17. The significance of nonlinearity on the responses of various loading conditions ............................................................................................... 108 vi LIST OF FIGURES Figure 1.1. Typical stress-strain curve behavior of fiber-reinforced composite material ............. 3 Figure 2.2. Fit of approximate stress-strain law for shear ........................................................... 23 Figure 2.5. Layer nomenclature for laminate .............................................................................. 32 Figure 4.2. Direct implementation of- ........................................................... 58 Figure 4.3. Number of Permutations (Np) vs. Combinations (Nc). ............................................ 60 Figure 4.4. Derivation of reverse Pascal triangle ........................................................................ 62 Figure 4.5. Combinations for which F is computed .................................................................... 65 Figure 4.6. Pascal triangle and RPT table ................................................................................... 66 Figure 4.7. RPT offset array needed for indexing ....................................................................... 67 Figure 4.8. Segment of code for computing . - .......................................................... 68 Figure 4.9. Code segment illustrating unrolling of Do-loops ..................................................... 69 Figure 4.10. Do-Loop Unrolling vs. Straight Implementation ...................................................... 70 Figure 5.1. One-ply plate loaded in tension ........................... . ..................... 73 Figure 5.2. Variation of absolute r. m. s. shear strain at the center of= element 2 with excitation load level .......................................................................................... 75 Figure 5.3. Variation of normalized r.m.s. displacement at free comer nodes with excitation load ................................................................................................... 76 Figure 5.4. Variation of normalized r.m.s. strains at the center of element 2 with excitation load level .......................................................................................... 76 Figure 5.5. Variation of normalized r.m.s. stresses at the center of element 2 with excitation load level .......................................................................................... 77 Figure 5.6. Two-ply laminated plate loaded in tension..... ..... - - - ....................... 78 Figure 5.7. Variation of absolute r. m. s. shear strain at the center of element 2 with excitation load level......................... - - - - - ........................ 80 Figure 5.8. Variation of normalized r. m. s. displacement at free comer nodes with excitation load level .......................................................................................... 80 Figure 5.9. Variation of normalized r.m.s. strains at the center of element 2 with excitation load level .......................................................................................... 81 Figure 5.10. Variation of normalized r. m. s. stresses at the center of element 2 with excitation load level.......................- - - -- - 81 Figure 5.11. Three-ply laminated plate loaded rn tension ............................................................. 82 Figure 5.12. Variation of normalized r.m.s. stresses at the center of element 2 with excitation load level .......................................................................................... 84 Figure 5.13. Variation of normalized r.m.s. strains at the center of element 2 with excitation load level .......................................................................................... 84 Figure 5.14. Twisting due to extensional loading for [30°], [30°l-30°] and [30°/0°/-30°] at So = 8000 ......................................................................................... 85 Figure 5.15. One-ply plate loaded in shear ................................................................................... 86 vii Figure 5.16. Variation of absolute r. m. s shear strain at the center of element 2 with excitation load level................... ...... - 88 Figure 5.17. Variation of normalized r. m. s. displacement at free comer nodes with excitation level ................................................................................................. 89 Figure 5.18. Variation of normalized r.m.s. strains at the center of element 2 with excitation load level ...... , .................................................................................... 89 Figure 5.19. Variation of normalized r.m.s. stresses at the center of element 2 with excitation load level .......................................................................................... 90 Figure 5.20. Tree-ply laminated plate loaded in shear .................................................................. 91 Figure 5.21. Variation of absolute r.m.s. shear strain at the center of element 2 with excitation load level .......................................................................................... 92 Figure 5.22. Variation of normalized r.m.s. displacement at free comer nodes with excitation load level ......................................................................................... 93 Figure 5.23. Variation of normalized r.m.s. strains at the center of element 2 with excitation load level .......................................................................................... 94 Figure 5.24. Variation of normalized r.m.s. stresses at the center of element 2 with excitation load level ......................................................................................... 94 Figure 5.25. One-ply plate with fiber orientation or ...................................................................... 95 Figure 5.26. Variation of absolute r.m.s. shear strain at the center of element 2 with excitation load level .......................................................................................... 97 Figure 5.27. Variation of normalized r.m.s. displacement at free comer node with excitation load level ......................................................................................... 97 Figure 5.28. Variation of normalized r.m.s. strains at the center of element 2 with excitation load level .......................................................................................... 98 Figure 5.29. Variation of normalized r.m.s. stresses at the center of element 2 with excitation load level .......................................................................................... 98 Figure 5.30. Two-ply plate with fiber orientation or ..................................................................... 99 Figure 5.31. Variation of absolute r.m.s. shear strain at the center of element 2 with excitation load level ........................................................................................ 101 Figure 5.32. Variation of normalized r.m.s. displacement at free comer node with excitation load level ........................................................................................ 101 Figure 5.33. Varariation of normalized r.m.s. strains at the center of element 2 with excitation load level ........................................................................................ 102 Figure 5.34. Varariation of normalized r.m.s. stresses at the center of element 2 with excitation load level ........................................................................................ 102 Figure 5.35. One-ply plate with fiber orientation (1 .................................................................... 103 Figure 5.36. Variation of absolute r.m.s. strain at the center of element 2 with excitation load level ........................................................................................ 105 Figure 5.37. Variation of normalized r.m.s. displacement at free comer node with excitation load level ........................................................................................ 106 Figure 5.38. Variation of normalized r.m.s. strains at the center of element 2 with excitation load level ........................................................................................ 106 viii Figure 5.39. Variation of normalized r.m.s. stresses at the center 0f element 2 with excitation load level .................................................................................... 107 1 . Introduction 1.1 General During the last two decades, research and development of laminated composite struc- tures has grown at an extremely rapid pace. It is becoming apparent and that more and more composite materials will be used in the design of structures, especially for applications in which the strength to weight ratio is of primary importance. Due to their high strength to weight and stiffness to weight ratios, advanced composite laminates made a direct impact in the aerospace industry over the last two decades, and are now slowly being introduced in automobile, ship building and other industries. For the most part deterministic dynamic analysis has been used in the analysis and design of composite components. However, re- cent advances in random vibration analysis allows more realistic techniques to be used since most composite components are exposed to stochastic dynamic loads. Space vehicle components made for NASA and the Air Force are now required to be designed for random loads. Major finite element program developers have begun to respond to these needs and have recently incorporated linear random vibration capabilities in their codes. However, very little work has been done on the random vibration analysis of elements made of com- posites which exhibit strongly anisotropic and moderately nonlinear behavior. There is an important need to address this deficiency, to develop suitable techniques for the random vi- bration analysis of composite components and to incorporate these techniques into general purpose finite element codes so that they may be widely used in analysis and design. One of the most important differences that filamentary composite laminates have over the traditional materials (such as aluminum and steel) used in aircraft, ships, etc., is their anisotropic behavior. Another important feature is that the stress-strain relations ex- hibit significant nonlinearities even for modest loads, when the loading is not parallel to the filaments or when the loading involves shear (Hahn and Tsai 1973, Hahn 1973). For the accurate analysis of such structures which exhibit nonlinear behavior arising due to geom- etry and/or material properties. It is necessary to include the nonlinearity in the analysis. In this study a procedure for the computation of the nonlinear random vibration re- sponse of laminated anisotropic plates modeled using finite elements is developed using the method of statistical linearization. Geometrical nonlinearity occurs due to structural con- figuration or large displacement. Only physical nonlinearity is included in this present study. This chapter describes the objectives of the present study, and presents a brief litera- ture review of related studies. 1.2 Objectives The main objective of the present work is to develop a procedure for the nonlinear elastic random vibration analysis of laminated composite plates. The exact solution of the nonlinear stochastic differential equations governing the dynamic response of systems is possible only for very simple systems (see literature review). For more complex systems, approximate methods must be used. Due to practical reasons it is crucial that the method be capable of dealing with structures modeled using finite elements, and the method of equiv- alent linearization is suited for this purpose. The excitation is assumed to be Gaussian. Furthermore, it is hoped that the material presented will stimulate and enhance further research on the random response of other laminated composite systems. The computer code written as part of this work can be easily generalized to analyze laminated shells, and can be modified to include higher order shear deformation. 1.3 Literature Review 1.3.1 Physlcal Nonlinearlty The linear elastic theory of fiber-reinforced composite materials is well developed (see, for example, Jones). However, most composite materials exhibit mildly nonlinear 0'1 0'2 1:12 A E‘ 2 i" I 2 v. r, v <— >51 >82 >7 12 Figure 1.1 Typical stress-strain curve behavior of fiber-reinforced composite material stress-strain behavior in at least one principal material direction. The stress-strain curve in the fiber direction of unidirectionally reinforced lamina is linear even at high stress level. However, the stress-strain response for loading transverse to the fibers is often somewhat nonlinear. Moreover, the shear response is quite nonlinear. The degree of nonlinearity varies from composite to composite and is due mainly to the nonlinear matrix material, which significantly affects the transverse modulus 15‘2 and the shear modulus 012 of the composite. The effect of the nonlinear matrix material on the longitudinal modulus E1 and Poisson’s ratio v12 is shown with micromechanics analysis to be negligible for normal combination of fibers and matrix materials. Specific examples of fiber-reinforced composite materials with nonlinear stress-strain behavior include boron/ epoxy with slight E2 nonlinearity but a strong G12 nonlinearity. On the other hand, metal- matrix composites such as boron/aluminum have strong E2 and G12 nonlinearities.Three- dimensionally fiber-reinforced composites such as carbon/carbon have nonlinearities in all principal material directions. The nonlinearities for all these materials are more apparent with increasing temperature and moisture content. Thus, analysis of composites should in- clude the effect of nonlinear stress-strain behavior. Various investigators have attempted to include material nonlinearities in the analysis of composite materials. Petit and Waddoups (1969) employed a piecewise linear method. According to this method, incremental stress-strain relations are first obtained at each state of strain and then the over-all behavior of the laminate is calculated by integrating the incremental stresses. The first increment in the laminate strains is calculated with the assumption that the lami- nate behaves linearly over the applied stress increment, i.e., [A2] = lAl,,lAcrl,,+1 (1.1) n+1 The increment in the laminate strains, As, is added to the previous strains to determine the current total laminate strain [2]“, = [ts],,+[Ae},,+1 (1.2) The individual lamina strains may be computed using rotational transformations. Account- ing for the orientation of the fibers in each lamina, the lamina constitutive equations are ex- pressed as follows: 0'1 Q11 Q12 0 81 (T2 = Q12 Q22 0 82 (1.3) 112 n O O Qoo n 712 n the stiffness matrix for (n+1) th stress increment can then be calculated. Hahn and Tsai (1973) derived a stress-strain relation which is linear in uniaxial load- ing in the longitudinal and transverse directions, but nonlinear in shear. Their theory was based on the strain energy density function which includes a fourth order term: —al: 1 l l l W = 55110? + 5S2”: + Slzcrlo2 + 5566th + 2576on2 (1.4) where W is the complementary energy density function. They have explicitly shown that only one fourth order constant in needed to account for the nonlinear shear behavior of an off-axis composite lamina. In their study, Kirchoff’s hypothesis that each lamina in the laminate is in the same state of membrane strain as that of the laminate was used. The stress-strain relation which takes account of frequently observed nonlinear behavior in in- plane shear is Sr 511 S12 0 Cl 0 {£2} = 512 522 0 {02} +§661f2{ O } (1.5) 712 0 0 S66 112 1:12 Their predictions of strain response under uniaxial off-axis loading agree fairly well with measurements by Cole and Pipes as well as the theory and experiments concerning the off- axis behavior. Hashin, Bagchi, and Rosen (1974) implemented the Ramberg-Osgood stress-strain relations to represent the nonlinear response of the lamina subjected to transverse and shear loads. The behavior in the direction of fibers was assumed to be linear. The Ramberg-Os- good model is quite flexible and able to represent varying degrees of nonlinearities as well as hysteresis. Sandhu (1976) used an approximation of stress- strain behavior under biaxial normal stress states to predict equivalent multiaxial strain increments. The incremental constitutive relationship used is defined under the following assumptions: 1. The increment of strain depends upon the strain state and the increment of stress. 