An Introduction to Nonlinear Finite Element Analysis
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J. N. REDDY Distinguished P...
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An Introduction to Nonlinear Finite Element Analysis
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J. N. REDDY Distinguished Professor Department of Mechanical Engineering Texas A&M University, College Station TX 77843–3123, USA
An Introduction to Nonlinear Finite Element Analysis
1
3
Great Clarendon Street, Oxford OX2 6DP It furthers the University’s objective of excellence in research, scholarship, and education by publishing worldwide in Oxford New York Auckland Cape Town Dar es Salaam Hong Kong Karachi Kuala Lumpur Madrid Melbourne Mexico City Nairobi New Delhi Shanghai Taipei Toronto With oﬃces in Argentina Austria Brazil Chile Czech Republic France Greece Guatemala Hungary Italy Japan Poland Portugal Singapore South Korea Switzerland Thailand Turkey Ukraine Vietnam Oxford is a registered trade mark of Oxford University Press in the UK and in certain other countries Published in the United States by Oxford University Press Inc., New York Oxford University Press, 2004 The moral rights of the author have been asserted Database right Oxford University Press (maker) First published 2004 Reprinted 2005 All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, without the prior permission in writing of Oxford University Press, or as expressly permitted by law, or under terms agreed with the appropriate reprographics rights organization. Enquiries concerning reproduction outside the scope of the above should be sent to the Rights Department, Oxford University Press, at the address above You must not circulate this book in any other binding or cover and you must impose this same condition on any acquirer A catalogue record for this title is available from the British Library Library of Congress Cataloging in Publication Data (Data available) Typeset by the author using TEX Printed in Great Britain on acidfree paper by T.J. International Ltd., Padstow, Cornwall ISBN 019852529X 10 9 8 7 6 5 4 3 2
9780198525295
To To my beloved teacher Professor John Tinsley Oden
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About the Author
J. N. Reddy is Distinguished Professor and the Holder of Oscar S. Wyatt Endowed Chair in the Department of Mechanical Engineering at Texas A&M University, College Station, Texas. Prior to the current position, he was the Clifton C. Garvin Professor in the Department of Engineering Science and Mechanics at Virginia Polytechnic Institute and State University (VPI&SU), Blacksburg, Virginia. Dr. Reddy is internationally known for his contributions to theoretical and applied mechanics and computational mechanics. He is the author of over 300 journal papers and dozen other books, including Introduction to the Finite Element Method (Second Edition), McGrawHill, l993; Theory and Analysis of Elastic Plates, Taylor & Francis, 1999; Energy Principles and Variational Methods in Applied Mechanics (Second Edition), John Wiley, 2002; Mechanics of Laminated Plates and Shells: Theory and Analysis, (second Edition) CRC Press, 2004; An Introduction to the Mathematical Theory of Finite Elements (coauthored with J. T. Oden), John Wiley, l976; Variational Methods in Theoretical Mechanics (coauthored with J. T. Oden), Springer—Verlag, 1976; The Finite Element Method in Heat Transfer and Fluid Dynamics (Second Edition; coauthored with D. K. Gartling), CRC Press, 2001; and Professor Reddy is the recipient of the Walter L. Huber Civil Engineering Research Prize of the American Society of Civil Engineers (ASCE), the Worcester Reed Warner Medal and the Charles Russ Richards Memorial Award of the American Society of Mechanical Engineers (ASME), the 1997 Archie Higdon Distinguished Educator Award from the American Society of Engineering Education (ASEE), the 1998 Nathan M. Newmark Medal from the American Society of Civil Engineers, the 2000 Excellence in the Field of Composites from the American Society of Composites, the 2000 Faculty Distinguished Achievement Award for Research and the 2003 Bush Excellence Award for Faculty in International Research from Texas A&M University, and the 2003 Computational Solid Mechanics Award from the U.S. Association of Computational Mechanics (USACM). Dr. Reddy is a Fellow of ASCE, ASME, the American Academy of Mechanics (AAM), the American Society of Composites, the U.S. Association of Computational Mechanics, the International Association of Computational Mechanics (IACM), and the Aeronautical Society of India (ASI). Professor Reddy is the EditorinChief of Mechanics of Advanced Materials and Structures, International Journal of Computational Engineering Science and International Journal of Structural Stability and Dynamics, and he serves on the editorial boards of over two dozen other journals, including International Journal for Numerical Methods in Engineering, Computer Methods in Applied Mechanics and Engineering, and International Journal of NonLinear Mechanics.
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ix
Contents Preface
xvii
1 Introduction 1.1 Mathematical Models 1.2 Numerical Simulations 1.3 The Finite Element Method 1.4 Nonlinear Analysis 1.4.1 Introduction 1.4.2 ClassiÞcation of Nonlinearities 1.5 The Big Picture References
1 1 3 5 7 7 7 11 12
2 The Finite Element Method: A Review 2.1 Introduction 2.2 OneDimensional Problems 2.2.1 Governing Diﬀerential Equation 2.2.2 Finite Element Approximation 2.2.3 Derivation of the Weak Form 2.2.4 Interpolation Functions 2.2.5 Finite Element Model 2.3 TwoDimensional Problems 2.3.1 Governing Diﬀerential Equation 2.3.2 Finite Element Approximation 2.3.3 Weak Formulation 2.3.4 Finite Element Model 2.3.5 Interpolation Functions 2.3.6 Assembly of Elements 2.4 Library of TwoDimensional Finite Elements 2.4.1 Introduction 2.4.2 Triangular Elements 2.4.3 Rectangular Elements 2.5 Numerical Integration 2.5.1 Preliminary Comments 2.5.2 Coordinate Transformations 2.5.3 Integration Over a Master Rectangular Element 2.5.4 Integration Over a Master Triangular Element
13 13 13 13 14 16 18 22 24 24 24 26 27 28 33 36 36 37 38 40 40 41 44 45
x
2.6 Computer Implementation 2.6.1 General Comments 2.6.2 OneDimensional Problems 2.6.3 TwoDimensional Problems 2.7 Closure Problems References 3 Heat Transfer and Other Field Problems in One Dimension 3.1 Model Diﬀerential Equation 3.2 Weak Formulation 3.3 Finite Element Model 3.4 Solution Procedures 3.4.1 General Comments 3.4.2 Direct Iteration Procedure 3.4.3 Newton’s Iteration Procedure 3.5 Computer Implementation 3.5.1 Introduction 3.5.2 Preprocessor Unit 3.5.3 Processor Unit 3.6 Closing Remarks Problems References 4 Nonlinear Bending of Straight Beams 4.1 Introduction 4.2 Euler—Bernoulli Beams 4.2.1 Basic Assumptions 4.2.2 Displacement Field and Strains 4.2.3 Weak Forms 4.2.4 Finite Element Model 4.2.5 Iterative Solutions of Nonlinear Equations 4.2.6 Load Increments 4.2.7 Membrane Locking 4.2.8 Computer Implementation 4.3 Timoshenko Beams 4.3.1 Displacement Field and Strains 4.3.2 Weak Forms 4.3.3 General Finite Element Model 4.3.4 Shear and Membrane Locking 4.3.5 Tangent Stiﬀness Matrix
46 46 48 52 53 57 59 61 61 62 62 64 64 65 68 73 73 73 76 82 82 85 87 87 88 88 88 89 95 97 100 101 102 110 110 111 113 115 117
xi
Problems References
124 126
5 Heat Transfer and Other Fields Problems in Two Dimensions 127 5.1 Model Equation 127 5.2 Weak Form 128 5.3 Finite Element Model 129 5.4 Solution Procedures 131 5.4.1 Direct Iteration 131 5.4.2 Newton—Raphson Iteration 131 5.5 Computer Implementation 132 5.5.1 Introduction 132 5.5.2 Numerical Integration 133 5.5.3 Element Calculations 135 Problems 139 References 140 6 Nonlinear Bending of Elastic Plates 6.1 Introduction 6.2 Classical Plate Theory 6.2.1 Assumptions of the Kinematics 6.2.2 Displacement Field and Strains 6.3 Variational Formulation of CPT 6.3.1 Virtual Work 6.3.2 Weak Forms 6.3.3 Equilibrium Equations 6.3.4 Boundary Conditions 6.3.5 Stress Resultant—Deßection Relations 6.4 Finite Element Models of CPT 6.4.1 General Formulation 6.4.2 Tangent Stiﬀness Coeﬃcients 6.4.3 Some Plate Finite Elements 6.5 Computer Implementation Aspects and Numerical Results of CPT Elements 6.5.1 Computer Implementation 6.5.2 Results of Linear Analysis 6.5.3 Results of Nonlinear Analysis 6.6 First—Order Shear Deformation Plate Theory 6.6.1 Introduction 6.6.2 Displacement Field 6.6.3 Weak Formulation
141 141 141 141 142 144 144 147 148 149 152 153 153 157 161 164 164 166 168 173 173 173 174
xii
6.7 Finite Element Models of FSDT 6.7.1 Virtual Work Statements 6.7.2 Finite Element Model 6.7.3 Tangent Stiﬀness Coeﬃcients 6.7.4 Shear and Membrane Locking 6.8 Computer Implementation and Numerical Results of FSDT Elements 6.8.1 Computer Implementation 6.8.2 Results of Linear Analysis 6.8.3 Results of Nonlinear Analysis 6.9 Theory of DoublyCurved Shells 6.9.1 Introduction 6.9.2 Geometric Description 6.9.3 Strain—Displacement Relations 6.9.4 Stress Resultants 6.9.5 Equations of Motion 6.10 Finite Element Analysis of Shells 6.10.1 Weak Forms 6.10.2 Finite Element Model 6.10.3 Linear Results 6.10.4 Nonlinear Results Problems References 7 Flows of Viscous Incompressible Fluids 7.1 Introduction 7.2 Governing Equations 7.2.1 Introduction 7.2.2 Conservation of Mass 7.2.3 Conservation of Momenta 7.2.4 Conservation of Energy 7.2.5 Constitutive Equations 7.2.6 Boundary Conditions 7.3 Governing Equations in Terms of Primitive Variables 7.3.1 Vector Form 7.3.2 Cartesian Component Form 7.4 Velocity—Pressure Finite Element Model 7.4.1 Weak Form 7.4.2 Finite Element Model 7.5 Penalty Finite Element Models 7.5.1 Introduction 7.5.2 Penalty Function Method
177 177 178 182 184 184 184 185 189 196 196 197 201 202 205 206 206 207 209 217 222 225 229 229 230 230 231 232 232 233 234 235 235 236 237 237 239 241 241 242
xiii
7.5.3 Reduced Integration Penalty Model 7.5.4 Consistent Penalty Model 7.6 Computational Aspects 7.6.1 Properties of the Matrix Equations 7.6.2 Choice of Elements 7.6.3 Evaluation of Element Matrices in Penalty Models 7.6.4 PostComputation of Stresses 7.7 Computer Implementation 7.7.1 Mixed Model 7.7.2 Penalty Model 7.8 Numerical Examples 7.8.1 Preliminary Comments 7.8.2 Fluid Squeezed Between Parallel Plates 7.8.3 Flow of a Viscous Lubricant in a Slider Bearing 7.8.4 WallDriven Cavity Flow 7.8.5 BackwardFacing Step 7.9 LeastSquares Finite Element Models 7.9.1 Introduction 7.9.2 Finite Element Model 7.9.3 Computational Aspects 7.9.4 Numerical Examples Problems References
244 245 246 246 247 249 250 251 251 254 254 254 255 258 261 267 269 269 272 274 275 283 283
8 Nonlinear Analysis of Time—Dependent Problems 8.1 Introduction
287 287
8.2 Time Approximations 8.2.1 Introduction 8.2.2 Parabolic Equations 8.2.3 Hyperbolic Equations 8.3 Stability and Accuracy 8.3.1 Preliminary Comments 8.3.2 Stability Criteria 8.4 Transient Analysis of Nonlinear Problems 8.4.1 Introduction 8.4.2 Heat Transfer 8.4.3 Flows of Viscous Incompressible Fluids 8.4.4 Plate Bending (FSDT) 8.5 Computer Implementation
288 288 289 292 295 295 296 297 297 297 298 300 304
xiv
8.6 Numerical Examples 8.6.1 Linear Problems 8.6.2 Nonlinear Problems Problems References 9 Finite Element Formulations of Solid Continua 9.1 Introduction 9.1.1 Background 9.1.2 Descriptions of Motion 9.2 Strain and Stress Measures 9.2.1 Deformation Gradient Tensor 9.2.2 Green and Almansi Strain Tensors 9.2.3 Polar Decomposition 9.2.4 Stress Tensors 9.2.5 Energetically—Conjugate Stresses and Strains 9.3 Strain and Stress Measures Between ConÞgurations 9.3.1 Notation 9.3.2 Conservation of Mass 9.3.3 Green Strain Tensors for Various ConÞgurations 9.3.4 Euler Strain Tensor 9.3.5 Relationships Between Various Stress Tensors 9.4 Constitutive Equations 9.5 Total Lagrangian and Updated Lagrangian Formulations of Continua 9.5.1 Principle of Virtual Displacements 9.5.2 Total Lagrangian Formulation 9.5.3 Updated Lagrangian Formulation 9.6 Finite Element Models of TwoDimensional Continua 9.6.1 Introduction 9.6.2 Total Lagrangian Formulation 9.6.3 Updated Lagrangian Formulation 9.6.4 Computer Implementation 9.6.5 Numerical Results 9.7 Shell Finite Elements 9.7.1 Introduction 9.7.2 Incremental Equations of Motion 9.7.3 Finite Element Models of a Continuum 9.7.4 Shell Finite Element 9.7.5 Numerical Examples Problems References
307 307 314 323 325 327 327 327 328 329 329 331 334 334 335 337 337 339 340 342 343 346 347 347 348 350 353 353 353 360 361 361 369 369 369 370 372 378 381 387
xv
10 Material Nonlinearities and Coupled Problems 10.1 Introduction 10.2 Nonlinear Elastic Problems 10.3 Small Deformation Theory of Plasticity 10.3.1 Introduction 10.3.2 Ideal Plasticity 10.3.3 Strain Hardening Plasticity 10.3.4 Elastic—Plastic Analysis of a Bar 10.4 NonNewtonian Fluids 10.4.1 Introduction 10.4.2 Governing Equations in Cylindrical Coordinates 10.4.3 PowerLaw Fluids 10.4.4 White—Metzner Fluids 10.4.5 Numerical Examples 10.5 Coupled Fluid Flow and Heat Transfer 10.5.1 Finite Element Models 10.5.2 Numerical Examples References
389 389 390 391 391 392 393 395 404 404 405 407 409 412 417 417 419 423
Appendix A1: Solution Procedures for Linear Algebraic Equations A1.1 Introduction A1.2 Direct Methods A1.2.1 Preliminary Comments A1.2.2 Symmetric Solver A1.2.3 Unsymmetric Solver A1.3 Iterative Methods A1.3.1 General Comments A1.3.2 Solution Algorithms References
425 425 427 427 428 430 430 430 432 433
Appendix A2: Solution Procedures for Nonlinear Algebraic Equations A2.1 Introduction A2.2 Picard Iteration Method A2.3 Newton—Raphson Iteration Method A2.4 Riks and ModiÞed Riks Schemes References
439 439 440 444 448 457
Subject Index
459
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Preface
The objective of this book is to present the theory and computer implementation of the Þnite element method as applied to simple nonlinear problems of heat transfer and similar Þeld problems, ßuid mechanics, and solid mechanics. Both geometric as well as material nonlinearities are considered, and static and transient (i.e. timedependent) responses are studied. The guiding principle in writing the book was to make the presentation suitable for (a) adoption as a text book for a Þrst course on nonlinear Þnite element analysis (or for a second course following an introductory course on the Þnite element method), and (b) for use by engineers and scientists from various disciplines for self study and practice. There exist a number of books on nonlinear Þnite elements. Most of these books contain a good coverage of the topics of structural mechanics, and few address topics of ßuid dynamics and heat transfer. While these books serve as good references to engineers or scientists who are already familiar with the subject but wish to learn advanced topics or latest developments, they are not suitable as textbooks for a Þrst course or for self study on nonlinear Þnite element analysis. The motivation and encouragement that led to the writing of the present book have come from the users of the author’s book, An Introduction to the Finite Element Method (McGraw—Hill, 1984; Second Edition, 1993; third edition scheduled for 2004), who have found the approach presented there to be most suitable for any one — irrespective of their scientiÞc background — interested in learning the method, and also from the fact that there does not exist a book that is suitable as a textbook for a Þrst course on nonlinear Þnite element analysis. The author has taught a course on nonlinear Þnite element analysis many times during the last twenty years, and the present book is an outcome of the lecture notes developed during this period. The same approach as that used in the aforementioned book, namely, the diﬀerential equation approach, is adopted in the present book to introduce the theory, formulation, and computer implementation of the Þnite element method as applied to nonlinear problems of science and engineering.
Beginning with a model (i.e. typical) secondorder, nonlinear diﬀerential equation in one dimension, the book takes the reader through increasingly complex problems of nonlinear beam bending, nonlinear Þeld problems in two dimensions, nonlinear plate bending, nonlinear formulations of solid continua, ßows of viscous incompressible ßuids in two dimensions (i.e. Navier—Stokes equations), timeapproximation schemes, continuum formulations of shells, and material nonlinear problems of solid mechanics. As stated earlier, the book is suitable as a textbook for a Þrst course on nonlinear Þnite elements in civil, aerospace, mechanical, and mechanics departments as well as in applied sciences. It can be used as a reference by engineers and scientists working in industry, government laboratories and academia. Introductory courses on the Þnite element method, continuum mechanics, and numerical analysis should prove to be helpful. The author has beneÞted in writing the book by the encouragement and support of many colleagues around the world who have used his book, An Introduction to the Finite Element Method, and students who have challenged him to explain and implement complicated concepts and formulations in simple ways. While it is not possible to name all of them, the author expresses his sincere appreciation. In particular, it is a pleasure to acknowledge the help of the author’s students Juan P. Pontaza with the leastsquares Þnite element analysis of ßuid ßow problems in Chapters 7 and 8, and Goy Teck Lim with the plasticity example in Chapter 10. The author expresses his deep sense of gratitude to his teacher, Professor J. T. Oden (University of Texas at Austin), to whom this book is dedicated and without whose advice, mentorship and support it would not have been possible for the author to modestly contribute to the Þeld of applied mechanics in general and theory and application of the Þnite element method in particular, through author’s teaching, research, and technical writings.