2. The increment of strain is proportional to the increment of stress. using these assumptions, the incremental constitutive law can be written as day. = S (8"!) do" (i,j, r,s = l, 2, 3) (1.6) ijr: where deij, do" are strain and stress increments and S if” is a function of strains 31;- Under generalized plane stress, the above equation reduces to: do. = So}. (so) dd}. (1.7) assuming that the lamina remains orthotropic at all load levels (this assumption was justi- fied experimentally) the above equation can be written as [do]. = [Cloldelo (1.8) This model is similar to the one used by Pent-Waddoups (1969) since both methods use lamination theory to generate the laminate compliance which is used to compute lami- nate strain increments under applied stress increment. These laminate strain increments are added to those obtained previously to detDrmine current strains. In the Pent-Waddoups technique these strains are used to compute the laminate compliance for the next load in- crement. This technique is essentially a predictor type. In Sandhu’s analysis, the strains are used to determine the average laminate compliance for the same load increment, and a new set of laminate strains are obtained. This procedure is repeated until the difference between two consecutive sets of laminate strains is less than a prescribed tolerance. The results of this approach are in good agreement with Cole and Pipes’ data and those of Hahn and Tsai as well. Jones and Nelson (1976) developed an orthotropic material model in which the non- linear mechanical properties are functions of the strain energy density, U 9 0i where MPi is the mechanical property (e. g. modulus of elasticity) for the i th stress- strain curve, and for lamina under plane stress U is given by U = (6181 +02£2+112712)/2 (1.10) where A o, B,- and C,- are the initial slope, initial curvature, and change of curvature of the ith stress- strain curve. The quantity U0 is used to nondimensionalize the strain energy por- tion of the mechanical property equation. The J ones-Nelson model is used in an iterative procedure which converges to the state of stress and strain corresponding to an equivalent linear elastic body. The mechanical properties in this model cannot be defined for strain energies greater than or equal to a specific value of strain energy U (value of strain energy where the ap- proximate mechanical property curve crosses the U axis). The value of U at which the me- chanical property becomes negative is U = 3"”. Thus, U is largest for stress-strain curves with low initial curvature and low rate of change of curvature. Accordingly, the Jones-Nelson model cannot be used as extrapolation for energies as large as U but must be restricted to energies less than or equal to U = %oé (6' is the maximum stress). This range is not sufficient to treat practical problems. Hence, modifications of this model are essen- tial. Jones and Morgan ( 1977) have extended the J ones-Nelson model to include treatment of more pronounced nonlinearities. Their approach is based on the concept that the stress- strain curve be connected to a straight line with equation 6 = me + 0'0 instead of merely specifying the slope m. This approach is useful in fitting the stress-strain curves while si- multaneously considering the statistical nature of failure data which can be described with such a line. A disadvantage of this approach is that the fit of the actual stress-strain data is not as good as with other approaches.Their computed strains agree with strains measured by Cole and Pipes about as well as the theories mentioned previously. Amijirna and Adachi (1978) presented a simplified method of predicting the nonlin- ear stress—strain curves for an unidirectionally orthotropic lamina, and symmetric biaxial laminates. The analytical procedure is based on linear elastic analysis with the application of classical laminated plate theory (L.P.T) to the small stress increments of the stress-strain curve. Because it is assumed that only the nonlinear component of the principal in plane shear response governs the nonlinearity of the unidirectional lamina stress-strain curves, the following relation between A712 and on was used (AUX) [00829 {1 - (611),, ( (511) ,o " (312) ,o) }1 (1.11) + [sin20 (516),, ( (311) n - (512) ")1 = (516),,(13712) The nth stress increment, (on) n, in the loading direction corresponding to the nth incre- ment of the in-plane shear strain component, (A712) n, is calculated from the above equa- tion. The complete nonlinear stress-strain curve was predicted for various laminations. The nonlinear stress-strain curves for various cases could be estimated and the results have con- siderably good agreement with experimental ones. Chou and Takahashi (1987) used the stepwise incremental analysis proposed by Petit and Waddoups (1969) to predict the non-linear stress/strain responses of flexible fiber com- posites. The uniaxial tensile stress/strain relations were obtained for several types of com- posites containing glass or kevlar fibers in an elastometric polymer. Their theoretical procedure considered the fiber geometric nonlinearity as well as the material nonlinearity. Chou and Lou (1988) developed a constitutive model based upon the Eulerian de- scription to account for material nonlinearity for flexible fibers. They used a complemen- tary energy function to derive the following material nonlinear stress-strain relation {3:} = {53110)} (Hz) where 511 = Slr+srll°r+srlll°i S12 = S12 = S21 S22 = 5221'5222021'522220221 816 = 516606’ 816 = 2516606 566 = S66+S6666°% S26 = 562 = 2522666206 the terms S11 , S22, S12 and Sm5 are needed for linear deformations; the terms Sm, S222 are needed for representing the birnodulus behavior in the axial and the transverse direction respectively; the terms S1111 , $2222 and S6666 are the nonlinear terms. Although this model resembles Hahn & Tsai’s model, the former has the capability of treating complicated prob- lems with more than one nonlinearity in the material property. 1.3.2 Shear Deformatlons Classical plate theory can yield significant error for moderately thick composite 1am- inae because transverse shear deformation is neglected. It is well known that transverse shear deformation is significant for thick plates, and this is especially true for composites since the shear moduli of polymer matrices are significantly lower than the extensional moduli. While a first-order shear theory (Reissner 1945, Midlin 1951) is adequate for plates made of conventional materials, a higher-order shear theory is usually required for compos- ite laminates (Reddy 1990, Noor and Burton 1989). Higher-order shear theories are usually adequate only for global modeling (i.e., prediction of displacements, natural frequencies and buckling loads), and are not sufficiently accurate for stress field computations. Local layer-wise models that represent each layer as a homogeneous anisotrOpic continuum are usually required for accurate stress computations, but these often greatly increase the size of the problem. Numerous higher-order shear theories have been proposed (e.g., Nelson 1974, Whit- ney and Sun 1974, Lo 1977, Reddy 1984, and many others). All of them use through-the- thickness displacement assumptions of the form (Reddy 1987) u (x1,x2,x3) u0 (xvxz) U (x1,x2,x3) {v(xl,x2,x3) } = {v0 (x1,x2) } +{ V(x1,x2,x3) } (1.13) w (x1. x2. x3) wo (x1. 21;) W (x1. x22. 1:3) Where “0’ and v0, and w0 are the displacement components of the reference plane x3 = 0, and U, V, and W are functions of x3 which vanish at 13 = 0. The different theo- ries can be identified by the assumed functional dependence of U, V, and W on x3. 1.3.3 Stochastlc Dynamlc Analysts General theory The general problem of random excitation of physical systems was first investigated theoretically by Einstein (1905) and was generalized and extended by Smoluchowski (1916) in context of the theory of Brownian motion. In 1931, Kolmogorov derived a precise mathematical formulation of the equations governing the probability densities satisfied by such processes. Contributions of major importance were also made by Fokker, Planck, Burger, Furth, Omstein, Uhlenbeek, Kramers and others. A number of important papers from this era have been collected in the book by Wax (1954). Stochastically excited linear systems have been studied in great detail and several an- alytical techniques exist for treating both stationary and nonstationary problems. Unfortu- nately, many structures of engineering interest cannot be considered linear and the techniques for analyzing nonlinear systems are not nearly so well developed. Some simple nonlinear systems can be analyzed exactly by means of the Fokker-Planck equation. How- ever, for most nonlinear systems exact solutions are not available. A number of approxi- mate techniques have been applied successfully to simple one DOF systems, but much less has been done in the analysis of dynamical systems with more than one DOF. Several po- tentially useful techniques for the analysis of such systems have been developed, but most of these techniques are either very difficult to apply or their application is restricted to only a small class of problems. The statistical linearization technique shows considerable prom- ise in this regard for it is not limited by the restrictions commonly imposed on the other ap- proaches. In addition, this approach can be made quite direct and relatively easy to apply. 10 The earliest work on the problem of random excitation of nonlinear system was that of Andronov (1933), who used the Kollnogorov-Fokker-Planck equations (Kolmogorov, 1931) to study the motions of general dynamic system subject to random disturbances. Kramers (1940) used this technique to study chemical reaction rates. Caughey and Dienes (1961), Lyon (1961), Klein (1964), and Herbert (1965) have used the Fokker-Planck equa- tion to study the response of nonlinear dynamical systems to white noise excitation. Barrett (1961), Merklinger (1963) and Stratonovich (1963) have applied the technique to solve nonlinear control problems. In almost all these investigations only first order statistical properties were obtained. While first order statistics are important parameters in the description of random processes, there are numerous applications where additional statistical information is required. For ex- ample, the spectral density of a random process requires a knowledge of the second order statistics of the process. A number of approximate techniques have been developed to ob- tain second order statistics for the response of nonlinear systems to random excitation. Booton (1954) and Caughey (1959) independently developed the method of equivalent lin- earization, which is simply the statistical extension of the well known equivalent lineariza- tion technique of Krylov and Bogolinbov (1937). Crandall (1961) developed a perturbational method based on classical perturbation theory. Payne (1967), Wong (1964), and Atkinson (1970) have developed approximate techniques based on eigenfunction ex- pansions and variational techniques. Many problems in mechanics and related fields, involving the response of dynamical systems to stochastic excitation can be modeled as systems of first order differential equa- tions of the form :11: = a(t,x) + Z bk(t,x)-g;wk(t) (1.14) k=l ll where x, a, bk, 1: = l, 2, ..., m are m vectors and the wk (1‘) for k = 1, 2, ..., n are inde- pendent processes of Brownian motion. In nonlinear stochastic differential equations the structure of the transition probability density function is usually much more complex than that of linear systems, and cannot be obtained in a direct fashion. The most common method of obtaining the transition proba- bility density function for nonlinear stochastic differential equations is through the use of the Kolmogorov-Fokker-Planck equations: 361 ._ .37 +a5q _ 0 (1.15) where ax is expressed in the following way a = 3—(A q) 342—09 -q) (1.16) 4‘ axo 1’ Zaxjflx,‘ 1" q must satisfy the initial condition q (x, to] go, to) = 8 (x - x0) Exact solutions of the Fokker-Planck-Kolmogorov equations have been found for two types of stochastic differential equations: 1. systems of linear equations 2. certain first order nonlinear equations. The steady-state probability density can always be obtained for first order nonlinear systems, and has been found for a certain class of coupled nonlinear oscillator problems. Caughey and Atkinson (1968), obtained the transition probability density function for a class of piecewise linear systems excited by Gaussian white noise. The exact steady-state probability density for first order nonlinear system excited by Gaussian white noise can readily be determined by direct integration. The Fokker-Planck equation for a stochastic differential equation of the type dx = —f(x) dt+dw (t) (1.17) 12 is given by where D is a positive constant. Direct integration of the above equation yields the steady- state probability distribution function q: (2:) providing that it satisfies the condition 34 if" q. (x) = C‘exp[-jf(§) mag] (1.19) 0 where C is the normalizing constant given by C = j exp[-jf(§) /Dd§]dx (1.20) 0 —00 Exact solutions of the steady state probability density function for nonlinear equa- tions of second order excited by white noise have been found only for equations of the form it'+f(H)Ji+g(x) = w(r) (1.21) E[dw(t) 21 = 2Ddt (1.22) H = $122+ {3 (man (1.23) The associated Fokker-Planck equation is easily shown to be 2 gs; = -x§§+§;1(stx) +f(H)i)ql +0373 (1.24) and by direct integration, the steady-state probability density function is given by 13 r H ‘ exp -(1/D) jf(n)dn 4.0.x) = ,, ,, ‘ ,3 ; (1.25) I [exp -(1/D)jf(n)dn dxdrt -- 0 The same technique can be applied to the system of coupled nonlinear equations. h The main advantage of the Fokker-Planck equation approach over all of the other ap- proaches considered here, including statistical linearization, is the exact solution it pro- vides. However, this advantage must be balanced by the fact that such solutions have only been found for certain restricted classes of problems. Caughey showed that the stationary Fokker-planck equation can be solved and the first order probability density of the Mark- ovian response process can be obtained provided: 1. The only energy dissipation in the system arises from damping forces that are pro- portional to the velocity. 2. The excitation is Gaussian white noise. 3. The correlation function matrix of the excitation is proportional to the damping ma- trix of the system. 