J. N. Reddy College Station, Texas
1 Introduction
1.1 Mathematical Models One of the most important thing engineers and scientists do is to model natural phenomena. They develop conceptual and mathematical models to simulate physical events, whether they are aerospace, biological, chemical, geological, or mechanical. The mathematical models are developed using laws of physics and they are often described in terms of algebraic, diﬀerential, and/or integral equations relating various quantities of interest. A mathematical model can be broadly deÞned as a set of relationships among variables that express the essential features of a physical system or process in analytical terms. The relationships that govern the system take the form of algebraic, diﬀerential, and integral equations. Mathematical models of physical phenomena are often based on fundamental scientiÞc laws of physics such as the principle of conservation of mass, the principle of conservation of linear momentum, and the principle of conservation of energy. Mathematical models of biological and other phenomena may be based on observations and accepted theories. Keeping the scope of the present study in mind, we limit our discussions to engineering systems that are governed by laws of continuum mechanics. Mathematical models of engineering systems are often characterized by very complex equations posed on geometrically complicated regions. Consequently, many of the mathematical models, until the advent of electronic computation, were drastically simpliÞed in the interest of analytically solving them. Over the last three decades, however, the computer has made it possible, with the help of mathematical models and numerical methods, to solve many practical problems of science and engineering. There now exists a new and growing body of knowledge connected with the use of numerical methods and computers to analyze mathematical models of physical systems, and this body is known as computational mechanics. Major established industries such as the automobile, aerospace, chemical, pharmaceutical, petroleum, electronics and communications, as well as emerging industries such as biotechnology, rely on computational mechanics—based capabilities to simulate complex systems
2
NONLINEAR FINITE ELEMENT ANALYSIS
Here, we consider the familiar example of a simple pendulum to illustrate how a mathematical model of the motion of the pendulum can be constructed using the principle of conservation of linear momentum, that is, Newton’s second law of motion. Numerical analysis of the problem will be considered in the sequel. Example 1.1.1 As an example of the mathematical model development, consider the problem of a simple pendulum. The system consists of a bob of mass m attached to one end of a rod of length l and the other end is pivoted to a Þxed point O, as shown in Figure 1.1.1. The primary goal of the mathematical model to be derived here is to have a means to determine the motion (i.e. angular displacement as a function of time) of the bob. Keeping the goal of the analysis in mind, we make several assumptions. (1) The bob as well as the rod connecting the bob to the Þxed point O are rigid; (2) the mass of the rod is negligible relative to the mass of the bob; and (3) there is no friction at the pivot. These assumptions may be removed to obtain a mathematical model that describes the system more accurately. The equation governing the motion can be derived using the principle of conservation of linear momentum, also known as Newton’s second law of motion, which states that the vector sum of externally applied forces is equal to the time rate of change of the linear momentum of the body: F = ma (1.1.1) where F is the vector sum of all forces acting on a system, m is the mass of the system, and a is the acceleration of the system. Applying Newton’s second law in the x direction (see Figure 1.1.1), we have Fx = −mg sin θ,
v=l
dθ dt
(1.1.2)
where θ is the angular displacement, v is the velocity along x, and t denotes time. Then, the equation for motion about the pivot becomes −mg sin θ = ml
O l
d2 θ dt2
or
d2 θ g + sin θ = 0 l dt2
R
(1.1.3)
x ma=mθ¨ l
θ mg
y
m
Figure 1.1.1 Simple pendulum.
INTRODUCTION
3
Thus, the problem at hand involves solving the nonlinear diﬀerential equation d2 θ g + sin θ = 0, l dt2
0 0, nonlinear) EP S = allowable error tolerance ² for the convergence test, IT M AX = maximum allowable number of iterations for convergence. (c) Read speciÞed boundary and initial data: N SP V , N SSV , N SM B, ISP V (I, J), V SP V (I), ISSV (I, J), V SSV (I), ISM B(I, J), BET A0(I), BET AU (I), UREF (I), and GU 0(I), where N SP V = number of speciÞed primary degrees of freedom (U), N SSV = number of speciÞed secondary degrees of freedom (Q ≡ nx a du dx ), N SM B = number of speciÞed mixed boundary conditions [Q + (β0 + βu U )(U − Uref )], ISP V (I, J) = array of speciÞed primary degrees of freedom (I = 1, 2, . . . , N SP V and J = 1, 2); ISP V (I, 1)= global node number at which the primary variable is speciÞed, and ISP V (I, 2) is the degree of freedom number at that node which is speciÞed (in the class of problems discussed in this chapter, N DF = 1 and therefore ISP V (I, 2) = 1 for all cases). V SP V (I) = array of speciÞed values of primary variable DOF in ISP V (I, J) [V SP V (I) should be in the same sequence as the speciÞed degrees of freedom in ISP V (I)], ISSV (I, J) = similar to ISP V (I, J), except for secondary variables, V SSV (I) = similar to V SP V (I), except for secondary variables, ISM B(I, J) = array of speciÞed mixed boundary conditions (I = 1, 2, . . . , N SSV ) and has meaning similar to arrays ISP V (I, J) and ISSV (I, J), BET A0(I) = array of speciÞed values of β0 ,
76
NONLINEAR FINITE ELEMENT ANALYSIS
BET A1(I) = array of speciÞed values of βu , U REF (I) = array of speciÞed values of Uref , GU 0(I) = initial (guess) solution vector (I = 1, 2, . . ., N EQ); N EQ=number of total primary degrees of freedom in the mesh. 2. Write out the input data and necessary warning messages.
3.5.3 Processor Unit Here we discuss mainly the calculation of element coeﬃcients, imposition of various types of boundary conditions, and iterative algorithm. The number of Gauss points needed to evaluate the coeﬃcients is determined by the highest polynomial degree, p [N GP = (p + 1)/2]. In the present case, the expression that dictates the degree of the polynomial is the integral involving c(x, u). If we assume c to be a linear function of u, and if u is approximated using linear polynomials, the integrand of the above integral expression is cubic (p = 3). Hence, a twopoint integration is needed to evaluate it; for quadratic interpolation of u, the integrand is a Þfth degree polynomial (p = 5), and hence it requires threepoint Gauss quadrature. Of course, the polynomial degrees goes up if a is a higher degree polynomial of x. The element coeﬃcients are evaluated using the Gauss rule. Based on the discussion presented in Section 2.7, the element coeﬃcients in (3.3.3a), for β = 0, can be expressed as ELK(I, J) = ELK(I, J) + (AX ∗ GDSF L(I) ∗ GDSF L(J) + BX ∗ SF L(I) ∗ GDSF L(J) + CX ∗ SF L(I) ∗ SF L(J))∗ GJ ∗ GAU SW T (N I, NGP ) which is evaluated inside a loop on N I = 1, 2, . . . , NGP and inner loops on I, J = 1, 2, . . . , N P E. Boxes 3.5.1 and 3.5.2 show the Fortran subroutines for the calculation of [K e ] and {f e }. The imposition of boundary conditions on a banded system of nonsymmetric equations is discussed next. The assembled coeﬃcient matrix [GLK] is stored a full band width (N BW ) form, and it is of the order N EQ × N BW , the last column GLK(I, N BW ) being reserved for the source vector. Note that GLK(I, N HBW ) is the main diagonal of the assembled matrix. Essential boundary conditions ˆI . Then the Ith equation of the Now suppose that UI is speciÞed to be U ˆI . This is done as follows: system is replaced with the equation UI = U GLK(I, J) = 0.0 for all J = 1, 2, . . . , N BW ˆI GLK(I, N HBW ) = 1.0, GLK(I, N BW ) = U This is repeated for all N SP V conditions.