4. The restoring force vector of the system is derivable from a potential. The solution of the time-independent Fokker-Planck equation under these conditions rep— resents a very significant accomplishment. However, it is a fact that most systems of prac- tical interest do not satisfy the above mentioned conditions. Since, in general, it is not possible to obtain exact statistics for the response of nonlinear system excited by white noise, a number of techniques have been developed to treat a broader class of problems. One of the approximate techniques based on the use of the Fokker-Planck equation known as the eigenfunction expansion was used by Wong (1964) and Payne (1967, 1968) for first order system, in which case the Sturm-Liouville theory applies. For many higher order systems the Fokker-Planck-Kolmogorov equations are of degenerate form. Atkin- 14 son(1970) has used eigenfunction expansion techniques for second order systems excited by white noise. Unfortunately, for many nonlinear systems of the second or higher order, the eigenvalue problem cannot be solved exactly. In some cases, perturbation techniques may be used to extend the class of systems which may be analyzed by this method. This requires that eigenvalues and eigenfunctions of an associated Fokker-Planck operator be known a priori, a situation which unfortunately occurs rather infrequently. The Rayleigh- Ritz method (Mikhlin, 1964) has been widely used to approximate the eigenvalues and eigenfunctions of the linear differential operators. An approximate technique that has been used is the normal mode approach for sta- tionary random response problems. This technique reduces a set of coupled nonlinear sec- ond order differential equations to a set of equations that include coupling only in the nonlinear terms, and is applicable for statistically uncorrelated excitations. The reduced equations may then be subjected to a number of approximate solution techniques such as the method of statistical linearization. In order to successfully apply this technique to a giv- en multi-DOF dynamical system, the system must satisfy the following two conditions: 1. The linear system obtained by neglecting all system nonlinearities must possess normal modes. 2. The correlation function matrix of the excitation must be diagonalized by the same transformation that diagonalizes the linear mass, damping and stiffness matrices. While the first condition may be acceptable in a number of situations, the second condition is quite restrictive. In particular, it precludes the application of the technique to all dynam- ical systems that are excited at only few points in space. However, such systems are of con- siderable interest physically and thus, the second restriction represents a rather severe limitation on the usefulness of the normal mode approach. Another approximate method is the perturbation approach where the Stochastically excited nonlinear system is treated in the same way as a deterrninistically excited system. 15 A power series expansion in terms of a small parameter which specifies the size of the non- linearity represents the solution for such systems. Substituting the assumed solution form into the original equations of motion and equating coefficient of like powers of the nonlin- earity parameter then yields a set of linear differential equations for the terms in the solution expansion. A first order approximation is obtained by solving two linear systems. The first is the system which is obtained by setting all nonlinearities equal to zero, and the second is a system having an excitation which is a function of the solution of the first system. Prac- tically speaking, it is almost impossible to extend this procedure beyond the first approxi- mation except in very trivial cases since the probability density of the first order correction term is non-Gaussian. In their analysis of nonlinear systems, Lyon (1960) and later Crandall (1963) applied the perturbation approach to a continuous nonlinear system. This proved useful in a variety of applications since the approach is not restricted to cases of uncorrelated excitation. The major source of difficulty however, arises in the solution of the equation for the first -order correction term. This is because generally, this equation has a non-Gaussian excitation. The complexity of this can be overwhelming even when a relatively simple multi-DOF system is considered. A scheme for reducing the computational difficulties was presented by Tung (1967). By using Foss’s complex-mode approach to solve for the first-order correction term the computational difficulties were alliviated, but the overall complexity of the approach is still considerable A third approximate technique is statistical linearization. This technique was devel- oped independently by Booton(1954) and Caughey (1959). It is applicable to nonlinear sto- chastic differential equations. This approach overcomes many of the limitations encountered in the previous methods for studying the stationary random response of multi- DOF systems. It is based on the concept of replacing the nonlinear system by an equivalent linear system while minimizing the difference between the two systems. This concept has 16 been widely used in control theory and in the study of multi-DOF dynamical systems with stochastic excitation. The accuracy of an approximate analysis based on statistical linearization is difficult to predict in general, but it is usually assumed that the approximate solution obtained is ac- curate to first order in the parameter specifying the size of the system nonlinearity. Wan and Yang (1972) showed that the accuracy of statistical linearization approach appear to be well within the limits of practical engineering usefulness. When studying a specific nonlin- ear system, the accuracy of statistical linearization should be assessed by comparing its re- sults with results obtained by Monte Carlo simulation. Applications to composites Much of the recent work related to analysis of composites has been on the develop- ment of suitable finite elements for composite plates and shells. Studies considering static loading far outnumber those that consider dynamic loading. There have been only a few studies considering stochastic dynamic loading, and some of these are briefly discussed in this section, along with their limitations. Witt and Sobczyk (1980) were the earliest researchers to study the random vibration of laminated plates. They used an analytical series formulation and modal analysis to study the stochastic bending response, but considered only linear behavior. Witt (1986) later ex- tended the formulation to include transverse shear deformation in the plate. Cederbaum, Elishakoff and Liberscu (1989) used an analytical formulation that in- cluded a first-order shear deformation theory together with modal analysis to study the lin- ear random vibration of composite plates. Cederbaum (1990) later extended this to include viscoelastic material behavior. Mei and Prasad (1989) appear to be the only researchers who have considered the non-linear random vibration of composite plates. They included transverse shear deforma- tion, but considered non-linearity arising from large deformations and not from the consti- 17 tutive laws. They used an analytical formulation together with the method of equivalent linearization, and considered only a single modal response equation obtained through an approximate Galerkin approach. There are several limitations in the works cited above: 1) None of the investigations include non-linear material behavior, which is quite im- portant for filamentary composites. 2) They all use analytical formulations, rather than finite element based formulations, and therefore are readily applicable only for plates with simple geometries and boundary conditions (e.g., rectangular plates with simply supported or fixed boundaries). 3) They cannot be easily used in existing large-scale computer programs which are predominantly based on the finite element approach. Harichandran and Hawaaari (1992) performed nonlinear random vibration analysis of filamentary composites loaded in extension. They approximated the nonlinear shear stress- strain law in terms of a power series. The result of their work has shown that the non-lin- earity in the constitutive law results in significant increases in the shear strains, but does not significantly affect the normal strains. 18 2. Finite Element Formulation 2.1 General The advent of computers has opened new horizons in the field of engineering design. In the realm of analysis for engineering design the finite element method has emerged as a powerful tool for modeling and analysis of solids and structures of complex geometries and variable material properties. Although the original applications of the finite element were in the area of stress analysis, its usage has spread to many other areas having similar math- ematical bases such as heat transfer, fluid flow, electric and magnetic fields and several oth- ers. The finite element method is a numerical procedure which enables a problem with an infinite number of DOF to be converted to one with a finite number in order to simplify the solution technique. The primary objective of the use of the finite element method in the analysis of struc- tures is to calculate approximately the displacements, strains, stresses and other responses of the structure. The power of the method resides mainly in its versatility. The method can be applied to a variety of structures with arbitrary shape, loads, and support conditions. The finite element mesh can mix elements of different types and physical properties. Today, the concept of finite elements is a very broad one. A most important formula- tion, which is widely used for the solution of practical problems, is the displacement-based finite element method. Due to its simplicity, generality and good numerical properties, this formulation has been used in major general-purpose analysis programs. The basic process of the displacement based finite element method is that the com- plete structure is idealized as an assemblage of individual structural elements. The element stiffness matrices corresponding to the global degrees of freedom of the structural idealiza- tion are calculated and the total stiffness matrix is obtained by addition of element stiffness matrices. The solution of the equilibrium equations of the assemblage of elements yields the element displacements which are used to calculate the strains and stresses. l9 In two-and three-dimensional finite element analyses, we basically use the Ritz anal- ysis technique with trial functions that approximate the actual displacements. the result is that the differential equations of equilibrium are not satisfied in general, but this error is re- duced as the finite element idealization of the structure is refined. 2.2 Constitutive Equations In the elementary theory of plates, certain assumptions are made regarding the stress distribution. Certain stresses are assumed to predominate, while others are neglected. In the theory of bending of beams and plates, the normal stress, 0'3 , which is perpendicular to the beam or plate midplane, is assumed to be negligible in comparison to the normal stresses, 61 or 02. In other words, due to the geometry of the plate, the magnitudes that 0'3 can as- sume are several orders of magnitude less than the values of 01 and 0‘2 which are induced by bending. Also, the assumption is made that any line perpendicular to the plate midplane before deformation remains perpendicular to the midplane after deformation, and it suffers neither extension nor contraction. As a result, the shear strains, 713 and 723 , and the normal strain, 83, are zero. The shear stresses, '1:13 and 1:23 , are also neglected. These assumptions, which are made in the classical theory of plates, are termed Kirchoff’s hypothesis. For thin plates, the hypothesis results in the existence of a plane stress state. Thus one pertinent as- sumption in establishing the constitutive or stress-strain relationships for the laminae of a laminated composite is that the laminae, when in the composite, are in a plane stress state - which is not to say that the interlaminar shear stresses, 113 and 123, will not be present between laminae once they are placed in the composite. However, these stresses may be neglected in establishing the laminae constitutive relations on which the laminae stress- strain relations will be formulated. It is commonly known that most uni-directional filamentary composites display orthotropic characteristics, and behave essentially linearly when loaded parallel to or per- pendicular to the fiber directions. However, when loaded in shear, they exhibit significantly 20 nonlinear behavior. Fig. 2.1 shows the global coordinate system x-y and material coordi- nate system 1-2 for a typical uni-directional composite element. In view of the advantages and the disadvantages of the various nonlinear stress-strain models described in chapter 1, the model proposed by Hahn and Tsai seems to obtain an explicit relation for e in terms of the loading and known material parameters including the nonlinear shear term. Hence, this model is adopted in this study. Based on experimental results, Hahn (1973) proposed the following strain-stress (or inverse) law for plane stress problems: 81 5 ll S12 0 or 0 Y12 0 0 S66 112 112 in which 81 and (II are the normal strain and stress in the l-direction (i.e., along the fiber direction), 62 and 0'2 are the normal strain and stress in the 2-direction, 712 and 112 are the engineering shear strain and shear stress corresponding to the material coordinates 1-2, and the square matrix on the right hand side is the linear compliance matrix [S] . The cubic "/////V \\ .s\\\ Figure 2.1 Coordinate systems 21 variation of 712 as a function of 1:12 in Eq. 2.1 describes the softening behavior of filamen- tary composites loaded in shear, and is sufficiently accurate. In finite element application, the stress-strain law is preferred. The inverse relation of the cubic shear strain-stress law may be written as 112 = g (712) , where g (712) is the real solution for 1:12 of the cubic 3 _ _ 5*66‘121'366112 712 ’ 0 , s (712) . . letting f (712) = T— - Q66, the stress-strain law may be wrltten as 12 61 Qll Q12 0 £1 0 {02} = Q12 Q22 0 {£2} +f(712) {0 } 1:12 0 0 Q“ 712 712 (2.2) Qll Qrz . 0 £1 0 0 0 81 = Q12 Q22 0 {£2} +f(712) [0 0 0] {82} 0 O Q66 712 O 0 712 or, more compactly as {0'} = [Q] {8'} +f(712) [diag(0.0.1)l {e'} (2.3) in which [Q] = [S] '1. In terms of elastic moduli and Poisson’s ratios, the elements of [S] and [Qlare 511 = l/E1,S22 = 1/52, 512 = -v12/E2 = —v21/El,and 566 = l/G12 Qll = El/(l-V12V21),Q22 = E22/(1—v12v21) Q12 = "1215'1/(l "V12V21) = v2152/(1'V12V2l) For Eqs. 2.1 and 2.2 to be exact inverse relations, the function f ( 712) is related to the solution of the cubic equation mentioned earlier, and contains terms involving fractional powers of 712. This poses computational difficulties in the method of equivalent lineariza- 22 tion used to perform the approximate random vibration analysis. In order to facilitate the analysis, it is assumed that the function f (712) may be approximated by 11% f 1 fl r ' I ' I ' I ' T 10000 — Hahn and Tsai ------ Fifth Order Polynomial ITFTI r. 8000 7000 - o o o o C o O o .0 a... o .0 0" 0.. 5000- Shear Stress (lb/ inz) 3000- 2000‘- 1000- - l E J l 441 A l L l n l n l n l 1 l n l n l 1 l n J 1 0 n 0.0 0.002 0.004 0.006 0.008 0.01 0.012 0.014 0.016 0.018 0.02 Shear Strain Figure 2.2 Flt of approximate stress-strain law for shw fIle) = a1 12+027i2+m+0n7i§ = 2017i; (2.4) i=1 Note that the nonlinear term in eq. 2.2 is f (712) 712, which is therefore being approximated by an odd-powered series starting with the cubic term. By suitable choice of the parameters a5, the stress-strain law can be made to approx- imate the law in Eq. 2.1 for realistic values of 712. Fig. 2.2 shows the shear stress-strain law given by Eq. 2.1 for Boron/ Epoxy Narmco 5505, and the approximate curve using Eq. 2.2 and 2.4 with n = 2. The material pr0perties used were: S u = 3.32 x 10"8 in2 lb"1, .122 = 3.48x10‘7 in2 lb'1,S66 = 1.25x10'6in21b"1,S66"' = 1.53x 10'14 in6 lb'2 23 a1 = -2.259x109 lb/inz, a2 = 3.505x1012 lb/inz. The fifth-order approximation is satisfactory for strains up to 0.02. The stress—strain law in terms of the global coordinate system, x-y, may be written as {0} = [-Q-l{€}+f([T31{€})[7*l{8} (2.5) inwhich to} = lo,o,o,,1T. {e} = legion 101 = [71"[Q1171’T.and['1'l is the orthogonal rotational transformation matrix given by cos 20 sin 29 2 sine c050 [T] = sin 29 cos 20 -2 sinOcosO (2.6) -sin0 c030 sin0cosO COS 29 - sin 29 [T3] is a row matrix consisting of the third row of [T]"T, andlr'l = lTl"ldiag(0.0.l)] [71'T.Notethatf([T31 {2}) =f(T,2) inEq.2.s. since 916 = Q26 = 0. A convenient form for the transformed lamina stiffnesses, a, has been given by Tsai and Pagano (1968): Q11 = U1 + Uzcos (20) + U3cos (49) Q22 = U1+ Uzcos (20) + U3cos (40) Q12 = U4-U3cos (40) (2.7) Q66 = US-U3COS (46) where 24 u1 = %(3Q11+3Q22+2Q12+4Q66) U2 = $49.. 