and J 6= I
(3.5.2)
NONLINEAR EQUATIONS IN SINGLE VARIABLE IN ONE DIMENSION
77
Box 3.5.1 Subroutine for the generation of the coeﬃcient matrices in the direct iteration method. SUBROUTINE COEFF (IEL,NPE,F0) IMPLICIT REAL*8(AH,OZ) DIMENSION GAUSS(5,5),WT(5,5) COMMON /SHP/ SFL(4),GDSFL(4) COMMON /STF/ STIF(3,3),ELF(3),ELX(3),ELU(3),AX0,AX1,AU C 1 2 3
DATA GAUSS/5*0.0D0,0.57735027D0,0.57735027D0,3*0.0D0, 0.77459667D0,0.0D0,0.77459667D0,2*0.0D0,0.86113631D0, 0.33998104D0,0.33998104D0,0.86113631D0,0.0D0, 0.906180D0,0.538469D0,0.0D0,0.538469D0,0.906180D0/
1 2 3
DATA WT /2.0D0,4*0.0D0,2*1.0D0,3*0.0D0,0.55555555D0, 0.88888888D0,0.55555555D0,2*0.0D0,0.34785485D0, 2*0.65214515D0,0.34785485D0,0.0D0,0.2369227D0, 0.478629D0,0.568889D0,0.478629D0,0.236927D0/
C NGP=IEL+1 EL=ELX(IEL+1)ELX(1) DO 10 I=1,NPE ELF(I)=0.0 DO 10 J=1,NPE 10 ELK(I,J)=0.0 C DO 80 NI=1,NGP XI=GAUSS(NI,NGP) CALL SHAPE (EL,ELX,GJ,IEL,NPE,XI) CNST=GJ*WT(NI,NGP) X=0.0 U=0.0 DO 20 I=1,NPE X=X+SFL(I)*ELX(I) U=U+SFL(I)*ELU(I) 20 CONTINUE C AX=AX0+AX1*X+AU*U DO 40 I=1,NPE ELF(I)=ELF(I)+F0*SFL(I)*CNST DO 40 J=1,NPE S00=GDSFL(I)*GDSFL(J)*CNST ELK(I,J)=ELK(I,J)+AX*S00 40 CONTINUE 80 CONTINUE RETURN END
78
NONLINEAR FINITE ELEMENT ANALYSIS
Box 3.5.2 Subroutine for the generation of the coeﬃcient matrices in the Newton—Raphson iteration method. SUBROUTINE COEFF (IEL,NPE,F0) IMPLICIT REAL*8(AH,OZ) DIMENSION GAUSS(5,5),WT(5,5),TANG(3,3) COMMON /SHP/ SFL(4),GDSFL(4) COMMON /STF/ STIF(3,3),ELF(3),ELX(3),ELU(3),AX0,AX1,AU
C
C
1 2 3
DATA GAUSS/5*0.0D0,−0.57735027D0,0.57735027D0,3*0.0D0, −0.77459667D0,0.0D0,0.77459667D0,2*0.0D0,−0.86113631D0, −0.33998104D0,0.33998104D0,0.86113631D0,0.0D0, −0.906180D0,−0.538469D0,0.0D0,0.538469D0,0.906180D0/
1 2 3
DATA WT /2.0D0,4*0.0D0,2*1.0D0,3*0.0D0,0.55555555D0, 0.88888888D0,0.55555555D0,2*0.0D0,0.34785485D0, 2*0.65214515D0,0.34785485D0,0.0D0,0.2369227D0, 0.478629D0,0.568889D0,0.478629D0,0.236927D0/
10
20
NGP=IEL+1 EL=ELX(IEL+1)ELX(1) DO 10 I=1,NPE ELF(I)=0.0 DO 10 J=1,NPE TANG(I,J)= 0.0 ELK(I,J) = 0.0 DO 80 NI=1,NGP XI=GAUSS(NI,NGP) CALL SHAPE (EL,ELX,GJ,IEL,NPE,XI) CNST=GJ*WT(NI,NGP) X=0.0 U=0.0 DU=0.0 DO 20 I=1,NPE X=X+SFL(I)*ELX(I) U=U+SFL(I)*ELU(I) DU=DU+GDSFL(I)*ELU(I) CONTINUE AX=AX0+AX1*X+AU*U DO 40 I=1,NPE ELF(I)=ELF(I)+F0*SFL(I)*CNST DO 40 J=1,NPE S00=GDSFL(I)*GDSFL(J)*CNST S10=GDSFL(I)*SFL(J)*CNST ELK(I,J)=ELK(I,J)+AX*S00 TANG(I,J)=TANG(I,J)+AU*DU*S10 CONTINUE CONTINUE
40 80 C C Compute the residual vector and tangent matrix C DO 100 I=1,NPE DO 100 J=1,NPE 100 ELF(I)=ELF(I)ELK(I,J)*ELU(J) DO 120 I=1,NPE DO 120 J=1,NPE 120 ELK(I,J)=ELK(I,J)+TANG(I,J) C RETURN END
NONLINEAR EQUATIONS IN SINGLE VARIABLE IN ONE DIMENSION
79
Natural boundary conditions ˆ I . Then the force in the Ith equation Next, suppose that QI is speciÞed to be Q ˆ is augmented with QI : ˆI GLK(I, N BW ) = GLK(I, N BW ) + Q
(3.5.3)
This is repeated for all N SSV conditions. Mixed boundary conditions Finally, we consider mixed boundary conditions of the form QI + βI (UI − Uref ) = 0
(3.5.4)
where βI is a function of u, say βI = βI0 + βI1 UI . The boundary condition may be implemented as follows: ³
GLK(I, N HBW ) = GLK(I, N HBW ) + βI0 + βI1 UI ³
´
´
GLK(I, N BW ) = GLK(I, N BW ) + βI0 + βI1 UI Uref
(3.5.5)
The above statement modiÞes the assembled direct stiﬀness matrix and source vector in direct iteration procedure. In the case of Newton—Raphson iteration, the assembled tangent matrix is modiÞed as follows: ³
´
GLK(I, N HBW ) = GLK(I, N HBW ) + βI0 + 2βI1 UI − βI1 Uref ³
´
³
(3.5.6) ´
GLK(I, N BW ) = GLK(I, N BW ) − βI0 + βI1 UI UI + βI0 + βI1 UI Uref Box 3.5.3 contains a listing of the subroutine that imposes various types of boundary conditions on an banded, unsymmetric system of equations. The major steps of the two iterative algorithms are summarized below. 1. Read in the convergence tolerance T OLR, maximum allowable number of iterations IT M AX, and the initial guess vector GU C(I). 2. Initialize the previous solution vector {GU P } = {0}. 3. Set the iteration counter, IT ER = 0. 4. Begin the iteration counter on IT ER = IT ER + 1. 5. Initialize the global coeﬃcient matrix [GLK] (and source vector {GLF } if used); when unsymmetric banded solver is used, the last column of [GLK] takes the place of {GLF }. 6. Calculate the element coeﬃcient matrices [ELK] using the last iteration solution {GU C} (or a weightedaverage of {GU C} and {GU P } when underrelaxation is used) and element source vectors {ELF } and assemble to obtain [GLK] (and {GLF }).
80
NONLINEAR FINITE ELEMENT ANALYSIS
Box 3.5.3 Subroutine for the implementation of various types of boundary conditions.
1 C C C C C C
SUBROUTINE BNDRYUNSYM(ITYPE,MXNEQ,MXFBW,MXEBC,MXNBC,NDF,NHBW, GLK, GLU,NSPV,NSSV,NSMB,ISPV,ISSV,ISMB,VSPV,VSSV,BETA0,BETAU,UREF) __________________________________________________________________ The subroutine is used to implement specified boundary conditions on BANDED UNSYMMETRIC system of finite element equations __________________________________________________________________
IMPLICIT REAL*8 (AH,OZ) DIMENSION ISPV(MXEBC,2),ISSV(MXNBC,2),ISMB(MXNBC,2),VSPV(MXEBC), 1 VSSV(MXEBC),UREF(MXNBC),BETA0(MXNBC),BETAU(MXNBC), 2 GLU(MXNEQ),GLK(MXNEQ,MXFBW) C NBW=2*NHBW C C Include specified PRIMARY degrees of freedom C IF(NSPV.NE.0)THEN DO 120 NP=1,NSPV NB=(ISPV(NP,1)1)*NDF+ISPV(NP,2) DO 110 J=1,NBW 110 GLK(NB,J)=0.0D0 GLK(NB,NHBW)=1.0D0 120 GLK(NB,NBW)=VSPV(NP) ENDIF C C Modify the source vector to include specified nonzero SECONDARY VARIABLES C IF(NSSV.NE.0)THEN DO 130 NS=1,NSSV NB=(ISSV(NS,1)1)*NDF+ISSV(NS,2) 130 GLK(NB,NBW)=GLK(NB,NBW)+VSSV(NS) ENDIF C C Implement the specified MIXED BOUNDARY CONDITIONS C IF(NSMB.NE.0)THEN DO 150 MB=1,NSMB NB=(ISMB(MB,1)1)*NDF+ISMB(MB,2) IF(ITYPE.LE.1)THEN GLK(NB,NHBW)=GLK(NB,NHBW)+BETA0(MB)+BETAU(MB)*GLU(NB) GLK(NB,NBW)=GLK(NB,NBW)+UREF(MB)*(BETA0(MB)+BETAU(MB)*GLU(NB)) ELSE GLK(NB,NHBW)=GLK(NB,NHBW)+BETA0(MB)+2.0*BETAU(MB)*GLU(NB) * UREF(MB)*BETAU(MB) GLK(NB,NBW)=GLK(NB,NBW)+UREF(MB)*(BETA0(MB)+BETAU(MB)*GLU(NB)) * (BETA0(MB)+BETAU(MB)*GLU(NB))*GLU(NB) ENDIF 150 CONTINUE ENDIF RETURN END
NONLINEAR EQUATIONS IN SINGLE VARIABLE IN ONE DIMENSION
81
7. Impose the boundary conditions (the essential boundary conditions must be homogeneous). 8. Solve the equations (solution is returned in the last column of [GLK]). 9a. In direct iteration, update the current and previous solution vectors {GU P } = {GU C} and {GU C} = {GLF } ({GLF } = [GLK]last ). 9b. In Newton—Raphson iteration, update the current and previous solution vectors {GU P } = {GU C} and {GU C} = {GU C} + {GLF }. 10. Calculate the residual or solution error, ERROR [see Eqs. (3.4.2)—(3.4.3)]. 11. If ERROR ≤ T OLR print IT ER, ERROR, and {GU C} and stop. If ERROR > T OLR, continue. 12. If IT ER > IT M AX, stop; otherwise, repeat steps 4 through 11. Example 3.5.1 Consider heat transfer in an isotropic bar of length L = 0.18 m. The surface of the bar is insulated so that there is no convection from the surface. The governing equation of the problem is the same as Eq. (3.1.1) with b = c = 0, u = T , the temperature. The conductivity a(x, T ) = k is assumed to vary according to the relation k = k0 (1 + βT )
(3.5.7)
where k0 is the constant thermal conductivity [k0 = 0.2 W/(m ◦ K)] and β the temperature coeﬃcient of thermal conductivity [β = 2 × 10−3 (◦ C−1 )]. Suppose that there is no internal heat generation (i.e. f = 0) and the boundary conditions are T (0) = 500◦ K,
T (L) = 300◦ K
(3.5.8)
Table 3.5.1 shows the linear and nonlinear solutions T (x). The results obtained with the direct iteration method and Newton—Raphson method with ² = 0.001 are tabulated in Table 3.5.1. In both methods, the convergent solution was obtained for three iterations. Both methods and both meshes give the same solution.
Table 3.5.1 Finite element solutions of a nonlinear heat conduction equation. DI/NR
Direct Iteration
Newton—Raphson
x
Linear
8L
4Q
8L
4Q
0.0000 0.0225 0.0450 0.0675 0.0900 0.1125 0.1350 0.1575 0.1800
500.00 475.00 450.00 425.00 400.00 375.00 350.00 325.00 300.00
500.00 477.24 453.94 430.06 405.54 380.35 354.40 327.65 300.00
500.00 477.24 453.94 430.06 405.54 380.35 354.40 327.65 300.00
500.00 477.24 453.94 430.05 405.54 380.34 354.40 327.65 300.00
500.00 477.24 453.94 430.05 405.54 380.34 354.40 327.65 300.00
82
NONLINEAR FINITE ELEMENT ANALYSIS
3.6 Closing Remarks In this chapter, Þnite element formulations of nonlinear boundaryvalue problems in one dimension are discussed, iterative methods of solving the nonlinear algebraic equations are studied, and computer implementation of the nonlinear Þnite element analysis is outlined. The simple class of problems discussed herein should give the reader a clear understanding of the workings of nonlinear Þnite element analysis steps. In the next chapter we will consider multivariable, onedimensional, nonlinear equations governing bending and stretching of straight beams.
Problems 3.1 Consider the secondorder diﬀerential equation −
³
du d µ dx dx
´
= f (x),
µ = µ0
³ du ´n−1 dx
(a)
where u(x) is the dependent unknown, f (x) is a known function of position x, and µ is a function of the dependent variable, as given in Eq. (a). Write the Þnite element model and derive the tangent stiﬀness matrix coeﬃcients. 3.2 Compute the tangent matrix for the case a(x, u) = ae0 (x) du , c(x, u) = 0, and β = 0 dx (see Example 3.4.2). 3.3 Consider the nonlinear diﬀerential equation −
h
i
√ du d (u + 2) = 1, dx dx
du (0) = 0, dx
0<x1, UPDATED Lagrangian formulation _________________________________________________________ . . . Initialize element force vector and stiffness matrix DO 10 I=1,NN F(I)=0.0 DO 10 J=1,NN ELSTIF(I,J)=0.0 CONTINUE
10 C C Numerical integration to evaluate the element matrices C DO 60 NI=1,NGP DO 60 NJ=1,NGP XI =GAUSSPT(NI,NGP) ETA=GAUSSPT(NJ,NGP) CALL SHAPE (NPE,XI,ETA,SF,GDSF,DET,ELXY) CONST=DET*GAUSSWT(NI,NGP)*GAUSSWT(NJ,NGP) C C Define the gradients of displacements and strains C X=0.0 U=0.0 UX=0.0 UY=0.0 VX=0.0 VY=0.0 DO 20 I=1,NPE L=(I1)*NDF+1 X=X+SF(I)*ELXY(I,1) U=U+SF(I)*ELU(L) UX=UX+GDSF(1,I)*ELU(L) UY=UY+GDSF(2,I)*ELU(L) VX=VX+GDSF(1,I)*ELU(L+1) 20 VY=VY+GDSF(2,I)*ELU(L+1) UX2=UX*UX UY2=UY*UY VX2=VX*VX VY2=VY*VY
364
NONLINEAR FINITE ELEMENT ANALYSIS
(Box 9.6.2 continued) C IF(LFORM.EQ.1)THEN UXP1=1.0+UX UXP2=UXP1*UXP1 VYP1=1.0+VY VYP2=VYP1*VYP1 C EX=UX+0.5*(UX2+VX2) EY=VY+0.5*(UY2+VY2) EXY=UY+VX+UX*UY+VX*VY S11=C(1,1)*EX+C(1,2)*EY S22=C(1,2)*EX+C(2,2)*EY S12=C(3,3)*EXY ELSE C C Compute the Almansi strains and Cauchy stresses C EX=UX0.5*(UX2+VX2) EY=VY0.5*(UY2+VY2) EXY=UY+VXUX*UYVX*VY S11=C(1,1)*EX+C(1,2)*EY S22=C(1,2)*EX+C(2,2)*EY S12=C(3,3)*EXY ENDIF C C Compute element force vector and stiffness matrix C II=1 DO 50 I=1,NPE C C Imbalance force coefficients for the TOTAL Lagrangian formulation C IF(LFORM.EQ.1)THEN F(II) =F(II) +(P*SF(I)UXP1*GDSF(1,I)*S11UY*GDSF(2,I)*S22 * (UXP1*GDSF(2,I)+UY*GDSF(1,I))*S12)*CONST F(II+1)=F(II+1)+(P*SF(I)VX*GDSF(1,I)*S11VYP1*GDSF(2,I)*S22 * (VYP1*GDSF(1,I)+VX*GDSF(2,I))*S12)*CONST C C Imbalance force coefficients for the UPDATED Lagrangian formulation C ELSE F(II)=F(II)+(P*SF(I)GDSF(1,I)*S11GDSF(2,I)*S12)*CONST F(II+1)=F(II+1)+(P*SF(I)GDSF(1,I)*S12GDSF(2,I)*S22)*CONST ENDIF JJ=1 DO 40 J=1,NPE SIG=S11*GDSF(1,I)*GDSF(1,J)+S22*GDSF(2,I)*GDSF(2,J) * +S12*(GDSF(1,I)*GDSF(2,J)+GDSF(2,I)*GDSF(1,J)) C IF(LFORM.EQ.1)THEN
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
365
(Box 9.6.2 continued) C C C
Stiffness coefficients for the TOTAL Lagrangian formulation * * * * * * * * * * * * *
C C C
* * * * *
ELSTIF(II,JJ)=ELSTIF(II,JJ)+(C(1,1)*UXP2*GDSF(1,I)*GDSF(1,J) +C(2,2)*UY2*GDSF(2,I)*GDSF(2,J) +C(1,2)*UXP1*UY*(GDSF(1,I)*GDSF(2,J)+GDSF(2,I)*GDSF(1,J)) +C(3,3)*(UXP1*GDSF(2,I)+UY*GDSF(1,I))* (UXP1*GDSF(2,J)+UY*GDSF(1,J))+SIG)*CONST ELSTIF(II+1,JJ+1)=ELSTIF(II+1,JJ+1)+(C(1,1)*VX2*GDSF(1,I) *GDSF(1,J)+C(2,2)*VYP2*GDSF(2,I)*GDSF(2,J) +C(1,2)*VYP1*VX*(GDSF(1,I)*GDSF(2,J)+GDSF(2,I)*GDSF(1,J)) +C(3,3)*(VYP1*GDSF(1,I)+VX*GDSF(2,I))* (VYP1*GDSF(1,J)+VX*GDSF(2,J))+SIG)*CONST ELSTIF(II,JJ+1)=ELSTIF(II,JJ+1)+(C(1,1)*UXP1*VX*GDSF(1,I) *GDSF(1,J)+C(2,2)*VYP1*UY*GDSF(2,I)*GDSF(2,J) +C(1,2)*(UXP1*VYP1*GDSF(1,I)*GDSF(2,J) +UY*VX*GDSF(2,I)*GDSF(1,J)) +C(3,3)*(UXP1*GDSF(2,I)+UY*GDSF(1,I))* (VYP1*GDSF(1,J)+VX*GDSF(2,J)))*CONST ELSTIF(II+1,JJ)=ELSTIF(II+1,JJ)+(C(1,1)*UXP1*VX*GDSF(1,J) *GDSF(1,I)+C(2,2)*VYP1*UY*GDSF(2,J)*GDSF(2,I) +C(1,2)*(UXP1*VYP1*GDSF(1,J)*GDSF(2,I) +UY*VX*GDSF(2,J)*GDSF(1,I)) +C(3,3)*(UXP1*GDSF(2,J)+UY*GDSF(1,J))* (VYP1*GDSF(1,I)+VX*GDSF(2,I)))*CONST
Stiffness coefficients for the UPDATED Lagrangian formulation
ELSE ELSTIF(II,JJ)=ELSTIF(II,JJ)+(C(1,1)*GDSF(1,I)*GDSF(1,J) * +C(3,3)*GDSF(2,I)*GDSF(2,J)+SIG)*CONST ELSTIF(II+1,JJ+1)=ELSTIF(II+1,JJ+1)+(C(3,3)*GDSF(1,I) * *GDSF(1,J)+C(2,2)*GDSF(2,I)*GDSF(2,J)+SIG)*CONST ELSTIF(II,JJ+1)=ELSTIF(II,JJ+1)+(C(1,2)*GDSF(1,I)*GDSF(2,J) * +C(3,3)*GDSF(2,I)*GDSF(1,J))*CONST ELSTIF(II+1,JJ)=ELSTIF(II+1,JJ)+(C(1,2)*GDSF(2,I)*GDSF(1,J) * +C(3,3)*GDSF(1,I)*GDSF(2,J))*CONST ENDIF 40 JJ=NDF*J+1 50 II=NDF*I+1 60 CONTINUE RETURN END
material properties used are E = 1.2 × 104 psi,
ν = 0.2
(9.6.40)
The beam assumed to be in the plane state of stress so that (no distinction is made between 1 Cij and 0 Cij for the two formulations, and they are assumed to remain constant during the deformation).