4222) U. = gramme-201240..) (2.3) U4 = %(Qll +Q22+2Q12—4Q66) Us = éIeri’sz‘lez‘i‘iQoo) Note that only three of the invariants U1, U 4 and U5 are independent. That is, these terms remain costant regardless of the angle 0. Thus, examining Eq. 2.8 reveals that each of the first four terms (independent terms) is composed of a constant term plus terms which change with the angle 0. Therefore, these are inherent lamina properties which are only de- pendent on the material being used and do not change with orientation of the lamina. This “invariant” concept is very useful in design with composite materials. 2.3 Finite Element Formulation using Classical Plate Theory Numerical methods, such as the finite element method, are necessary in practical ap- plications as they are able to model general geometries, boundary conditions, loading and materials. In this section, the derived expressions needed for the evaluation of the in-plane stiffness and mass matrices of the isoparamerric four-node rectangular element shown in Fig. 2.3 , and the plate bending stiffness and mass matrices of the Melosh-Zienkiewicz- Cheung (MZC) noncomforrning four- node rectangular element shown in Fig. 2.4. 2.3.1 ln-plane formulation Isoparametric finite elements are used for the in-plane extensional response. The el- ement displacements are interpolated as 4x = N'qul '1’” 2912*”. 34x3 ‘1‘” 4%: (29) q), = N‘lqy1+N‘2qy2 +N'3qy3 +N‘4qy4 (2.10) 25 where qx and q), are the local element displacements at any point of the element, and q”. and qyo, (i = l, . . ., 4) , are the corresponding element displacement at the element nodes. The interpolation functions N‘ ‘- are defined in the natural coordinate system of the element, which has variables g and n that each vary from -1 to +1. The fundamental property of the interpolation function N ' i is that its value in the natural coordinates is unity at node i and is zero at all other nodes. Using these conditions the functions N ' ,- corresponding to a specific nodal point layout could be solved for in a systematic manner. Figure 2.3 Nodal displacements for isoparametric plane stress element As shown in Figure 2.3 the nodal displacements are: {q.e} = {qquyi} (i=1,2,3,4) 26 the in-plane displacements at the reference plane of a laminated composite plate can be ex- pressed as qx(C,n) _ [M1 0 N2 0 M3 0 N4 0 ] {q' } quCrTI) O N'1 0 M2 0 N3 0 ”‘4 e in which {q'e} is a vector of eight element nodal displacements, and N‘ o are the interpo- lation functions: N'1(§.n) = 31-(1-é) (l-n) (2.11) N'2(§.n) = 71(1+§)(1-n) (2.12) N'3(§,n) = §(1+§)(1+n) (2.13) N'4(§.n) =-,-1,(1-§) (1+n) (2.14) The strain field within the element may be expressed as {8} = [3'] {q'.} (2.15) where the strain-displacement matrix is given by ~a 0. ”la 0 ‘ U; a (la—E 1 a Mix 0 [3'] = 0 33' [N'] = 0 5871' [N'] = 0 Ni, (2.16) .9. 8 la 1 a Mr, N‘... _ y 37c. 1,3; 03'; where N‘ i, x and N ' o, y represent partial derivatives of N' o with respect to x and y. i’ 3): a i’ (I ’ ’ ’ ) (. ) N -—i--1N‘ --1234 218 fly a I 3"." (I 9 r 9 ) (° ) 27 "‘57 or :. .unl or more explicitly, or” = 71150-11) Mo, = -fi(l-§) or“: alga—n) N'o,,=-Zl-5(l+§) N'.,. = 3‘;th N3, = gonad) ~'.,.=--41—a(1+n) N'.,,= 21-50—51 2.3.2 Out-oi-plane formulation . Consider the plate-bending element in Fig. 2.4. It was originally developed by Melosh , Zienkiewicz and Cheung (Melosh 1963, Zienkiewicz and Cheung 1964). As with other elements of this type, it has only one displacement in the z-direction. This element is said to be nonconforming because it does not have normal-slope com- patibility at the edges. There are some practical reasons for considering nonconforming plate-bending elements. General-purpose structural analysis programs usually permit six e1; 2 D N t T QIH aw, 3x Figure 2.4 Nodal displacements for rectangular plate-bending element 28 ”fl fir:- f nodal degrees of freedom, three displacements and three rotations. Compatible ele- ments(C1 elements) are capable of preserving interelement continuity of the field and its first derivatives at the nodes, but not interelement continuity of all second derivatives of the field. Such elements posses an additional nodal degrees of freedom Ge. 39%;) which does not fit well into a typical general-purpose program. On the other hand, several incompatible plate elements exist with only nodal displacements and rotations as degree of freedom. These elements fit well into general-purpose computer codes and permit the analysis of a variety of general structures. Such elements are particularly appealing because they can be used with plane stress elements to model plates and shells. The displacement function chosen for this element is a complete cubic of ten terms plus two quadratic terms. w = C1 + c2§ + C3T] + c4§2 + c5§n + c6112 + 0,? + csfizn 2 3 3 3 (2°19) + c9§n + c101] + cu§ t] + c12§n Since the element does not preserve normal slope continuity between adjacent elements boundaries, it violates one of the conditions for convergence to the exact solution with a refined mesh. Convergence, however, has been proved by Walz, Fulton and Cyrus(l968), and numerical results obtained by Zeinkiewicz (1977) demonstrate the convergence rates and accuracy for both displacements and bending moments. It should be mentioned that rectangular elements are the most straight forward type of plate-bending elements. Trian- gular and quadrilateral elements are more versatile for general structural analysis. Zienk- iewicz presents an excellent discussion of other plate-bending element formulations and types. The displacement shape functions for the above element can be expressed as follows Mo = (1+to) (1+no) (2+§o+no-§2-n2) (2.20) OOH- 29 . r71 it 19; r- N".-2 = --;-bn,(1+§o)(l-no) (1+no)2 N"a = gnarl—to) (1+no) (1+§o) whet-tel;0 = fig and no = non (i=1. 2. 3. 4) The curvatures at any point within the element are given by xx {19} = VZW'] {q".} = 13"] {4",} K xy where the generalized strain-displacement matrix [B"] is 1v"t'l,xx N" [3"] { 32 32 2 321 [N"l N' N" i = 2 2 a a i = i1.” 1’2.” 3): By J: y 2N" 2N‘ 12, xx t'l,xy i2,xy The elements of [B"‘.] are: H l N il,xx = 2? (l +7191) (€2-3Sgg" 1) N" =0 i2,xx Nni3,xx = '4—lo,§?(1+II,-Tl)(1+3§t§) _1_ Woo, = 4b2(1+§,-§)(hf-311,114) mm, = -§n?(1 +§,.§) (1 +1191) (1 412112) N‘ 0 i3, yy = N" N" 2N‘ 13, xx i3, yy 0 1'3, xy 2N3“, = halt—b [n,§o(4 - 352 - 3712) + 25,71,- (§o§+n,-n) —2 (ht-+11%,” 30 (2.21) (2.22) (2.23) (2.24) (2.25) (2.26) (2.27) (2.28) (2.29) (2.30) (2.31) F—mflm“f1 , l 2- ..., = 751131.11 +1151) (1 -3n,-n) (2.32) 2N»... = 4—1,,§%n,<1+§,§) (1—3tot) (2.33) The full out-of-plane strain-displacement matrix [8"] can be written as [3"] = 41127411931 13".] [3'31 [19"..113x12 (2.34) where 3:11-me 0 <1-3§)(1-n)ab2 18".) = 3n(1-§)a2 -(1-§)(l-3n)a2b 0 (4—3§2-3112)ab -(1-n)(1+3n)ab2 -(1-§)(1+3§)a’b —3(1-n)§b2 0 -(1-n)(l+3§)ab2 13",] = 3n(1+§)a2 -(1+§)(1-3n)a2b 0 -(4-3§2-3n2)ab -(l-n)(1+3n)ab2 -(1+§)(1—3§)a2b -3§(1+Tl)b2 0 (1+n)(l+3§)ab2 [3"3] = -3n(1+§)a2 (1+§)(1+3n)a2b 0 (4—3t2—3n2)ab -(1+n)(1-3n)ab2 (1+§)(1-3§)a2b 3§(1+n)b2 0 (1+n)(1--3§)ab2 13".] = —3n(1—§)a2 (1-§)(1+3n)a2b 0 —(4-3§2-3n2)ab (1+n)(1-3n)ab2 (1—§)(1+3§)a2b 2.3.3 Formulation of the linear elemental stiffness matrix [K,] Composite laminates are constucted by bonding two or more laminae. Laminate de- formations are assumed to be small with respect to the laminate thickness. Using the theory of thin laminates, strains are considered to very linearly through the thickness and interlam- inar deformations may be considered small at interior regions. 31 For combined in-plane and out-of-plane behavior, assuming plane section remain plane after bending, the strain at any point in the plate is {e} = {8°} +z{x} = [18'] +le"]] {4.} = [B] {q.} (2.35) The strain in material coordinates is {2'} = [Tl'TiB] {q.} = [TJ’TilB'HziB'Tl {q.} (2.36) The shear strain in material coordinates (i.e., the third element of {e'} is “foo = [T3][31{q.} = [T3] [13'] +ziB"l] {4.} (2.37) in which [T3] is the third row of [T] “T Figure 2.5 Layer nomenclature for laminate As has been described previously, the plate element used in the formulation has 20 DOF: three displacements and two rotations at each of the four nodes. The linear elemental stiffness matrix of a laminated plate with N laminae is 32 1K.1 = 1 1317151 [BldV V N ab Z; = 2 H j tlB'lT+le"1T1télllB'l+le"l1dxdydz (2.38) k=100z = 2 H I [[B'lT+z[B"]T] [0'1 [[B']+z[B"ll|J|d§a‘ndz k: 1-1-12._l where N is the number of laminae, zk is the height from the reference plane to the bottom of the kth lamina (20 being the height to the top surface of the laminate as shown in figure 2.4), and Ill = STUFF—l)- (x, y) is the determinant of the jacobian matrix for transformation from the x-y coordinate system to the E- 11 system. The integral with respect to z in Eq. 2.39 may be performed in closed-form, and the double integral with respect to 5,. and n is normally computed by numerical Gauss integration. Expanding Eq. 2.38 the elemental stiffness matrix can be written as 1V [Kol = 2 llK',1 +1K",l+ [K”’,] + 1K"",11 (2.39) k==l in which 1 1 We] = (a-z.-.) j j lB'lTlél [B']|J|d§dn (2.40) —l-l zi-Zi—l l 1 — [K21 = *7— j j lB'lTlol [3"]llld§d|‘l (2.41) -l-l Zi-Zz_1 l 1 _ 1K"',1 = ——§—— 1 ] 18"17191 [8'1 lJldtdn (2.42) -l-r 4-44“ — 1K"",l = ——3— j [8"]TIQ] [B"lllld§a‘n (2.43) -l-l 33 The Jacobian matix [J] is (2.44) 2.3.4 Formulation of the elemental consistent mass matrlx The consistent mass matrix for the plate element is [M] e I pNTNdv V 1v ab)“ 2 II I pthT+le"lTl [[N'l +zIN"]]dxdydz k=looz‘_l N’ 1 1 Q 2 I I p[[N]T+z[N"]T] [[N’] +z[N"]]IJld§drldz k = 1’14“” (2.45) Performing the integral with respect to z analytically, the elemental mass can be ex- pressedas .N [14.1 = z ltM',l + [M”,] + lM'".l +1M"”,11 (2.46) k==1 inwhich l l 1M',1 = (are.-.) I I lN'lTl‘Q‘l 1N1 lJldtdn (2.47) , -l-l ZI'Zi-l l 1 — [M”el = 2 I [N'lTlQl [N"]lJ|d§dn (2.48) -l—l ZI'Zi-l 1 l — .[M”’,l = -—2— I I lM'lTlQJ [N']|Jld§dn (2.49) -l-1 3 l l 3— 1M"",l = ff—z’ifl I I [It/"17121 [N"]lJld§dn (2.50) 3 —l-l 2.3.5 Numerical integration In the integration of finite element matrices, a subroutine is called to evaluate the un- known function given in Eq. 2.52 at given points, and these points may be anywhere on the element. A very important numerical integration procedure in which both the positions of the sampling points and the weights have been optimized is Gauss quadrature. Hence, for the two dimensional plate bending problem we have 1 l I I ¢(§.n)d§dn~2W.-W,-¢<§,njl (2.51) h} -l-l where W; and W]. are the integration weights. The above mentioned scheme is directly ap- plicable to the evaluation of matrices of rectangular elements in which all integration limits are -1 to +1. The order of numerical integration to be used in the evaluation of stiffness and mass integrals depend on the degree of precision that the order of the numerical Gauss integration must satisfy. Since the shape functions have up to fifth order terms, we must satisfy the con- dition 2n - 1 2 5. Therefore, the order of integration is taken to be n = 3 x 3 . 35 3. Equivalent Linearization 3.1 Introduction Random vibration analysis of mechanical systems has become an important subject in recent years for various engineering applications. Forces due to earthquakes, turbulence in air or water, storm waves, and the forces experienced by a vehicle traversing rough ter- rain are examples where an understanding of random vibration theory is essential to the successful design of the structure. Composite materials used in high speed flight vehicules are usually exposed to fluctuating loads caused by the flow of turbulent air or rocket en- gines. A common feature of such problems is that the excitation is often so complex that is can be described only statistically. In addition, most physical systems behave in a linear manner only for a limited range of the excitations, and since under random excitation larger responses can be expected, at least occasionally, it is often necessary to study nonlinear re- sponses due to random excitations. Exact solution of non-linear random vibration problems is possible only for simple systems. For realistic engineering problems an approximate method must be used. One of the most widely used approximation techniques for nonlinear random vibra- tion problems is equivalent linearization (or statistical linearization) in which the original nonlinear system is replaced by an effective linear system. This approach has proved quite useful for a broad range of engineering problems. Other methods such as Gaussian closure and energy balance are closely related and generally give similar response results as equiv- alent linearizaan while approaches such as the perturbation method may give somewhat different results. The essence of the method of equivalent linearization is to replace a given non-linear system by a linear system in such a way that the difference between the two systems is min- 36 imized for all possible solutions of the associated linear system. The solution of the linear system is then taken as an approximate solution of the original nonlinear system. The min- imization of the difference in the equations of motion with respect to the linear system pa- rameters does not necessarily guarantee that a minimization of the difference in the corresponding solutions will be achieved. It has been shown that the method of equivalent linearization gives good results even for strong nonlinearities. 3.2 Formulation Consider the response to external load of a nonlinear multi-DOF vibratory system: Generally, The dynamic equations of motion of a composite plate discretized into finite el- ements may be written as [M] {ii} + [C1 {C1} + [K] {(1} +¢(q. <1. it) = {Q(t)} (3.1) in which [M] is the consistent or lumped mass matrix obtained by assembling the element mass matrices, [C] is the damping matrix (usually specified indirectly through modal damping ratios), [K] is the linear stiffness matrix obtained by assembling the linear ele- ment stiffness matrices, {4)} is a vector of nonlinear terms obtained by assembling the el- ement load vectors {the} , {q} , {t1} , and {a} are the displacement, velocity and acceleration vectors and {Q (t) } is an excitation vector. It is assumed in this study that {Q ( t) } is a zero mean Gaussian vector random process. In the case when (I) (q, q, a) is non-zero and nonlinear, attention is directed toward techniques of approximate analysis. Equivalent linearization offers a systematic and readily automated method for generating an approximate solution of Eq. 3.1. In the process of obtaining an approximate solution of Eq. 3.1, let us consider an aux- iliary (equivalent) system which is described by a linear differential equation of the form ([Ml + [M.l) {ii} + ([C1 + (Col) {<1} + UK] + [K.]) {(1} = {Q (0} (3.2) where [M e] , [C e] , and [Kc] are deterministic mass, damping and stiffness matrices. 37 The difference {5} between Eq. 3.1 and 3.2 may be written as (Spanos, 1976; Spa- nos and Iwan, 1978) {6} = Mq+Cq+Kq+¢— (M+Me)21'+(—(C+Ce)t1)-(K+Ke)q = ¢(q: 4’ q) TMeq- Ceq-Kgq (3.3) In order to determine the matrices [M e] , [Ce] and [K e] of the equivalent linear system it is necessary to establish a criterion for the minimization of {5} , based on a suit- able norm of this vector. Here the Euclidean norm defined as II {61113 = {6} WE} will be used as a measure of {5} . The minimization of 5 is performed according to the criterion E[ {5} T{5}] = E{5f+5§+ +53} = minimum where 51], 52,... 5" are the elements of 5. Using the linearity of the expectaan operator E [ I , Eq 3.5 can be written as n 2D;2 = minimum i=1 where i = 1, 2, 3, ..., n and D‘- is defined by n 2 vi = 316%} = EH" 2 (mirir+c?jér+k34r>] I i=1 (3.4) (3.5) (3.6) (3.7) Minimization is with respect to the class of functions of t which are solutions of Eq. 3.2. The necessary conditions for Eq. 3.7 to be true are ieEltsthsn = low?) = 0 8m”. 29ij 38 (3.8) iEliWiz‘tll = 597w?) = o (3.9) Be; cu —a;El{5}T{5}l = 497(1),?) = o (3.10) where mfj, cf]. and [:3 are the (i, j) elements of the matrices [M e] , [Ce] and [K e] , re- spectively. The expansion of Eqs. 3.8, 3.9 and 3.10 gives the following [16“.] T E{¢,-?1} = 13121217} [CZ-1T (3.11) [MAT where [Mr] , [Cf] and [KI-I] are the irh rows of the matrices Me, C, and Kc, respec- tively, and {a} = lq. q. a] T - (3.12) So far the method of statistical linearization has been presented irrespective of the particular probability density which is used in computing the expectations appearing in Eq. 3. 1 1. The formulation is facilitated by assuming that the excitation of the original nonlinear system is Gaussian. Therefore, the response of the equivalent linear system to this excita- tion is also Gaussian. Utilizing Kazakov’s formula (1965) for Gaussian random vectors: Etftnln} = “1111715inth (3.13) we obtain 39 ‘ fl I... I J .83: 0Q -~-1 (3.14) o O V Bloom] = 5112} {2105+ #3: Q we Q.) m 1Q we b d Comparison of Eqs. 3.1] and 3.14 shows that the elements of the matrices [M' ] , [C' ] and [K' ] are given by the simple expressions 3(1). * _ r m tj -- E{r{q’j}} (3-15) * - E L" 316 M. at- _ 1 These results are due to Kazakov (1965). They were first used for stationary nonlinear ran- dom vibration analysis by Atalik and Utku (1976). Subsequently, Spanos (197 8, 1980) pointed out that they are also applicable for non-stationary problems. For the laminated composite plate, the nonlinear force vector is only a function of the generalized displacement coordinate q. Thus the nonlinearity will affect only the stiffness matrix and consequently, [C'] = [M'] = [0]. 3.3 Derivation of the Nonlinear Elemental Stiffness Matrices Consider virtual displacements {54¢} . The elemental virtual work done is 5W = {54,} T {R.} where {R e} are the nodal loads on the element. The internal virtual work is 5U = I{52}T{o}dv setting 5W = 5U and using the strain displacement relations {58} T = {561,}7181T and the constitutive law in Eq.2.5 yields {64.17%} ItSq.}TlBlT{ 151 {a} +f(712) 17*] {a} 14V} (3.18) {6a,}TI I [817151 {£}dV+ I 13leth [7‘1 {e}dV] V V since Eq. 3.18 must hold for any arbitrary virtual displacements {89¢} T, the following equation must hold: {R.} = I I [BlT{ [51 131cm] {4.} + I131 Tftto) 17*] [B] {q.}dv V V (3.19) = [Kol {4.} + {4.} where [K e] is the elemental stiffness matrix given in Eq. 2.39 and {4,} = I 1317mm) [1*] [B] {q.}dv V N “b “' (3.20) = 2 ll lma) twirl [B] {q.}dxdydz i=1002,-. is the elemental vector of nonlinear terms. For the functional form of f (712) given by fun) = Zuni; (3.21) 1': 1 the nonlinear vector in the above equation may be written as 41 {4,} = 2 {to} (3.22) 1:1 in which ab 11 11>] = )3 2:: I I I 755131 17*] [B] {q.}dxdydz (3.23) k: 11': l iOOz,_, Since the nonlinear vector (I) is a function of the displacement vector {q} only, the nonlinearity will affect only the stiffness matrix. Hence, c' = m' = 0. The resulting equivalent stiffness matrix can be written as N n l l 11: [16“.] = BIN—La }{¢ }] = 2 Eta-I Ilz‘IdBthT’l [B] [Alllld§dndz (3.24) k=li=l and requires the evaluation of [A] = E[-a———{aq fl}; {qe} I (3.25) The partial derivative is My 814. —755{q.} = 755111 +2iiq.}ti5 8’“qu (3.26) and since 712: [T3] [B] {qe}, Eq. 3.25 becomes [A] = E{([T3] [Bl {2.112‘111 +2i{q.} ([Tol [B] {q.})2“‘lT31 {Bl} (3.27) Consider the first term of the above equation 5([7'31131 {u.})2‘= ([T3l [[B'l +le"ll {4.})2‘ (3.28) = ([T31 [13'] {(1.} +zIT31 [B"ll {qo})2‘ Using the Binomial expansion 42 2i . (a + bz) 2" = 2 (2")a2“’b’z’ (3.29) (=0 Eq. 3.28 may be expanded as 2i 1511751 [B] {2.1)2i = 2‘, [flag] [3'1 {qo})2""([T3] 13"] {q.l)'z’ (=0 21' 21-1 = 25 52)“ OLE” ZTEU'B H1192, ) (3.30) I=() j =lk' =1 3 20 I x [[2 2 T31...B"j..k..qe’k..) ] = 1k" =1 or in condensed form . 21 51([T3IIB] {q.}>2*1 = 2a,? (3.31) i=0 where orI is given by 3 20 3 fl) 3 20 = 2 Z 2 2 2 2 E[qe,k'qe,k2qe.k3" 'tqe km] j|=lk|=lj2=lk2=l j21=1k21=1 (3.32) XL!)111731..)(:I:-I:B",..Ic,..)2i 3"”) =2i- 1+1 For the case where i=2, which corresponds to a fifth order approximation to f (712) , the expanded form of Eq. 3.31 is 43 2t 3 20 3 20 3 20 3 20 I I I D 2 alz _ 2 2 2 Z 2 2 2 T3j1811 ‘5 113.128 12k2T313B 13k3T3j4Bk J4l‘4 i=0 11=1kl=1j2=lk2=1j3=1k3=lj4=1k4=l x E lqe, 12,42, kzqe, we, k‘] 3 3 20 3 20 +42 2 Z Z 2 Z 2 2 2 TsjB' flax-23'} Tsj,B'j3k,TsJ-.B"m ' —1k1=1j2=lk2=1j3=1k3=1j4=1k =1 X E [qe, k'qe, kzqe, ksqe, 1:4] 3 3 20 +622 2 20 2 20 Z 20 2 2 T3jiB'Jik1T3123)“:T3laanak3T3jIan4k4 j,,=1k =1j2=lk2= lj3=lk3= 1j4=1k =1 X E [qe, kl qe, kzqe, k3qe, k4] 3 3 3 +423 2 E 2 20 2 E Z i 731.3 1. kT3sznjzk 2"T3jaB jsk kT3J' Built jH=lk =1j2=1k2=1j3=1k3=1j‘=1k =1 X E [qe, k‘qe, kzqe, k3qe, 1:4] 4 3 20 3 20 3 20 3 20 + Z 2 2 2 Z 2 2 2 2 T3; 3 " 731;” "1255133 "131:, T31.” "141:4 j|=1k1= 1j2=1k2= 1j3=1k3=1j4=1k4=l X E [qe, k‘qe, kzqe, k3qe, k‘] (333) Now consider the second term in Eq. 3.25 which can be expressed as follows: E[{q,} ([T3] [3]) {qe} )2“1 [T3] [3]] 2i-1 , = E{qe} 2 (Z'I'IJUTal [B'] {qe})2"‘"’([T3] [3"] {qe})’ (=0 x [T31 [[B'] z‘+ [3"12” ‘1 (3'34) 2i-1 2 {5,} [[B']z’+ [B"1z’+‘1 [=0 where {B,} is given by 2i-l {5,} = E({q.} 2 (2‘;‘)([T31[B'1{q.})2‘-1-'([T31 [8"] {q.})') (3.35) i=0 The pm element of { B1} is 3 20 3 20 3 20 B113: 2 2 2 Z 2 E[qe,k,qe,k2qe,k3'“qe,1:214] j,=1k,=1j,=1k,=1 j2,_'=1ku_'=1 (2. I) 2i-1 2i-l—l 25-1 (3.36) l- o n x I (m T3}.)( H BIJ.) H 311.!) =1 m=1 =2i-l I resent 2ith-order moments of the zero-mean Gaussian vector {q e} . The higher-order even where 12') indicates the binomial coefficient. The expectations in Eqs. 3.32 and 3.36mp- moments may be expressed in terms of the covariances as follows: E [9,, hqe, k2° ' 'qe, k2,] = 2E [qe, que, k2] E [qe, k3qe, k‘] ° ' 'E [9,, k2,_ lqe, 2i] (3°37) in which the summation is taken over all possible ways of dividing the 2i variables into i combinations of pairs [the number of terms in the summation is 1.3.5...(2i-3)(2i-1)]. The effort required to compute the a, and [3, p coefficient by Eqs. 3.31 and 3.34 is substantial. However, a special algorithm described in chapter 4 that carefully takes into account the symmetries involved in the summations has been developed, and the computa- tional effort has been reduced by a factor of more than 20. Finally the equivalent stiffness matrix can be written as 2a.. J j 1 [3mm [3] [Alllldtdndz (3.38) n l 1 ll =1 i-l-IZ.-' N [16“.] = 2 i=1 where [A] is given by 45 2i 2i—1 [A] = 2092’ [I] +2: 2 {[3,} [T3] [[B']z’+ [B“]z’“] (3.39) i=0 i=0 Expanding [B]T[T"] [B], [K'] canbesplit into [K21 = [K'*.1+[Ic".1 +[K'"*.1 (3.40) where N n 11 m2] = Z aJ I [B']T[7*][B'][A1]|J|d§dn (3.41) k=li=l -1-1 N n 11 mt] = 2 2a.. 1 [WWW] [B"1+[B"]Tn*1 [3'11 [Azlmdzdn (3.42) k: li=l 1-1-1 N n 11 [K'"*.1 = 2 ad I [B"1’[1*1{B"][A31md§dn (3.43) k=li=1 _1_1 and [A1] , [A2],and [A3] are 9 [A1] = J [Aldz 2:“ (+1 (+1 _ Zk -zk—l ‘,§.°‘t(——z+1 )m 25-1 21+1-Zic+11 21+2-21+21 +21. 2 {BI} [T3] [13'] (Ti-‘4)+ [3"] (7%)] i=0 1: [A2] = j 2 MM: zk-l 2.‘ z1+2 “21:21 :22)“ °‘:(-——— 1+2 )[I] (3.45) 2i- 1 Zl+2'2£+2 21+3-zi+31 +2i’20{fll} [T3][[B](—T_—2—i)+13"1(-—1T3—-—):l (3.46) 31 [A3] = I 22 [A] dz 2i 21+ 3 _ 21+ 3 = 31‘” 1174—”) m (347) 23-1 Z£+3 21:31 21:4“ 21+: +2: 2 {[3,} [T31 [[81 {—13:74} +[B"] {—M—“H (=0 The general expression for [A q] , q = l, 2, 3 can be written concisely as 47 1: [Aq] = Izq'l[A]dz zk-l 2‘ dig-zit", = 2a‘( Ha: )[I] (3.48) (=0 2i— 1 Z(+q_zi:ql zik+q+l_z£-:ql+l +2i 120 {13,} [T3] [13'] (—_l+q )+ [B"l( ”(1+1 J] 3.4 Random Vibration Analysis Since computation of [K' e] requires the mean and covariance of the displacement responses to be known, an iterative approach must be used, in which each iteration consists of a linear random vibration analysis. Linear random vibration analysis is well-known and is only summarized here. Time-domain or frequency-domain techniques can be used for the analysis, and the main steps using a frequency domain approach are as follows: 1. Using the stiffness matrix [K e] + [K' c] (with [K' e] =[O] in the first iteration) and mass matrix [M] , determine the frequencies, to], and mode shapes, {vi} , for a chosen number of modes (say n modes). 2. Perform a linear random vibration analysis to determine the covariance matrix of the nodal displacements {u}. The rsth element of the covariance matrix is given by Elq,q.1 = 2 2'; w“): 2 way ,,,,. IH(- com (w)S,,,.(m)dm (3.49) j=1k=1M kl=lm=l in which V”. are elements of the mode shape matrix, M j = {wj} T[M] {Wj}. is the jth modal mass, H}. (m) = (0)]? - o) + 21101.03).l is the jth modal frequency re- 48 sponse function, and S1," (to) is the cross spectral density function for the excita- tions P, and Pm. Note that for synchronous loading only the auto spectra are non- zero, and the double summation over ( and m may be reduced to a single summation. For certain classes of excitation spectra, closed form solutions can be used to rap- idly compute the integrals in Eq. 3.49 (Harichandran 1992), while for more general cases numerical integration must be implemented. 3. Compute the equivalent element stiffness matrices [K' e] from Eq. 3.40 through 3.47, and assemble the global equivalent stiffness matrix [K' ] . The three steps outlined above are repeated until convergence is reached in the cova- riances of the nodal displacements. One method of checking for convergence is by using the nodal displacement variances, and the mth iteration is assumed to have converged if _ 2 If; (6%“ agnm'l) O.2 j; 90’" < e (3.50) in which Gq‘ = ,/E [qiqi] . The covariances of the strains within an element may be computed by replacing w”. and v“ in Eq. 3.49 with strains corresponding to modes j and k in the final iteration. The covariances of the stresses may be computed from the covariances of the strains and the nonlinear constitutive equations. 3.5 Computation of Strains and Stresses The multidirectional composite laminate consists of laminae of various fiber orienta- tions. In this form the stiffness of each lamina may differ significantly from adjacent lam- inae. Since the strain components in thin laminates vary linearly through the laminate thickness, discontinuities in the in-plane stress components will occur at laminae interfaces. 49 fe Tn and Hence, it is imperative that the detailed state of stresses be established for each lamina. In this study strain and stress components are computed within each lamina using the tech- niques outlined below. In global coordinates, the strain at any point (x,y,z) in the element is computed as fol- lows {8} = [[B'] +z[B"]] {(1,} (3.51) The covariance matrix of the strains in global coordinates is given by [28] = [{8} {8} T] = [[B'] +23") [29.] [[B'] +z[B"]]T (3.52) in which [Zq ] is the covariance matrix of the nodal displacements. In local coordinates, the strain can be expressed as {8'} = [Tl'T{8} = [Tl‘TllB'] +z{B"]] {4,} (3.53) The covariance matrix of the strains in local coordinates is computed through [28.] = [T] ‘71 [3'] + z [3"11 [th1 [[B'] + z [3"] ] TIT] '1 (3.54) For stresses, it is easier to compute the stresses in local coordinate first and then trans- fer to global coordinates. In local coordinates the stresses at any point (x,y,z) are given by O O O {o'} = [Q]{8'}+f(712)[0 o OJW} (3.55) 0 0 l The normal stresses in local coordinates are 0' Q11 912 3'11 { ”l =[ I (3.56) 0'22 Q21 * 922 {8'22} and the shear stress is 50 =Qoo7'12 +f (712) 112 = Qoo'Y'lz + Z“ 72'2“ (3°57) From Eq. 3. 54 the covariance matrix of the normal stresses is Q11 Q12 Q11 912 T 2 . = 2. = Z . T 3.58 [ 0N] [Q21 Q22][ 1|:Q21 Q22] [QM] [ 2”] [QN] ( ) From Eq. 3.55 the covariance matrix of the shear stress can be expressed as follows E [on] = E [Qggy'f 1 + 2Q6620 E [y2;+2] + 2a .2a 5 [y2;+21'+2] (3.59) Since 712 is a gaussian random variable, E [7%] can be expanded as follows wagl =1x2x...x(2k-1)x(o§2)' (3.60) l in which , a; = E [fiz] . Note that a: was obtained in the computation of the strain in 12 12 " the local coordinates using Eq. 3.52. In global coordinate the stresses can be computed easily through [2,] = [7‘1"[20] O m" (3.61) 0 steel in which [20.] is the covariance matrix of the stresses in local coordinates, computed by using Eqs.3.58 and 3.59. 3.6 Laminate Strength Analysis From a design point of view, it is important to be able to predict failure due to exces- sive strains or stresses. The maximum strain criterion is one of the failure criterion used in the analysis of unidirectional fiber composites. In this criterion the orthotropic lamina is characterized by six ultimate strain allowables. 51 If any one of the ultimate strains is exceeded in any lamina, it is deemed to have failed. U1- timate strains in material coordinates for Narmco 5505 (the material used in the numerical examples presented in Chapter 5)at room temperature are 81+ = 0.0040 82+ = 0.0027 712+ = 0.01 1 81- = 0.0065 92. = 0.0038 712' = 0.011 As each ply fails, the laminate stiffness is recalculated to reflect the deletion of the failed lamina. The lamina failure strains can be used to predict of the laminate ultimate strength. Alternatively, failure in composite laminates have also been expressed in terms of stresses. The most common ones are the Tsai-Hill and Tsai-Wu criteria (Jones 1975). 52 4. Optimizing Computational Effort 4.1 Introduction Finite element programs can be computation-intensive. For example, even small problems of size n may require a computation time in the order of 0 (n4) . In nonlinear problems the computation time becomes worse because convergence iteration is required. The nonlinear random vibration analysis outlined in the previous chapter is extremely com- putation-intensive and unless the computational effort is alleviated, the user will be restrict- ed to analyze only small problems. In order to investigate a reasonably large problem, there is an urgent need to drastically reduce the required number of computations and conse- quently minimize execution time. In this chapter a novel optimization technique is developed to make use of the sym- metricity inherent in the mathematical expressions. The improved performance of this tech- nique is presented later in this chapter and is compared to the straight forward FORTRAN implementation. A number of simple techniques is also used to reduce the computation time for the types of formulas that involve operations on multiple matrices. 4.2 FORTRAN Implementation This section provides a general description of the FORTRAN code deveIOped for this problem. The program is logically divided into a number of different modules. Each module corresponds to a separate logical computation unit as defined in the general flaw-chart of Fig. (4.1). This logical separation was adopted in order to simplify the implementation and greatly enhance the readability of the code. The modular design should also greatly facili- tate future modifications and extensions to the existing code. This is true even if major changes and additions are required. The corresponding modifications may be added be- tween the units of Fig.4.l or within each unit without affecting other units. 53 C9 (9 C9 C9 @629 Get input parameters i Initialize Variables i l Compute Linear Mass & Stiffness ’i Compute Eigenvalues & Covariance matrix Convergence reached ? Compute Alpha & Beta V Compute Nonlinear Stiffness matrix F ' Compute Strains & Stresses Figure 4.1 General flow-chart for Plate.f program Figure 4.1 shows the general flow-chart for the main program. Each step in the flow- chart is described in detail below: 1. The user is prompted to input a number of parameters that completely control the size, variation and behavior of the problem. The implementation is quite automated and allows for a large variety of problem sizes, loading conditions, support condi- tions, number of laminae, and different thicknesses and fiber orientation angles for each lamina. 