366
NONLINEAR FINITE ELEMENT ANALYSIS
Table 9.6.1 contains the numerical results obtained with the updated Lagrangian formulation for the center deßection at the free end (also see Figure 9.6.1), while Table 9.6.2 contains (also see Figure 9.6.2) the Cauchy and second Piola—Kirchhoﬀ stresses at the Þxed end of the beam (at the Gauss point located nearest to the top left end). The load was distributed equally at the top and bottom of the beam. Surprisingly, the solution without iteration is very good, except for initial oscillations. It took only 2 or 3 iterations to converge for each load step. Table 9.6.1 Transverse deßections of a cantilevered beam under uniform load (applied equally at the top and bottom), obtained with the updated Lagrangian formulation (5 × 1Q8 mesh). −q0
x
y
u(10, 0.5) v(10, 0.5)
1.0
9.914 9.968
−0.730 −0.287
−0.086 −0.032
−1.230* −0.787
2.0
9.674 9.822
−1.884 −1.291
−0.326 −0.178
−2.384 −1.791
3.0
9.327 9.530
−2.893 −2.373
−0.673 −0.470
−3.393 −2.873
4.0
8.920 9.145
−3.750 −3.327
−1.080 −0.855
−4.250 −3.827
5.0
8.493 8.699
−4.460 −4.153
−1.507 −1.301
−4.960 −4.653
6.0
8.070 8.244
−5.045 −4.831
−1.930 −1.756
−5.545 −5.331
8.0
7.286 7.391
−5.926 −5.831
−2.714 −2.609
−6.426 −6.331
10.0
6.609 6.656
−6.540 −6.506
−3.391 −3.344
−7.040 −7.006
12.0
6.035 6.052
−6.981 −6.971
−3.965 −3.948
−7.481 −7.471
14.0
5.550 5.557
−7.309 −7.305
−4.450 −4.443
−7.809 −7.805
16.0
5.137 5.142
−7.562 −7.559
−4.863 −4.858
−8.062 −8.059
*The Þrst row corresponds to iteration and the second row to the no iteration.
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
367
16.0
Without iteration
12.0
8.0 0
Load, q
With iteration
4.0
0.0 0.0
2.0
4.0
6.0
8.0
Transverse deflection, v(10,0.5)
Figure 9.6.1 Transverse deßection v(10, 0.5) versus load q0 for a cantilever beam under uniform load of intensity q0 . Table 9.6.2 A comparison of the stresses 1 σxx and 10 Sxx obtained with the updated Lagrangian formulation. −q0
1.0 2.0 3.0 4.0 5.0 6.0 8.0 10.0 12.0 14.0 16.0
1σ
xx
153.55 300.14 434.44 555.38 662.83 758.08 918.14 1046.77 1152.26 1240.32 1314.87
1S 0 xx
161.76 332.20 503.43 671.46 833.34 988.19 1277.32 1542.59 1788.70 2019.60 2238.26
1σ
yy
23.15 43.65 61.50 77.11 90.87 103.17 124.61 143.15 159.78 175.11 189.50
1S 0 yy
28.88 65.41 107.05 151.82 198.02 244.66 337.16 427.19 514.33 598.72 680.62
Table 9.6.3 contains the numerical results for the center deßection at the free end and the second Piola—Kirchhoﬀ stresses at the Þxed end of the beam (at the Gauss point located nearest to the top left end). The results were obtained with the total Lagrangian description and with nonlinear iteration. Results are included only for selective loads. Only 2 to 3 iterations were taken to converge at each load step.
368
NONLINEAR FINITE ELEMENT ANALYSIS
Table 9.6.3 Transverse deßections of a cantilevered beam obtained with the total Lagrangian formulation. −q0 *
1σ
1S 0 xx
143.84 263.28 426.04 509.29 539.85 536.14 509.23 465.82 410.10
151.97 294.79 538.12 728.24 877.75 998.51 1098.78 1184.04 1257.97
u(10, 0.5) v(10, 0.5)
1.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0
−0.087 −0.331 −1.091 −1.942 −2.721 −3.391 −3.957 −4.434 −4.839
−1.230 −2.382 −4.237 −5.516 −6.380 −6.979 −7.408 −7.726 −7.971
xx
*The linear solution at q0 = −0.5 is v(10, 0.5) = −0.6227.
16
Cauchy stress (five Q8 elements)
Load, q0
12
8
Second PiolaKirchhoff stress (five Q8 elements)
4
(updated Lagrangian formulation) 0 0
400
800
1200
1600
2000
2400
Axial stress, σ xx
Figure 9.6.2 Normal stresses versus load for a cantilever beam under uniform load of intensity q0 . This completes the development of Þnite element models of nonlinear continua. The procedures developed herein can be readily extended to axisymmetric problems, threedimensional problems, problems with assumed displacements and/or strains [7—9], and composite plates and shells [10]. The extension of the formulations to transient problems is straightforward.
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
369
9.7 Shell Finite Elements 9.7.1 Introduction In previous chapters, the beam and plate elements were developed using beam and plate theories that were derived from an assumed displacement Þeld. Such theories are limited to geometrically linear analyses and nonlinear analysis with small strains and moderate rotations. The Þnite element models to be developed in this section are based on threedimensional elasticity equations, and the geometry and the displacement Þelds of the structure are directly discretized by imposing some geometric and static constraints to satisfy the assumptions of a beam or shell theory. Both shell and beam elements can have a variable number of nodes, and the shell element can be modiÞed as transition elements to model shell intersections or solidtoshell transition regions. Such formulations would appear to be especially applicable to material and geometrical nonlinear analysis of shelltype structures in which large displacements and rotations are experienced (see [2,10—27]).
9.7.2 Incremental Equations of Motion Consider the motion of a body in a Þxed Cartesian coordinate system. Assuming that the body may experience large displacements and rotations, we wish to determine deformed conÞgurations of the body for diﬀerent times/loads. We assume that all conÞgurations from time t = 0 to the current time t, both inclusive, have been determined, and the conÞguration C2 for time t = t + ∆t is sought next. The total Lagrangian description with the principle of virtual displacements (9.5.17) is used to express the dynamic equilibrium of the body in the conÞguration C2 . For dynamic analysis, the principle of virtual displacements (9.5.17) must be modiÞed to include inertial terms. In this case we have Z
2 2V
ρ 2u ¨i δ 2 ui d 2 V =
Z
0
0V
ρ 2u ¨i δ 2 ui d 0 V
(9.7.1)
and hence the mass matrix can be evaluated using the initial conÞguration of the body. Using Hamilton’s principle we obtain the equations of motion of the moving body at time t + ∆t in the variational form as Z
0V
0
2
2
0
ρ u ¨i δ( ui ) d V +
Z
0V
2
0 Cijrs 0 ers
= R− where δ(2 ui ) = δui .