2. All static variables are initialized, and the topography of the plate based on the input parameters is generated. Elements are generated along a rectangular grid. The val- ues of Z, and 2,, _ 1 based on the number of laminae are also computed. These val- ues represent the distances from the mid-plane to the top and bottom of each lamina L... The signs for Z]: and Zr _1 are determined according to the position of lamina Li with respect to the mid-plane (i.e. if L,- is below the mid-plane, Zk and 2,, _ 1 will both be negative). Finally, 3’ and B” are computed according to the formulation previously introduced in chapter 2. These computations are done for each Gauss point within an element. 3. The elemental stiffness and mass matrices are computed using 3 x 3 Gauss quadra- ture. These matrices are then assembled using the topography matrix computed in step]. The global structural mass and stiffness matrices are then obtained by elimi- nating the restrained DOF specified by the user. 4. The eigenvalues are computed using the structural mass and stiffness matrices de- termined in step 3 for the first iteration or in step 6 for subsequent iterations. In the first iteration, only the linear mass and stiffness matrices are used. In subsequent it- erations, the stiffness matrix includes both linear and nonlinear terms. The eigen- values are then sorted and the corresponding eigenvectors are computed. The DOF chosen by the user are loaded with excitations having the user specified spectral densities S ((0). Finally, the structural covariance matrix is computed using Eq. 55 3.49 and convergence based on Eq. 3.50 is checked using a tolerance of 0.001. Co— variance matrices corresponding to elemental DOF are extracted from the structural covariance matrix as needed. 5. The coefficient on and B, as introduced in Eqs. 3.32 and 3.35, are computed. These intensive computations are performed for each element and at each Gauss point. and is the most time-consuming part of the whole program. It is here, that two major op- timization techniques that drastically reduce the number of computations are intro- duced. These techniques are described in details in the following sections. The individual values of the or and B coefficients for any given pair of indices may be computed independently and this will facilitate the parallelization of this step as discussed in chapter 6. 6. Using the values of or and B, the elemental nonlinear stiffness matrix for each lam- ina is computed. These matrices are assembled and the restrained DOFs are elimi- nated to obtain the final nonlinear stiffness matrix for the whole laminate. The nonlinear stiffness matrix is then added to the linear one to obtain the total stiffness matrix. 7. Steps 4 through 6 are repeated until convergence is obtained. 8. After convergence, the final covariance matrix is used to obtain the covariances of the local and global strains and stresses. These values are computed at the center of the element chosen by the user for each lamina. 4.3 Optimization Techniques The time needed to evaluate any given formula is directly proportional to the number of arithmetic Operations performed by the corresponding code. When computing the non- linear stiffness matrix by statistical linearization, the execution time was found to be large especially for computing the or and I3 coefficients.The actual time to execute a given in- struction that involves arithmetic operations also depends on the precision of the variables 56 involved (i.e., the multiplication of double precision numbers requires about twice the amount of time needed for the multiplication of single precision numbers). Hence, mini- mizing the number of arithmetic operations performed by the program can greatly reduce the total computation time. Since code normally represents the direct implementation of a given problem, any technique to Optimize such code will have to preserve the integrity of the implementation. Thus, optimization has to be done at the semantic level. For example, a computation that involves the multiplication of a number of variables may be totally skipped if at least one of these variables has a value of zero. Checking for zero values within a matrix V is only justified if it is known a priori that V is mostly sparse. The rest of this section describes in details a number of techniques developed and implemented to mini- mize the required number of instructions and thus optimize the execution time. Also, a de- tailed comparison between the non-optimized and the optimized versions of the code is presented. 4.3.1 Optimizing or and IS The computation of Eqs. 3.31 requires the evaluation of five sub-equations of the fol- lowingtype 3 20 3 20 3 20 3 20 =22222222F (4.1) j =lk =lj2=1k2 =lj3=1k3= ljg=lkg=l whereFis F = 731.3 ' T3123 .12k2T3lsB '1; 3T3jrB M kE [q, 2 q, que *3 q, ’2] (4’2) A direct implementation of Eq. 4.1 is shown in Fig 4.2, where each Do-loop repre- sents the corresponding summation in the equation. Based on this implementation the ex- pression F has to be evaluated approximately N times, where N s 13 x 106. The time required to execute the code in Fig. 4.2 is approximately Ta‘ = N x C (F) , where C (F) represents the computation time required to multiply the twelve double precision variables 57 DO J2 = , 3, 1 DO K2 = 1, 20, 1 DO J3 = 1, 3, 1 DO K3 = 1, 20, 1 DO J4 = l, 3, 1 DO K4 = 1, 20, 1 tttcov = cove(ielem,k1,k2)*cove(ielem,k3,k4)+ +cove(ielem,k1,k3)*cove(ielem,k2,k4)+ +cove(ielem,k1,k4)*cove(ielem,k2,k3) alpha2(ic,ielem,1) = alpha2(ic,ielem,1) + +tt3< dint(i2,j2) 69 -— Do-Loop Unrolling / j -- Straight Implementation / 5 - / - / . / , // 4 / / / / 4 u \ Actual time in sec (x 100) N t—i 2 Number of elements per row/column Figure 4.10 Do-Loop Unrolling vs. Straight Implementation. is totally independent of the DOJoops corresponding to 1'3 and 1'3. Thus, C may be moved outside these DO-loops resulting in the saving of ops (C) x (ndofz - 1) , where 0PS(C) is the number of arithmetic operations in the computation. When all the DO-loops for the above implementation are considered, the total number of operations saved by this simple technique is approximately OPS (C) x (ndofz - 1) x 604. The actual timing for the above illustration was measured on a Sun SPARC ELC workstation and is shown in Fig. 4.9 where it is compared to the straight forward implementation for different DO-loop di- mensions. The computation time was measured using the UND( time command. Because of its wide applicability, this technique was used throughout the program. The interested reader is encouraged to go through the program listing to realize the impor- tance of this technique in reducing the overall execution time of the program. Table 4.8 presents a final comparison between a straight-forward implementation and the optimized version of the program. Four seperate runs were made with the same loading 70 conditions and geometry while varying the number of layers. Note that the total time for each run is directly dependent on the number of iterations required for convergence and hence, in the above table, the time for 5 laminae (4 iterations) is less than the time for 4 laminae (5 iterations). The fourth column of the table, i.e. the ratio of non-optimized to op- timized time, suggests an average ratio of 40. For this specific example, (with 4 laminae), the optimized version takes only around two hours, while the straight implementation takes TABLE 4.2 Comparison of total times. Stacking Sequence Optimized (sec) Non-optimized (sec) Non-Opt/Opt [30l-30] 4100.6 150871 36.8 [30/0/-30] 3734.4 1701365 45.5 [301301-301-30] 6968.4 2992303 43.0 [30/30/01-3OI-30] 5558.2 2801876 50.4 roughly 3.5 days. All times in the table were measured on Sun SPARC 10 workstations. 71 5. Numerical Results 5.1 General In this chapter a number of numerical examples of in-plane and out-of-plane respons- es of laminated cantilever plates are presented. Various types of loading, fiber orientations and stacking sequences are considered, and displacement, strain and stress responses are computed. In addition, the twisting effect due to shear coupling in unsymmetrical laminat- ed composites is discussed. The basic units of length and force are taken to be inches and pounds. 5.2 In-Plane Loading 5.2.1 Extensional loading To study the effect of material nonlinearity on the response of a composite laminated plate loaded in extension, a cantilevered plate made of Boron/Epoxy Narmco 5505 was considered. The plate was modeled with nine finite elements and excited by the boundary loads shown in Fig. 5.1, 5.5 and 5.9, with P being a zero-mean white noise excitation. A white noise has a spectral density function (SDF) that is constant at all frequencies. i.e., S((o) = so , —oo 50001b2 see. This 86 is because for very high load levels, the shear response exceeds the range for which the ap- proximate fifth-order shear stress-strain law is applicable. TABLE 5.7 first five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level so = 4000 11:2 sec. [30°] [60°] Mode First Iteration Last Iteration First Iteration Last Iteration 1 n 25.5 HZ 24.7 HZ 15.7 HZ 15.5 HZ 2 52.8 50.6 47.6 46.7 3 96.7 95.5 74.5 72.0 4 139.4 133.2 95.2 94.0 5 170.5 166.1 101.5 98.5 TABLE 5.8 Number of iterations required for convergence Lead by“, so No. of Iterations (lb 3) [30°] [60°] 1000 4 4 2000 5 4 3000 5 4 4000 5 4 5000 3 4 6000 12 4 7000 21 5 The variation of the absolute shear strain in the material coordinates at the center of element 2 with excitation load level is shown in Fig. 5.16. The responses for the [30°] ply exhibits more nonlinearity than that of the [60°] ply. For So = 40001b2 sec, the r.m.s. shear strain is about 0.0062 for the [30°] ply. The peak shear strain would be expected to be in excess of three times the r.m.s. strain, which places it near the upper limit for which the fifth-order shear stress-strain law of Fig. 2.2 is applicable. For shear strains exceeding 0.02, the fifth-order law increases very rapidly, resulting in poor convergence. 87 The variation of the normalized r.m.s. y-displacement of the corner nodes with the ex- citation spectrum level So is shown in Fig. 5.17. The figure illustrates that the proportional increase in the displacement due to the nonlinearity increases with the excitation level. The apparent increase in stiffness of the [30°/-30°] laminate as So exceeds 4000 lb2 sec is again due to the fact that the approximate fifth-order stress-strain law breaks down for the high loads. Figs. 5.18 and 5.19 show the normalized r.m.s normal and shear strains and stresses in material coordinates at the center of element 2 with the excitation level. The nonlinearity in the constitutive law has a significant effect on the shear strain for the [30°] ply (about 13%), but it shows negligible effect for the [60°] ply (less than 2%). Nonlinearity has a neg- ligible effect on all stresses. 0.007 I I r , — a: 300 ‘ —- a: mo 0.006 - a0.005 - d b I E0004 - - a b ”3 0.003 - u i 0.002 - _ om“ —————J P ..., —————————————— * 0.0 —-"‘""'1” 4 . 1 . 1 . 0 1000 2000 3000 4000 Input So (“32 sec) Figure 5.16 Variation of absolute r.m.s shear strain at the center of element 2 with excitation load level 88 G - I ll «50 c. l. 1 F503: at: :4 m2 Va SUN—.1352. 2 Tile v..C_r..:.r... m2 Hm 30.12.3223le no 1.1 I I I — ar=30° --- 0:600 ‘5 o 1.08 - - E 8 .2 8‘ :6 1.06 "' 2”; g 1.04 - - 8102 ,.. ”’.—,” .. 2 ’II” P ””’ 1 1.0 ’II" 1 1 1 0 1000 2000 3000 4000 Input 30 (1h2 sec) Figure 5.17 Variation of normalized rms. displacement at free comer nodes with excitation level _ Eu {01' a= 30° —" C" for a= 60° -—' 7.2 for 0: 30° m - - 112 fora=60° ..... —. .5112 ------ enfora=30° ./-—- —————— _ —- enfora=60° If. ./' 331.08 - ,.z _ 1:11.04 - .8 '3 a 1.0 E 0.96 - 1 0.92 - . . 1 _ I l 1 0 1000 2000 3000 4000 Input So (le sec) Figure 5.18 Variation of normalized r.m.s. strains at the center of element 2 with excitation load level 89 I aUaanfifi $.21 305:.3: .32 int 351' It) (111 iii dis Iht‘ lei 1.002 I fi I I -— an for at: 30° J —- 01110! a: 60° ’" LWIS °—° 7'12 for a= 30° .’,/' d "' - T12 {“03 60° .’,/" ------ an fora: 30° ’./” 1.001 —' "22‘0”“ 60° ,,-” ............... A Normalized RMS Stresses Input So (le sec) Figure 5.19 Variation of normalized r.m.s. stresses at the center of element 2 with excitation load level Case 11: Three-ply laminated plate with fiber orientation of or, 0 and —or in top, middle and bottom layers, respectively, as shown in Fig. 5.17. The total plate thickness is the same as in Case LThe level of excitation was increased from 5000 to 300001b2 see. Table 5.9 shows the first five undamped natural frequencies from the linear and non- linear analysis (first and last iteration) of the three-ply laminated plate for the load spectrum level So=30000 lb2 sec. Due to the softening effect of the shear nonlinearity, the natural fre- quencies show about 3 to 7% decreases. The laminate with the [30°I0°/-30°] arrangement is stiffer than the [60°/0°l-60°] laminate, as indicated by the higher natural frequencies. Table 5.10 shows the number of iterations required for convergence of the r.m.s. nodal displacement. For the [30°/0°l—30°] laminate the number of iterations remain constant with the excitation load level, but the number of iterations increases with the excitation spectrum level for the [60°/0°l-60°] laminate. A large number of iterations are required for the [60°/0°! 90 \ s / 2 / 5 / . / a s 1 4 7 ‘1‘” § 0=ct° § e=0° § 0=-or° s s 15‘ -60°] laminate at so=2s,ooo 1h2 sec. This indicates that the fifth order shear stress-strain law is not applicable for the [60°/0°l-60°] laminate at any load level about and beyond 25000 Figure 5.20 Tree-ply laminated plate loaded in shear 1b2 sec. TABLE 5.9 first five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level S0 = 30000 lb2 sec [30°/0°/-30°] [60°/0°/-60°] Mode First Iteration Last Iteration First Iteration Last Iteration 1 EL 23.4 HZ 21.8 HZ 18.0 HZ 17.7 HZ 2 81.6 79.7 65.3 64.0 3 101.6 100.4 80.7 78.1 4 147.9 137.7 110.0 106.6 5 261.3 253.6 215.3 213.3 91 TABLE 5.10 Number of iterations required for convergence Load bye], 50 No. of Iterations (”’2“) [30°/0°/—30°1 [60°/0°/-60°] 5000 4 5 10000 5 6 15000 5 6 20(X)0 5 6 25000 5 9 30000 5 14 Fig. 5.21 shows the variation of the absolute r.m.s. shear strain in material coordinates at the center of element 2 with excitation load level. The responses are clearly nonlinear. The effect of nonlinearity is comparable for both the [30°/0°l-30°] and [60°/0°l-60°] lami- nates. 0.008 . . fl , . , - , —- a=30° 0.007 '“ “=60 4 . ,,’: 0.006 - /”’ - G r /’/’ ea // 1:10.005 " /// '- m _ / 1a //’ 00.004 - / - £3 / m D / m // 50.003 - // a » / 0.002 - / - 0.001 - ., . 1 0.0 4 j 1 I a L a I a l a 0 5000 10000 15000 20000 25000 3000 Input 30 (le sec) Figure 5.21 Variation of absolute r.m.s. shear strain at the center of element 2 with excitation load level 92 the 3‘51 re: 00 uchF- firu-u—fhi -¥\ 0‘1a ’\€.l U I I- . EIII! ~l The variation of the normalized r.m.s. y-displacement due to nonlinearity with exci- tation level, So, is shown in Fig. 5.22. There is a steady increase in the displacement with the excitation spectrum level due to the effect of nonlinearity. The fifth order approxima- tion is not applicable for the [60°/0°l-60°] laminate at load levels above200001b2 sec. Fig 5.23 shows that the nonlinearity in the constitutive law has a significant effect on the shear strain of the [30°/0°l-30°] laminate and a negligible effect on the shear strain of the [60°/0°l-60°] laminate. The normalized normal strain in material direction 1 (all) is rel- atively insensitive to nonlinear effects, whereas the normalized normal strain in material di- rection 2 (622) shows a significant increase for both angles. Fig. 5.24 shows that nonlinearity is again negligible for all stresses. 1.08 ’ I V l ' I ' l -— a=30° —- a=60° ‘1'? g 1.06 - - .9. O. .."3 ______ \ 'O // \\ / \ 3“” “ ,/ N. / // § / .... // a 1.02 - / a Q / Z // / / 1.0 n l a L n L A J 1 0 5000 10000 15000 20000 25000 Input 30 (le sec) Figure 5.22 Variation of normalized r.m.s. displacement at free comer nodes with excitation load level 93 H1..-11.1.li..|.|..l l. m5..5.5V. m2“ UQNIflCF—CZ 1 0. 0. ”Hug 1|: 1|... 1|,“ F1 aOmaUu-W Viz“ VONZQFCLCZ , I , '— Cu fora=30° —_ _______-_: 1.16 —" E“ fora=60° .’././ . 1.14 '_' 712f0ra=30° /././ _ rn "" 712f0fa=60° ./. - ‘ ‘3 ° / /- -—- -— ~- ‘ n81.12 ----- fizzfora=3o .I. /. _”.: 1:: ll —- enfora=60° /.