Z
0V
1 0 Sij
0
δ 0 eij d V + δ 0 eij d 0 V
Z
0V
1 0 Sij
δ 0 ηij d 0 V (9.7.2)
370
NONLINEAR FINITE ELEMENT ANALYSIS
9.7.3 Finite Element Model of a Continuum Equation (9.7.2) can be used to develop the nonlinear displacement Þnite element model for any continuum. The basic step in deriving the Þnite element equations for a shell element is the selection of proper interpolation functions for the displacement Þeld and geometry. In the case of beam and shell elements, the approximation for the geometry is chosen such that the beam or shell kinematic hypotheses are realized. First, we derive the Þnite element model of a continuum and then specialize it to shells [24—26]. It is important that the coordinates and displacements are interpolated using the same interpolation functions (isoparametric formulation) so that the displacement compatibility across element boundaries can be preserved in all conÞgurations. Let 0
1
xi = ui =
n X
k=1 n X
ψk 0 xki ,
1
ψk 1 uki ,
xi =
ui =
k=1
n X
ψk 1 xki ,
k=1 n X
2
xi =
n X
ψk 2 xki
(9.7.3)
k=1
ψk uki
(i = 1, 2, 3)
(9.7.4)
k=1
where the right superscript k indicates the quantity at nodal point k, ψk is the interpolation function corresponding to nodal point k, and n is the number of element nodal points. Substitution of Eqs. (9.7.3) and (9.7.4) in Eq. (9.7.2) yields the Þnite element model 1 ¨e 0 [M ]{∆ }
+ (10 [KL ] + 10 [KNL ]){∆e } = 2 {R} − 10 {F }
(9.7.5)
where {∆e } is the vector of nodal incremental displacements from time t to ¨ e }, 1 [KL ]{∆e }, 1 [KNL ]{∆e }, and 1 {F } time t + ∆t in an element, and 10 [M ]{∆ 0 0 0 are obtained by evaluating the integrals, respectively: Z Z
0 0V
0V
2
2
0
ρ u ¨i δ ui d V,
1 0 Sij
0
δ 0 ηij d V,
Z
Z
0V
0V
0 Cijrs 0 ers
1 0 Sij
δ 0 eij d 0 V
δ 0 eij d 0 V
Various matrices are deÞned by 1 0 [KL ]
=
1 0 [KNL ]
=
1 0 [M ] 1 0 {F }
= =
Z
0A
Z
0V
Z
0V
Z
0V
T 1 1 0 [BL ] 0 [C] 0 [BL ]
d 0V
1 T 1 0 [BN L ] 0 [S] 0 [BNL ] 0
ρ 1 [H]T 1 [H] d 0 V
T1 ˆ 1 0 [BL ] 0 {S}
d 0V
d 0V
(9.7.6a) (9.7.6b) (9.7.6c) (9.7.6d)
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
371
In the above equations, 10 [BL ] and 10 [BNL ] are the linear and nonlinear strain—displacement transformation matrices, 0 [C] is the incremental stressstrain material property matrix, 10 [S] is a matrix of 2nd Piola—Kirchhoﬀ stress ˆ is a vector of these stresses, and 1 [H] is the incremental components, 10 {S} displacement interpolation matrix. All matrix elements correspond to the conÞguration at time t and are deÞned with respect to the conÞguration at time t = 0. It is important to note that Eq. (9.7.5) is only an approximation to the actual solution to be determined in each time step [see Eq. (9.5.15)]. Therefore, it may be necessary to iterate in each time step until Eq. (9.5.15), with inertia terms, is satisÞed to a required tolerance. The Þnite element equations (9.7.5) are secondorder diﬀerential equations in time. In order to obtain numerical solutions at each time step, Eq. (9.7.5) needs to be converted to algebraic equations using a time approximation scheme, as explained in Chapter 8. In the present study the Newmark scheme (see Section 8.2.3 for details) is used. In the Newmark time integration scheme, the displacements and velocities are approximated by [see Eqs. (8.2.22) and (8.2.23)] ú + (∆t)2 [( 1 − β) t {∆} ¨ +β {∆} = ∆t t {∆} + t {∆} 2 t+∆t ú ú + ∆t[(1 − α) t {∆} ¨ + α t+∆t {∆}] ¨ {∆} = t {∆} t+∆t
t+∆t
¨ {∆}] (9.7.7)
where α = 12 and β = 14 for the constant—average acceleration method, and ∆t is the time step. Rearranging Eqs. (9.7.5) and (9.7.7), we obtain [10 [M ] ≈ 20 [M ]; see Eq. (8.2.15)] 1 ˆ 2 ˆ (9.7.8) 0 [K]{∆} = {R} where {∆} is the vector of nodal incremental displacements at time t, {∆} = and
t+∆t {∆} − t {∆}, 1 ˆ 0 [K] 2
= a3 10 [M ] + 10 [KL ] + 10 [KNL ] ³
(9.7.9a) ´
ˆ = 2 {R} − 10 {F } + 10 [M ] a3 t {∆} + a4 t {∆} ú + a5 t {∆} ¨ {R} (9.7.9b) a3 =
1 1 , a4 = a3 ∆t, a5 = −1 2 β(∆t) 2β
(9.7.10)
Once Eq. (9.7.8) is solved for {∆} at time t+∆t, the acceleration and velocity vectors are obtained using [a1 = α∆t and a2 = (1 − α)∆t; see Eqs. (8.2.19)] ¨ = a3 {∆} − a4 t {∆} ú − a5 t {∆} ¨ {∆} t+∆t ú t ú t+∆t ¨ t ¨ {∆} = {∆} + a1 {∆} + a2 {∆} t+∆t
(9.7.11)
372
NONLINEAR FINITE ELEMENT ANALYSIS
The Þnite element equations (9.7.8) are solved, after assembly and imposition of boundary conditions, iteratively at each time step until Eq. (9.5.15) is satisÞed within a required tolerance. The Newton—Raphson method with Riks—Wempner algorithm (see Appendix A1) is used in the present study.