< /' ..................... . (I) - /- / .......... _ m ’ /'/‘ ........ . 1. L /" _______ q 08 _ //../ ......... % ul.06 - -/. / ..... .. 0.96 - “\___ ________ _____—— l l l . 1 . l . 0 5000 10000 15000 20000 25000 30000 Input so (1b2 see) Figure 5.23 Variation of normalized rrn.s. strains at the center of element 2 with excitation load level E E 3" O 0.9998 Normalized RMS Stresses 0.9996 - l 1 J A l 0 20 40 60 Input So (1b2 sec) Figure 5.24 Variation of normalized r.m.s. stresses at the center of element 2 with excitation load level 5.3 0U1 “'16 the same z-diI‘ECtil Ca used. Fl /////////////////; ’7”, - and nc ply ha Stiffer 116in t a1 dii load 5.3 Out-Of-Plane Loading The same cantilever plate used in the in-plane extensional loading is considered with the same material properties and the same geometry, but now the out-of-plane DOF in the z-direction are loaded. The level of excitation was increased from 5 to 80 lb2 sec. Case I: One-ply plate with fiber orientation at. Two values of at, 30° and 60° were used. Figure 5.25 shows the plate and the loaded DOF. >1 Figure 5.25 One-ply plate with fiber orientation 0: Table 5.11 shows the first five undamped natural frequencies of the plate from linear and nonlinear analysis for the excitations level corresponding to So = 801b2 sec. The [30°] ply has higher natural frequencies than the [60°] ply which indicates that the former ply is stiffer. Due to the softening effect of the shear nonlinearity, the natural frequencies show negligible decrease for the [60°] ply, and a slight decrease for the [30°] ply. Table 5.12 shows the number of iterations required for convergence of the r.m.s. nod- al displacements. The number of iterations remains approximately constant for the range of loads and comparable for [30°] and [60°] 95 TAE ter: of e} give 1th [on ma]. chat resu TABLE 5.11 First five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level So = 80 lb2 sec [ 30°] [60°] Mode First Iteration Last Iteration First Iteration Last Iteration 1 p 25.5 24.6 15.7 15.4 2 52.8 51.3 47.6 47.1 3 96.7 95.5 74.5 71.7 4 139.4 135.8 95.2 93.7 5 170.5 166.6 101.5 99.3 TABLE 5.12 Number of iterations required for convergence Lead by“, So No. of iterations ("’2“) [30°] [60°] 5 n 3 3 10 3 3 20 4 4 40 4 4 60 4 4 80 4 4 The variation of the absolute r.m.s. shear strains in material coordinates at the center of element 2 are shown in Fig. 5.26. The responses are clearly nonlinear. Note that for any given excitation level, the shear strain for the [30°] laminate is less than that for the [60°] laminate. The variation of the normalized r.m.s. z-displacement of the comer nodes with the excitation spectrum level, So, is shown in Fig. 5.27. The nonlinearity is less pronounced for the fiber orientation of [30°] than for [60°]. The variation of the normalized r.m.s. nor- mal and shear strains and stresses in material directions at the center of element 2 with ex- citation level are shown in Figs. 5.28 and 5.29. The nonlinearity in the constitutive law results in a slight increase of most strains and stresses. 96 0.012 Ill 0 0 00: Atll ] .It. WU. n. 0 Hum—«haw hdUn—mfl met—2N— thu 2.05.0057—37 r02! UON:-::CZ 0.012 r v I fi fi -— a= 30° —- a- 60° 0.01 " -l i- ’I’4 ’ P 0.008 - //”’ - I V) II/ a //’ 00.006 P // ' a / m / 50.004 "' " . /’ l // 0.m2 " / d o / 0.0 a I L I A l n 0 20 40 60 80 Input So (lb2 sec) Figure 5.26 Variation of absolute rrn.s. shear strain at the center of element 2 with excitation load level §§§ ‘é Normalized RMS displacement s E .H O H — a: 30° . —- a= 60° / ———————— /// // ' // u / / 1 / .. / / / / 'l l" -t / / D / d / , / / I A I A I A 0 20 40 60 80 Input So (1b2 sec) Figure 5.27 Variation of normalized rms. displacement at free comer node with excitation load level 97 1.12 . . . . . _ €11 fora: 30° ‘ _" Cu fad: 60° 1.1 "" 712 fora=30° .- rn - — 712 f0fa= 60° l”—____- '31 03 """ ezzfow=30° I’,’ _ b ' —' £22 for a= 60° //’ 106 " // ’ ’ v a’ d E // ’ ’ ’ .......... .::_ ..g-q U // ’ I ’ ......... ;.:-...’u ..—— —— 31.04 — // ’ ’ ’ ,lI” " a r- / I I ........’.o...’.’o H102 /f’ ’ ..n,"I’ - ° ' _ / I 5" __- _- __ __ _- _- _. -_ Z _ ”4;...57 _.- .. -—-- -- 1" ’- ...- I... 1.0 L 0.98 ‘ 1 n 1 L 0 20 40 60 Input so (11)2 sec) Figure 5.28 Variation of normalized r.m.s. strains at the center of element 2 with excitation load level 1.12 I l — an fora=30° —- a“ fora: 60° 11 ’_' Tu fOfO=30° q § - - 1'12 for a= 60° 0 “1.08 """ 022 fora: 30° .- g —- an forar=60 ”‘ m afffld” ‘ $1.06 - //// 1 /// ....--°--'::.‘ '° / ----------- ::;.--—-—' '- / .......... ___..—- .81“ /// ...o...._:.’.:'.’ ’ ’I' - - ’1 / ........ 0’ ’ ........ 1 / ...... ,"’ — a v "’ '— Ig // ...... -°,". '1 .. -- ' 81'02 - // ...... :a‘ .— -- "' " - —_- __ _- __ __ _: z b // ..vé'fwzkj .——- ——-- ’- ...- /’fl£- " 10 “—— .. 0.98 ‘ J m I L 0 20 40 60 Input So (lb2 sec) Figure 5.29 Variation of normalized r.m.s. stresses at the center of element 2 with excitation load level 98 Case 11: Two-ply laminated plate with fiber orientation or and -0t in top and bottom layers, respectively, as shown in Figure 5.30. The total plate thickness is the same as in P1 Case I. 15" _ VI Figure 5.30 Two-ply plate with fiber orientation (1 Table 5.13 lists the first five natural frequencies from linear and nonlinear analysis for the excitation level So = 80 1b2 sec. Due to the softening effect of the shear nonlinearity, the natural frequencies show a slight decrease. Note that the laminate [30°/-30°] is stiffer than that of [60°/-60°] as indicated by its higher natural frequencies. Table 5.14 indicates that the number of iterations required for convergence of the [30°/80°] laminate are comparable to those of the [60°/-60°] laminate. The number of iter- ations is approximately the same at all load levels. Fig. 5.31 shows that the r.m.s. shear strain exceeds 0.007 at So values of about 75 lb2 sec for the [30°/-30°] laminate and about 45 1b2 sec for [60°/-60°] laminates. As described earlier, for load levels in excess of these values the approximate fifth-order shear stressj strain law breaks down. TABLE 5.13 first five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis rot load level 30 .. 80 11:2 sec [30°/—30°] [60°/-60°] Mode First Iteration Last Iteration First Iteration Last Iteration u 1 24.0 112 23.1 HZ 15.6 HZ 15.2 112 2 74.5 73.3 46.4 45.8 3 92.2 91. 86.7 85.3 4 152.5 148.2 95.0 92.7 5 215.8 212.6 103.7 101.4 TABLE 5.14 Number of iterations required for convergence Load 19"., so No. of Iterations “b2" [30°/40°] [60°/—60°] 5 3 3 10 3 3 20 4 4 40 4 4 60 4 3 80 4 4 Fig. 5.32 shows that nonlinearity causes the r.m.s. z-displacement of the comer nodes to increase by about 6% for the [30°/-30°]laminate and about 3.5% for the [60°/60°] lam- inate at So values of 80 and 60 1b2 sec, respectively. Figs. 5.33and 5.34 shows the effect of nonlinearity on the normal and shear strains and stresses. They indicate that the nonlinearity in the constitutive law significantly affects most strains and stresses. 100 0.012 . . , . l f ——agm° —- a=60° 0.01 " J ’z’I” ‘ G // 130008 '- ///’ a // 30.006 - // - m /// VJ // 50.004 - // 1 / / / 0.002 l- / .l o / 00 I 4 n l . I . 20 40 6O 80 Input so (11:2 sec) Figure 5.31 Variation of absolute rms. shear suain at the center of element 2 with excitation load level 1.07 I l f —- a=30° . —- a=60° 5 g t ..‘3 1.05 a Q h .m 6 1.04 - ,__~ - m y”’ “~~“. / / E 1.03 .. // .. g _ / o // Erm- // - o ' // Z _ / .. 101 // P / 1.0 I . I n I . 20 40 60 80 Input So (le sec) Figure 5.32 Variation of normalized r.m.s. displacement at free comer node with excitation load level 101 1008 I ' I ' I _ C“ fora= 30° -- enfora=60° . '—' 712 fmfi30° - " 712 fora=60° ------ e22 fora: 30: O, "' -- e22 fora=60 /,/ § E I" O N Normalized RMS Strains 1.0 Input so (1b2 sec) Figure 5.33 Varariation of normalized runs. strains at the center of element 2 with excitation load level 1.12 I I I q I — an fora=30° -- unfora=60° 1'1 '—' 1'12 fora=30° ‘ o m - ‘- 7121.01'0360 "—~~_: 8 ----- a fora=30° ,I” ”108 22 ” - o - __ e ” I: unfora=60 / m // .— —- " " .- ----- d 0,106 " // r "' ’ ‘- ’ "' / , ’ E h / ’ oooooooooooo 2H / ’ ’ ......... :2.“ __..—— ”10:“ / ,I’ ----------- .21: 531’” .- E // ’ ’ ’ ..... ;.;;..’.’. /// I I ...,;“L:"' 1 E102 l- //’ ’ ..o0":...’.:’ .1 E ,”.---I"’ . ...- .—-- —- —- —- —' —- —- _. - /, .v/°""_..- I" "" '— ‘ ’53: ' 1.0 4 ‘ 0.98 a I a L a I 0 20 40 60 Input So (le sec) Figure 5.34 Varariation of normalized r.m.s. stresses at the center of element 2 with excitation load level 102 Case HI: Three-ply laminated plate with fiber orientation of at, 0 and -0t in the top, middle and bottom layers, respectively The total plate thickness is identical to that used in Cases I and II. =I Figure 5.35 One-ply plate with fiber orientation a Table 5.15 shows the first five undamped natural frequencies of the plate from the first and last iteration of the analysis, for the excitations level So = 80 lb2 sec. The values from the first iteration correspond to the case where nonlinearity is neglected, while the values from the last iteration show the effect of nonlinearity. Due to the softening effect of the shear nonlinearity, the natural frequencies show slight decreases. The ply arrangement with [30°/ 0°/-30°] is stiffer than the arrangement with [60°/O°/-60°], as indicated by the higher natural frequencies for the former case Table 5.16 shows the number of iterations required for convergence of the root—mean- square (r.m.s) nodal displacements according to the criterion in Eq. 3.50 with a = 10's. The number of iterations required for convergence increases with the excitation load level 103 TABLE 5.15 First five natural frequencies from linear (first iteration) and nonlinear (last iteration) analysis for load level So = 80 lb2 sec [30°/0°/-30°] [60°/O°/—60°] Mode First Iteration Last Iteration First Iteration Last Iteration 1 23.2 HZ 22.8 112 18.0 HZ 17.7 112 2 81.4 80.8 65.3 64.6 3 101.5 100.9 80.7 79.4 4 146.9 143.4 110.0 108.6 5 260.6 258.2 215.3 214.4 TABLE 5.16 Number of iterations required for convergence Load I”... So No. of Iterations (”’2“) [30°/0°/-30°] [60°/0°/—60°] 5 4 3 10 4 3 20 5 3 40 6 3 60 5 3 80 6 3 since the nonlinearity becomes more pronounced for higher loads. For any particular, load level the number of iterations required for the ply arrangement with [30°/0°l-30°] is more than the corresponding number for the [60°/0°l-60°]1aminate. This indicates the nonlinear- ity is less pronounced for [60°/O°/-60°]. The variation of the absolute shear strain in material coordinates at the center of ele- ment 2 with the excitation load level is shown in Fig. 5.36 and the responses are clearly non- linear. For any given excitation level, the shear strain for the [60°/0°l-60°]laminate is significantly less than that for the [30°/0°l-30°] laminate. 104 The variation of the normalized r.m.s. z-displacement of the corner nodes with the ex- citation spectrum level, So, is shown in Fig. 5.37. The displacement is normalized dividing by the linear response (fer which [K'] = [0] ). The figure illustrates that the proportion- al increase in the displacement due to nonlinearity steadily increases with the excitation level. The effect of nonlinearity is less pronounced for the [60°/0°-60°] laminate than for the [30°/0°/~30°] laminate. This is because the smaller shear strains in the former case re- duce the overall level of nonlinearity. The variation of normalized r.m.s. shear and normal strains and stresses in material coordinates at the center of element 2 with the excitation level are shown in Figs. 5.38 and 5.39. Again the norrnalizations have been performed by dividing by the corresponding lin- ear responses. The nonlinearity in the constitutive law results in a significant increase in the shear and normal strains and stresses for the [30°/O°/-30°] laminate, but it does not affect the normal strains and shear strain for the [60°/0°/-60°] laminate. 0.012 r 1 f I I — a=30° 4 —- a=60° 0.01 - . l' £3 0.008 - ‘ b ”‘ U) ’,-I" 530.“)6 '- ””’ " /’ m //’ 50.004 - /,I’ - / I // // 0.002 " / - 00 a l . l . I . 0 20 40 60 80 Input So (lb2 see) Figure 5.36 Variation of absolute r.m.s. strain at the center of element 2 with excitation load level 105 1.1 :- t- :- 2 8 8 Normalized RMS displacement S N 1.0 Input So (lb2 sec) Figure 5.37 Variation of normalized r.m.s. displacement at free comer node with excitation load level .—I O u— N H U y—e f" O 0 Normalized RMS Strains '2 § 1 .02 — e" fora=30° —- E" fora: 60° °—° ‘yufora=30° - - 7,2 four-60° '''''' e22 fora=30° -" £22 fora=60° I 1 .43'4 .4". .4" .4" " 0’s... .4". .4" .I.-' 04.. - '4'. .4” .fi" ./... - Input so (le sec) Figure 5.38 Variation of normalized rms. strains at the center of element 2 with excitation load level 106 :- :- r- :- 2 8 8 E 33 b N Normalized RMS Stresses Figure 5.39 _ U“ for a= 30° '_"' U" for a= 60° Tn for a= 30° r12 rota= 60° ------ on for a= 30° 022 for a= 60° O O .. 107 Variation of normalized r.m.s. stresses at the center of element 2 with excitation load level lililll i rim Frill! Table 5.17 provides an overall summary of the responses for which nonlinearity is important for the Narmco 5505 material. TABLE 5.17 The significance of nonlinearity on the responses of various loading conditions Responses for which nonlinearity is significant Loading Stacking Sequence Low Moderate High less than 5% 5 - 10% more than 10% [30°] 011: 022, 811 : 822 712: 112 ‘1 [60°] 011: 022: 811 : 822 712,112 '1 III-plane [30°/30°] 911: 522: 311: 712: 112: 922: 0 ”mm“ [60°l-60°l 022: 1512 712 011: 311: 922:“ [30°/WW] 311: £22: 712: 011: 022: 112: II \ [60°/0°l'50° 1 811: 1522: 712: 0'11: 022, 112: '1 [30°] s11: 622. 011: 112: 022 V 712 Impm [60°] E11: 822: 712: V: 011: 1312: 022 “W [30°/0940"] 0'11: 112: 022 Bin" 922’ 712 \ [60°/0990"] 811: 712. 011: 1:12: 022 V 822 [30°] 911: 822: 712: w: 0'11: 112: 022 [60°] 822, 012: 022 712: w: 011 811 Out-of- [30°/'3OO] 222,011 8111271322“ plane [60°/~60°l 811: 822: 712: W: 022 011: 1512 [30°/0°I-30°] 011:311:112:W 022, 822: 712 \ [60°/Wm 011,022, 112: 311: 922: 712: W 108 6. Conclusions and Recommendations 6.1 Conclusions A general formulation for the nonlinear random vibration analysis of laminated com- posite plates modeled using finite elements and classical plate theory is presented. Only nonlinearity in the shear stress-strain law, which is most significant for filamentary com- posites, is considered. An approximate representation of the non-linear shear stress-strain law in terms of an odd-powered polynomial of arbitrary order results in a tractable formu- lation that is sufficiently accurate for practical purposes. The solution is performed itera- tively using linear random vibration analysis during each iteration. Although classical laminate theory is often inadequate for composite laminates, it was used as a starting point in this study. An overview of the finite element discretization is presented. The plate ele- ment considered is a four noded one having 5 degrees-of-freedom per node. The bending-extension coupling which always exists for unsymmetrical laminates was investigated. The numerical examples presented indicate that the effect of nonlinearity on the responses for any given load level depends on the ply-arrangement, and as expected becomes more significant for higher loads. The responses were computed for Narmco 5505 material, and root-mean-square displacements and strains were found to increase as much as 20% for certain ply arrangements and loads. For other composite materials with different degrees of nonlinearity, the results could be significantly different. To realize the above mentioned objectives, a computer program was developed and implemented for the nonlinear random vibration analysis described above. Several optimi- zation techniques were developed and used, including an efficient indexing scheme using a variation of the pascal triangle, for the computation of the covariance matrix of the nodal displacements. The multidimensional symmetricity in the calculation of the covariance ma- trix was fully exploited to drastically reduce the actual number of computations. The for- mulation presented herein requires the use of a large number of nested loops. Testing for 109 zero entries of the strain-displacement matrices within the outer loops made it possible to skip a number of intermediate nested loops and thus eliminate a large number of unneces- sary computations. While the above mentioned optimization techniques were able to great— ly reduce the computational time, it may still be unacceptable for large problem. Parallel processing techniques appear to be well-suited for this type of analysis and are discussed in the next section in the context of recommendations for future work. 6.2 Recommendations for future work The amount of computations for the nonlinear random vibration analysis presented in this work tends to increase rapidly with the increasing size of the problem. For large struc- tures, and more general formulations involving shear deformation and interlaminar shear stresses, the calculation of the nonlinear stiffness matrix discussed in Chapter 3 is even more computation-intensive. Even with moderate size problems, the computations become excessive on currently available uniprocessor workstations. For practical analysis using finer finite element idealization, the most promising computers are supercomputers and massively parallel machines. While supercomputers can greatly speed up all vector and ma- trix operations, parallel computers offer great potential in the near future. There are a number of stages in the analysis described in this work where parallel computation may be effectively employed. Some initial ideas along this front are explored. There are three stages during each iteration in which intensive computations are re- quired. Possibilities for parallelizing each of these stages are discussed below: 1. The use of modal analysis requires the solution of an eigenvalue problem. The eigensolutions vary only slightly from one iteration to the next, and may be com- puted from the old ones using the inherently parallel homotopy continuation meth- od. The solution of the eigenproblem for the very first cycle may also be parallelized using this method (Zhang and Harichandran 1989, Chu 1984). 110 2. The coefficients 01 and B needed in the computation of the nonlinear stiffness ma- trix must be evaluated at each Gauss point for the numerical quadrature. Since the coefficients are estimated on an element by element basis, elements can be divided into as many groups as there are processors, and the computations for different groups of elements can be performed in parallel. 3. When a large number of modes, u, must be considered owing to their natural fre- quencies being in the dominant excitation frequency range, considerable time is re- quired to compute the integrals corresponding to the n (n + l) / 2 pairs of modes in order to evaluate the r.m.s. responses. Since each integral is independent, the com- putations can be performed in parallel. While the techniques outlined above do provide for parallelism, they also require a fair amount of communication between processors. In step 1 the system matrices must be ac- cessible to all processors, and in step 2 the nodal covariances for each group of elements needs to be accessible to the corresponding processors. Therefore, while the proposed schemes should be efficient for shared memory computers, they may need to be further en- hanced for scalable distributed memory computers. Research and implementation is needed to address these questions. The classical plate theory used in this study can yield significant error even for mod- erately thick composite laminae because transverse shear deformation is neglected. It is well known that transverse shear deformation is significant for thick plates, and this is es- pecially true for composites since the shear moduli of polymer matrices are significantly lower than the extensional moduli. While the first-order shear theory (Reissner 1945, Mindlin 1951) is adequate for plates made of conventional materials, a higher-order shear is usually required for composite laminates (Reddy 1990, Noor and Burton 1989). Even higher-order shear theories are usually adequate only for global modeling (i.e., prediction of displacements, natural frequencies and buckling loads), and are not sufficiently accurate for stress field computations. Local layer-wise models that represent each layer as a homo- 111 geneous anisotropic continuum are usually required for accurate stress computations, but these often magnify the size of the problem. For random vibration analysis, both global and local models are of interest since one or the other may be applicable in a specific situation. It appears therefore that separate solution schemes using both types of models should be developed. Composite laminates are typically used in either plate or shell configurations. The analysis using laminated shell elements is much more complicated than that using laminat- ed plate elements, primarily because their geometry is more complicated. Although com- posite shells are widely used in the design of aircraft and automobile components, very little work has been done on the nonlinear random vibration analysis of elements made of composites. 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"The finite element method for analysisof elastic isotropic and orthotmpic slabs,” Proceedings of the Institute of Civil Engineers, 28, 471-488. 115 APPENDIX PROGRAM 116 .ou.uovc.uovH.U«.>ouunu.nuou.Hu:¢.uu.onuva:Huou Hana .usnca uous you H m n u a...“ on ozu on Dzm 00.0 I “D.Hvxuun 00.0 I AU.H.Buu H.0w.dub on «Hava.o I AHV>ouu a.oo.HIH on A.c30:x=:.nu:unun.ennuuonuu.mauuwadvcoao .moEdc caau uuouenunn usacH. 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I .503 .5.) I 0500 00:0 ..x.) .00. .503. «5 5.2:.5I8 00 5.5IEc.5I5 o0 00350> 00050 «0 0050000 00 000 .500008 \ .H.uu50 I .503 5.2:.5I5 00 55005.. unnwou Bouu 550u5 0:9. .I.N5.005u3 .550u5.0005.ec .N.0006.auon.5u50.0u50.5000.fl¢.nn0.EC.000.EC.unnmou 5500 .Oflhu. I NUQE 0035m> c0050 0030E00 00000 00000 00000 00000 00000 00000 00000 00000 129 oo.o . .n.«.u a.ow.«-fi on H.oo.fiua oo on uco on one on vac .n.x.::u . .fl.x.uu>o«o + .fl.«.u . .fi.«.u a.oo.a.x on oo.o I .n.«vu H.ou.fiuh on a.oo.fi.« on .ow.oo.u ..oo.ow.u .Aow.oo.uu>u«o ..oo.ow.:xu couucoaao An-o.£-ov mud-o» uanHnfia Ame m AuU>an.xxu.neso onuusoundu 02m zmbhmm ...Nflvouah3 Eduo\E:n.. n ..ESuo..\ ..Edu.. I Esaoxada. ...«avouauk .uuaOu oucouuo>cou. «I.Na.ou«u3 A. .NHVOU.—h3 «I.Ndvou«h3 «a new on 92m .H.H.>ou I AH.>ouu H.ow.aIH on «can .osuu. I Dah>co coca .m-o.a .ua. 25:0 \ Bauv u“ .29uo.uuvu I Educ ASSuvuuoo I Eda on 92m ~...H.H.>ou. ~c..H.>oou o Sana I ESmo NOOAAHV>OUU I AH.HV>OUV 4 630 I EDI a .ow .a I H on 00.0 I Esme oo.o I Eda u“ 0:0 zmbpmm nou>cu..a ouu>cu AI 06“» uuuau. .c...ouau) .OIHau. I Eaunuu can» .Equuuu. u“ .ouaau. I vou>cu 0826 .8393 goofing .oo.>oou ..ow.ow.>ou cofincoaav .u-o.:-a. ceauauoun cannon uaUaaOfia .Uou>cu.EHuauu.EuHm0«.>ouu.>ou.>:Uu¢b ocuuaoundu Dzm zmbfimu “A. ..N.m0.m.xH.unfihou 00 0:0 00 0:0 00 0:0 00 0:0 ..!.u«.v«.~x.oa.cw.x>ou I .=.x.u«.0>oo H.0N.HIE 00 H.o~.an on nooaaua u.>:.nun 00 H.xE.uIH on once .o>ou ousofiou. ...c.0uwu3 av 0:0 H+Odl¢d on one aoowuvd .o«..~.>ou - .n.H.x>oo a ..H+xE.u.mI.a+>=.. .a+.mc.a+>n..In 0U a I 0« fl ..H+xevc.mc.a+>:.. .ao.mc.uo>:..IH at «and .x>ou OuSQEOU. ...c.0unh3 00 U60 0° ”CO 00.0I.5.H.x>ou n ..H§xfivo.mc~u+>cv..auh 0v H .A«+xfivc.mcfia+>:...alH ow .x>ou uwCH. ...I.0uwh3 u“ on» on azu .H.H.5¢Hmu«.o>ou I .H.oo>ouu a .o~ .fi a H on can» .EIUIIu .uo:.. "a unug.uun:ofi.uonfi.mu>H«.auHmU«.ec.>:.XE \anwx CQanu Ao~.oo>00u cowucceav flo~.m~.0«.«o~.o«.m~.v>ou.hoo.om.x>oo..ow.oo.>ou cowacueav Saunuu HavauoH .uoo.:ua. mIH-uu uuUuaasu .va.Ewuuuu.Uo>oou.o>ou.x>oo.>ou.o>ouu unannounaa 130 ~+Cwa I :«H H.0N.and on H‘ON.HI& on H.0N.AIH on H.0N.HIH on o I :«H .>uuua ocuxooc« ozu ouaaoum. ...o.ouau3 .>ouuo >m onu nu“) uuouuuo onu ousasou cu >muuc ucaxocc« on» ouanoum ...ou a“ I 309« .o>oo voyagEou uo nonesc Houou I >0 :« acwuuco uo hogan: x4: ...u on 02m on ozm 0o 92m 00 02m 09 92m .6.X.Eda0wvo>ou c AJ.H.EGH0«.0>OU + o Ad.n.EdH0w.o>oo I ax.H.EOHo«.o>OU + . .q.x.euaofi.o>ou . .n.H.eofioa.o>ou . .c«.aoaofl.>o « . a“ . ca H.o~.x.q on ~.°~.nax on a.o~.H.n on H.o~.H.H on o u a“ H.>c.xa.fl.eoHoH on .oafla> uouooaxm «usanuoua. .....ou«p) OD 02m 0o ozw oo 02m 0Q 92m 00.0 I .b.x.q.HvNauon H.v.fiux on on ozw 00.0 n .6.x.A.H.Hauo£ n.~.«ux OD H.om.aun co H.mN.an on H.a.HIH on on ozm on ozm 0O ozu 00.0 n AA.U.H.NQSQHI a.m.nnq OD “.mN.HIfi 0Q H.o.«IH on on ozm on cam on pzm ou.o I .q.n.uvaoznfla a.n.HIq on H.m~.nIo on H.a.HIH on .n.0§aqvma I AMVMBB .N.o:«q.na I .Nvmea .a.vc«q.na I Auvmee .OConUc« c“ o>au ou nab ou .QCaQ.n9 >300 0a 92m on nzm .a+d.H.aESU + AU.H+HVQE=h I Ab.H.QE:h H-.H.QHID on fiu.H.mIH on .mH.m.QESU .ma.N.QE5n Aoa.dvofisn .on.n.0§5fi .om.~vgfisn .o~.a.0&5n on 92m HnON I ~H.¢.QESH Hu.H.cNIH on OOOHHH ..>o. m unwxoUCa am on: “cu oHocmfiuu Hmuuam .oauo>ou. Ousoaou unaq.uuu:o«.uonw.au>flfl.auflmu«.ac.>c.xa \xan«\ cgagou .n.mea .Ao~.m.n:nu ..o~.m.~cpu coaacosuo ...uuoaa ..ooooma.uauuofi .Aom...oasn ..ooooa.m~.>o coaucoeIu .o~.v.m~.m.~nuon ..o~.~.m~.m.fiauon coaucoe«o .m.m~.m.~anaHI ..m.m~.a.aonnfla coaucosau .m.~u ..m.m.ma .Ao~.m.m.mcn ..o~.n.a.~cn ..o~.o~.m~.o>oo coaucIEAc .mv>..m.x co«uco24o .u-o.:-u. n.44um anonquH A>Ix.Nu.mF.nCD.Nr—Q.O>OU .~auun.Hauon.~uzn~u.flanaflu.aumua< mzHaoommsm ozm :.19329§Z§3u 00 0:0 00 on» 00 0:0 .n.x.uu>0wo . .x‘«.u . An.«.u I .fi.«.u H.oo.HIx ov IoccU «ICU Hoa 131 .nKIMHVNGAu «dun lawn me.~h.Ncnu I .HK.«6.N¢AU I Hun o« I .HIHx. I ma co. I HHINx. I «a can» HFIv.o .uo. .an.mn.mcnuvunu. «H con» .n-u.o .uu. .an.nn.mcnuvuna. uH H .ou .H I n: on H .o« .H I «a on .mn.mae IHIu INIu Hmnvnea I .annea I H.» H .m .H I an on H .n .H I an on .ax.~n~«:nu I “Ha.an~:nu IHIn coon I .HIHx. I Ha coo I HHINx. I «A can» AFIv.o .uu. ..Hx.Hn.mcnu.Ina. «H con» HIIv.o .uo. ..mu.~n.~cnu.aau. uH H .o~ .H I H: on H .OH .H I «a on H .H .H I H» on .mn.nea I HHn.naa IHIu H .m .H I «a on on cam coon I .HIHx. I HH on ozm can» Ho-o.o .uu. ..Hx.Hn.~:nu.InI. HH .n.H.oH.~:n I .n.H.~cnu H .ow .H I Hg on H.ON.HIn on H .n .H I Hm on H.m.HIH on o I cHH .oconccH cH o>II cu «an» ou .uH.ch anou ..IIU on ozm H.>chE.HIEoHoH on on ozm .>c x 33. oucosoHo HHI Ion IIIIu .n.H.uH.ch I .n.H.ncnu H.ou.HIn on uH..IuH "II-no. ...I.uuHI3 H.H.HIH on .oconvcH cH I>II ou menu 0» .uH.mcn zaoo IIIIu H I 0H I 0H H .325... .H I a on on azm H .IuosoH .H I xH on on nzm o I 0H 3 0:0 .559“. «mono no good IIIIU on ozw on ozm oo nzm uH one on ozm on ozm on can on can on can HH 0:» .....uuoua.v.aesn I HHH.IIoIH.H.assn I on ozm I ..~.uuoua.~.aszw I H.H.uuouH.qussn. I aoaH I .cHH.uIHuoH oa ozm mg m on ozm «H cco pH I HmH.quIH HH vac Hm4.uuoua I AonH.quIH Hvx.vn.chIIHInIHI>axIHIuIHH.aIHoH.uH.~IsaHIIHH.aoHIH.uH.~IanI mg m on ozm can» .HH-v.o .uu. .Ha>ax.uno. HH HH 0:» ..HH I «a I mg I Ix.uIHHoH.aoHoHv>o I Ho>ax onHH.quIH I I; coca Hp-o.o .uo. ...x.vn.~cnu.InI. HH mg I onH H .o~ .H I Ix on can» Aug .uH. .mxvuuouqv HH .vn.mee I HI» I may H .v .HImH I mx on H .H .H I In On .ma.uuoua I uH .nx.nn.chu I Hun I mun mg I onH OH I .H-nx. I ma H .n .H I an on cozy .I-o.o .uo. ..nz.nn.~cnu.ana. HH 4 I ...quIH H .o~ .H I 0. on x I .2983 .Hn.naa I HI» I «I» n I .~.uuoIH H .m .H I Ha on H I .H.quIH 132 .mx.~n.mcnu I .Hx.Hn.ncnu IHIn co. I HHIHx. I «a coca Hh-o.o .uo. ..nx.~n.ncnu.IAI. uH H .om .H I «x on Hun.naa I .Hn.nsp IHIu H .H .H I an on ooom I .HIHx. I Ha can» Hp-u.° .uo. ..Hx.Hn.m:nu.InI. HH H .o~ .H I H: on H .H .H I Hn on o I cHH HH can on ozu on azm “H oco ..x.vn.ncnuI m IHIn IHIII...EIHIH.uH.~InnHII.I.eIHIH.uH.~I;aHI ..Hq I Ha I H; I Ix.quHoH.eoHIH.>o I m cogu .p-n.o .uo. ..vx..n.mcnuvuna. HH H .0” .H I I: on ..nvnse .muu IHII H .n .H I In on .Hx.nn.ncnu IHun I~In OH I .H-Hx. I ma coca Hp-c.o .uo. ..mx.mn.ncnu.una. HH H .OH .H I n: on .Hn.Hee IHIu IHII H .H .H I n» on wa.~n.mcnu I .Ha.Hn.~:nu IHIn oo. I HH-~x. I «a coca .p-o.o .uu. ..mx.~n.ncnu.InI. uH H .oN .H I N: on .mn.mea I .Hn.Hep IHIu H .H .H I Ha on ooom I .H-Hx_ I Hg con» Ah-u.o .uo. ..Hx.Hn.~cnu.InI. HH H .o~ .H I H: on H .H .H I Hn on o I :HH on Qzu on 92w «a 0:0 «H cc» ..x.In.chuI m IHIn IHIII.H.aoHoH.uH.manaHIIAH.eIHoH.uH.HIIQHI ..HH I mg I mg I Ix.uIHHoH.EIHIH.>I I m can» .p-o.o .uo. .HIx.In.chu.InI. HH H .o« .H I Ix on .Invnea .«Iu IHIu H .n .H I In on .Hx.mn.ncnu IHIn IHIn cm I HH-nx. I ma cozy Hp-o.o .uu. ..mx.nn.mcnu.InI. HH H .on .H I nx on .Hn.naa IHII IHII H .H .H I ma on .mx.~n.~cnu I .Hx.Hn.~cnu IHIn co. I .HIHu. I Hg coca .h-u.o .uo. ..mx.~n.~cnu.una. HH H .o~ .H I mg on .mnvnep I .Hn.Hsa IHI» H .H .H I an on ooom I .H-Hx. I HH coca .I-v.o .uo. ..Ha.Hn.~:nu.InI. HH H .om .H I Hz on H .H .H I Hn on o I cHH on 92m 0o 92m «H 0:0 on 92m 00 DZE mu 0c0 09 02m OD Dzm «H 0:0 on ozw OD azw «H 0:0 Hvx.vd.mcnuI m Immn Inuu I AN.EGH0«.UH.NQ£QHNI.~.EUH0H.UHV~o£Qaa ..HA I NJ I mg I vxvuuquw.EoHoHv>0 I m cozy abofl.o .uo. Aavx.vn.mcnu.an~. aw H .ON .a I vx on .vhvnfih INuu Inna H .n .H u ch 09 133 .Nx.mh.mcnu I HHK.HDvncnu IHon 00' I .Hnmx. I Na cozy Abuv.o .uu. .HNx.Nnvmcnu.und. «H H .ON .H I fix on .thn99 I .annha IHuu H .H .H I Nn on ooom I .HIHxv I HA cozy Ahuv.o .uo. ..Hx.Hh.mcnu.undv «H H .ON .H I Hz 09 H .n .H I Hn on o I CHH OD 92m on 92m «H 0:0 on 92m 0o 02m «H van on ozm on 92m «H flco on ozu on can «H 0:0 .vx.vnvmcDUI m Imun IMQUIHQ.EOHOH.UvacnnHaIHv.EoHoH.UHvNunnHa HAHA I «A I mg I vxvuuuuoH.EoHoH.>o I m coca .hIU.o .uo. ..vx.v0.ncnu.unu. «H H .on .H I v& on thvmfih INuu Inna H .n .H I v6 on me.mnvncnu IHun Inna cm I .Humx. I ma cozy .hub.o .uo. .an.mnvncnu.undv «H H .om .H I mg on Hmhvnefi IHuu In.» H .m .H I n6 on .Nx.~d.mcnu I HHx.Hn.N:nu IHun 00. I HHIHx. I NJ conu .nufl.o .uo. .Hflx.Nn.mcDu.unu. «H H .0” .H I N: on .NUVMBP I .annfik IHau H .n .H I Nd on ooom I HHIHx. I HA con» 86-0.0 .uu. ..Hx.HD.N:nu.ana. «H H .ON .H I Hz on H .n .H I H6 09 o I cHH on 02m 0a 02m «H 0:0 on can on ozm «H as. on ozm on nzm «H use on azm on ozu «H on» .Ix.In.ch«I u IHIn InIuI.H.eIHIH.uH.«InaHII.H.aIHIH.oH.«IgnHI A.HH I «a I mg I vauu««oH.aoHoH.>o I m can» .p-o.o .uo. ...x.vn.mcnu.IAI. «H H .oN .H I .2 oo .In.nae .HIu IHII H .H .H I In on .nx.nn.mcnu IHIn IHIn o~ I .H-nx. I Hg cunu .h-o.o .uo. ..nx.nn.ncnu.una. «H H .om .H I mg on .Hn.Hae IHIu IN.» H .H .H I mm on .«x.~n.~cnu I .Hx.Hn.~cn« IHIn co. I .H-~x. I HH con» Hp-o.o .uu. ..mx.~n.~cn«.onuv «H H .om .H I mg on Amn.nea I .Hn.Hee IHIu H .H .H I no on ooom I .H-Hx. I HH :Icu .n-o.o .uu. ..Hx.Hn.~cnu.InI. «H H .ON .H I H: on H .H .H I Hn on o I cHH on azm 0o Dzw «H 0:0 OD sz OD 92m «H one on Dzm OD 92m «H 0:0 0o 02m on 02m «H 0:0 .vx.vw.mcnuI m INun Imau I HN.EOHOHIUH.NonaHuIA~.EvHoH.UHVNQSQHU ..HA I NJ I mg I oxvuu««OH.EoHoH.>o u m cozy .huv.o .uo. .Hvx.v6.mcnu.unav «H H .ON .H I vx OD thvmbb INnu unau H .n .H I v5 00 133 H.0N.HIQH on on nzu OD Q23 on ozm «H 060 OD 92m on 02m «H 0:0 on 92m on Dzm «H 0:0 .mXIMhVNcnuImINquHQH.H.EUHOHIUHvNauonIHQH.H.EoHoH.UHVNuuon ..HA I NJ I ma I nxvuu««OH.EfiHoHv>o I m cozy .buv.o .uu. HHMK.nn.Ncnu.nnov «H H . 0N .H I n: on ~N¥.thmcnu I anhvmfia IHnu “Nuu can“ «5-0.0 .uu. H .n .H I Mn 00 o~ I .Huflx. I ma HANMIthmcnuvuna. «H H .om .H I «x on HHx.HbvNcnu I Amnvneb I .Hhvmfifi IHuu can» Ap-o.o nua- H .n .H I ~n oo oo. I .H-Hx. I ma ..Hx.Hn.~cnu.InI. «H H .o~ .H I H: on H .n .H I Hn on ooom I .H-nH. I H; H.o~.HIaH on on nzm OQ 92m «H Use on ozm OD 92m «H 0:0 .~x.H:.EoHoH.o>ou IHNx.NO.ncnuIQUI.m.EoHoH.UH.HQ£QH0IHM.EUH0H.UH.annHa can» .h-o.o .uo. ..Nx. Nnvmcnu.nno. «H H.0N.HuNz 0O .Hx.HUvmcnu I .Nfivmsk I .annhfi I nu conu .hnfl.o IUD. H.n.HI~n on AAHx.HU.ncnu.¢na. «H HIONIHuHx on H.m.HuHD on on ozw 0o 92m «H 0:0 00 ozm 0o 02m «H 0:0 Hats— Ifl—OHOHvOerU I IANKINU.ncnuIauIHNIBOHOHIUHVHannHaIHN.EdHoHIUHangnHI coca Hva.o .uo. AHNIIND.ncnuv¢nI. «H H.0N.HIN¥ on HHxIHbvwcnu I Anbvan I .Hbvnab I nu H.n.HI«b on cozy Hhuc.o .uu. HHHzIHDVNcDuVInnv «H H.0N.HIHK on HIMIHIHO on on 02m on ozm ANv—IHMIEOHOHVOIIOU I IANx.Nn.NcnuuquHH.EoH0H.uH.HunaHauHH.EdHoHIUHvHOLQHa can» thc.o .uo. HHNX.«U.Ncnu.unm. «H H.om.HINx 0o .Hx.Hn.~cnu I .Nnvnpb I HHDVMEB I nu H.n.HINb on cozy Ah»o.o .uo. 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