9.7.4 Shell Finite Element The shell element is deduced from the threedimensional continuum element by imposing two kinematic constraints: (1) straight line normal to the midsurface of the shell before deformation remains straight but not normal after deformation; (2) the transverse normal components of strain and stress are ignored in the development. However, the shell element admits arbitrarily large displacements and rotations but small strains since the shell thickness is assumed not to change and the normal is not allowed to distort [22, 24—26, 28]. Consider the solid threedimensional element shown in Figure 9.7.1. Let (ξ, η) be the curvilinear coordinates in the middle surface of the shell and ζ be the coordinate in the thickness direction. The coordinates (ξ, η, ζ) are normalized such that they vary between −1 and +1. The coordinates of a typical point in the element can be written as ∙
n X
1+ζ k 1−ζ k xi = (xi )top + (xi )bottom ψk (ξ, η) 2 2 k=1
¸
(9.7.12)
where n is the number of nodes in the element, and ψk (ζ, η) is the Þnite element interpolation function associated with node k. If ψk (ξ, η) are derived as interpolation functions of a parent element, square or triangular inplane, then compatibility is achieved at the interfaces of curved space shell elements. DeÞne ˆk3 = V3k /V3k  V3ik = (xki )top − (xki )bottom , e (9.7.13) where V3k is the vector connecting the upper and lower points of the normal at node k. Equation (9.7.12) can be rewritten as xi =
n X
k=1
ψk (ξ, η)
∙
(xki )mid
¸
∙
n X ζ k ζ + V3i = ψk (ξ, η) (xki )mid + hk ek3i 2 2 k=1
¸
(9.7.14)
where hk = V3k  is the thickness of the shell element at node k. Hence, the coordinates of any point in the element at time t are interpolated by the expression ∙ ¸ n X ζ 1 xi = ψk (ξ, η) 1 xki + hk 1 ek3i (9.7.15) 2 k=1
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
x3
eˆ 3k
x2
ˆ ˆ E2 E 3
eˆ 1k
eˆ k2
ˆ E 1
373
Element nodes
x1
Figure 9.7.1 Geometry and coordinate system of a curved shell element. and the displacements and incremental displacements by 1
ui = 1 xi − 0 x1 = ui = 2 ui − 1 ui =
n X
ψk (ξ, η)
k=1
∙
1 k ui
∙
¸
´ ζ ³ ψk (ξ, η) uki + hk 2 ek3i − 1 ek3i 2 k=1 n X
¸
´ ζ ³ + hk 1 ek3i − 0 ek3i 2
(9.7.16) (9.7.17)
Here 1 uki and uki denote, respectively, the displacement and incremental displacement components in the xi direction at the kth node and time t. For small rotation dΩ at each node, we have ˆk1 + θ1k 1 e ˆk2 + θ3k 1 e ˆk3 dΩ = θ2k 1 e
(9.7.18)
ˆk3 can be written as the increment of vector 1 e ˆk3 = 2 e ˆk3 − 1 e ˆk3 = dΩ × 1 e ˆk3 = θ1k 1 e ˆk1 − θ2k 1 e ˆk2 ∆1 e
(9.7.19)
Then Eq. (9.7.17) becomes ui =
n X
k=1
ψk (ξ, η)
∙
uki
¸
´ ζ ³ + hk θ1k 1 ek1i − θ2k 1 ek2i 2
(i = 1, 2, 3)
(9.7.20)
ˆk1 and 1 e ˆk2 at node k can be obtained from the relations The unit vectors 1 e 1 k ˆ1 e
=
ˆ 2 × 1e ˆk3 E , ˆ 2 × 1e ˆk3  E
1 k ˆ2 e
ˆk3 × 1 e ˆk1 = 1e
(9.7.21)
374
NONLINEAR FINITE ELEMENT ANALYSIS
ˆ i are the unit vectors of the stationary global coordinate system where E 0 0 ( x1 , x2 , 0 x3 ). Equation (9.7.35) can be written in matrix form as {u} = {u1 u2 u3 }T = 1 [H]3×5n {∆e }5n×1
(9.7.22)
where {∆e } = {uki θ1k θ2k }T , (i = 1, 2, 3, k = 1, 2, . . . , n, and n is the number of nodes) is the vector of nodal incremental displacements (Þve per node), and 1 [H] is the incremental displacement interpolation matrix ⎡
. . . ψk = ⎣... 0 ... 0
0 ψk 0
0 0 ψk
1 1 k 2 ψk ζhk e11 1 1 k 2 ψk ζhk e12 1 1 k 2 ψk ζhk e13
− 12 ψk ζhk 1 ek21 − 12 ψk ζhk 1 ek22 − 12 ψk ζhk 1 ek23
⎤
... 1 [H]3×5n ...⎦ ... (9.7.23) For each time step or iteration step one can Þnd three unit vectors at each node from Eqs. (9.7.19) and (9.7.21). From Eq. (9.3.17) the linear strain increments {0 e} = {0 e11 0 e22 0 e33 20 e12 20 e13 20 e23 }T can be expressed as {0 e} = 1 [A]{0 u}
(9.7.24a)
where {0 u} is the vector of derivatives of increment displacements, {0 u} = {0 u1,1 0 u1,2 0 u1,3 0 u2,1 0 u2,2 0 u2,3 0 u3,1 0 u3,2 0 u3,3 }T ⎡ 1 + 1u
0 1,1
⎢ ⎢ 1 [A] ⎢ = 6×9 ⎢ ⎣
0 0 1u 0 1,2 1u 0 1,3 0
0 1u 0 1,2 0 1 + 10 u1,1 0 1u 0 1,3
0 0 1u 0 1,3 0 1 + 10 u1,1 1u 0 1,2
... ... ... ... ... ...
1u 0 3,1
0 0 1u 0 3,2 1 + 10 u3,3 0
0 1u 0 3,2 0 1u 0 3,1 0 1 + 10 u3,3
⎤ 0 0 ⎥ 1 + 10 u3,3 ⎥ ⎥ ⎥ 0 ⎦ 1u 0 3,1 1u 0 3,2
(9.7.24b)
The dots correspond to the entries (not displayed due to the page width) 1u ... 0 2,1 ... 0 ... 0 = ... 1 + 10 u2,2 1u ... 0 2,3 ... 0
0 1 + 10 u2,2 0 1u 0 3,1 0 1u 0 2,3
0 0 1u 0 2,3 0 1u 0 2,1 1 + 10 u2,2
and 0 ui,j = ∂ui /∂ 0 xj . The vectors {0 u} and {0 e} are related to the displacement increments at nodes by {0 u} = [N ] {u} = [N ] 1 [H]{∆e } {0 e} = 1 [A]{0 u} = 1 [A][N ] 1 [H]{∆e } ≡ 10 [BL ]{∆e } 1 1 1 0 [BL ] = [A][N ] [H]
(9.7.25a)
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
375
where [N ]T is the operator of diﬀerentials ⎡
∂ ∂ 0 x1
[N]T = ⎣ 0 0
∂ ∂ 0 x2
∂ ∂ 0 x3
0 0
0 0
0
0
0
∂ ∂ 0 x1
∂ ∂ 0 x2
∂ ∂ 0 x3
0
0
0
0 0
0 0
∂ ∂ 0 x1
∂ ∂ 0 x2
⎤
0 0 ⎦
∂ ∂ 0 x3
(9.7.25b)
The components of 1 [A] include 10 ui,j . From Eq. (9.7.16) the global displacements are related to the natural curvilinear coordinates (ξ, η) and the linear coordinate ζ. Hence the derivatives of these displacements 10 ui,j with respect to the global coordinates 0 x1 , 0 x2 , and 0 x3 are obtained through the relation ⎡ ∂ 1u
1
⎢
[10 ui,j ] = ⎢ ⎣
∂ 0 x1 ∂ 1 u1 ∂ 0 x2 ∂ 1 u1 ∂ 0 x3
∂ 1 u2 ∂ 0 x1 ∂ 1 u2 ∂ 0 x2 ∂ 1 u2 ∂ 0 x3
∂ 1 u3 ∂ 0 x1 ∂ 1 u3 ∂ 0 x2 ∂ 1 u3 ∂ 0 x3
⎤
⎡ ∂ 1u
1
⎥ 0 −1 ⎢ ⎥ = [J] ⎢ ⎦ ⎣
∂ξ ∂ 1 u1 ∂η ∂ 1 u1 ∂ζ
∂ 1 u2 ∂ξ ∂ 1 u2 ∂η ∂ 1 u2 ∂ζ
∂ 1 u3 ∂ξ ∂ 1 u3 ∂η ∂ 1 u3 ∂ζ
⎤ ⎥ ⎥ ⎦
(9.7.26)
The Jacobian matrix 0 [J] is deÞned as ⎡ ∂ 0x
∂ 0 x2 ∂ξ ∂ 0 x2 ∂η ∂ 0 x2 ∂ζ
1
0
⎢
[J] = ⎢ ⎣
∂ξ ∂ 0 x1 ∂η ∂ 0 x1 ∂ζ
∂ 0 x3 ∂ξ ∂ 0 x3 ∂η ∂ 0 x3 ∂η
⎤ ⎥ ⎥ ⎦
(9.7.27)
and is computed from the coordinate deÞnition of Eq. (9.7.15). The derivatives of displacements 1 ui with respect to the coordinates ξ, η, and ζ can be computed from Eq. (9.7.16). In the evaluations of element matrices in Eqs ˆ should be (9.7.6ad), the integrands of 10 [BL ], 0 [C], 10 BNL ], 10 [S], 1 [H], and 10 {S} expressed in the same coordinate system, namely the global coordinate system (0 x1 , 0 x2 , 0 x3 ) or the local curvilinear system (x01 , x02 , x03 ). The number of stress and strain components are reduced to Þve since we neglect the transverse normal components of stress and strain. Hence, the global derivatives of displacements, [10 ui,j ] which are obtained in Eq. (9.7.26), are transformed to the local derivatives of the local displacements along the orthogonal coordinates by the following relation ⎡ ∂ 1 u0 ⎢ ⎢ ⎢ ⎣
1 ∂x01 1 ∂ u01 ∂x02 ∂ 1 u01 ∂x03
∂ 1 u02 ∂x01 ∂ 1 u02 ∂x02 ∂ 1 u02 ∂x03
∂ 1 u03 ∂x01 ∂ 1 u03 ∂x02 ∂ 1 u03 ∂x03
⎤
⎥ ⎥ 1 ⎥ = [θ]T 3×3 [0 ui,j ] [θ]3×3 ⎦
(9.7.28)
where [θ]T is the transformation matrix between the local coordinate system (x01 , x02 , x03 ) and the global coordinate system (0 x1 , 0 x2 , 0 x3 ) at the integration
376
NONLINEAR FINITE ELEMENT ANALYSIS
point. The transformation matrix [θ] is obtained by interpolating the three ˆ1 , 1 e ˆ2 , 1 e ˆ3 ) at each node: orthogonal unit vectors (1 e ⎡ Pn ψk 1 ek11 Pk=1 n [θ] = ⎣ Pk=1 ψk 1 ek12 n 1 k k=1 ψk
e13
Pn ψk 1 ek21 Pk=1 n ψk 1 ek22 Pk=1 n 1 k k=1 ψk
e23
Pn ⎤ ψk 1 ek31 Pk=1 n ψk 1 ek32 ⎦ Pk=1 n 1 k k=1 ψk
(9.7.29)
e33
Since the element matrices are evaluated using numerical integration, the transformation must be performed at each integration point during the numerical integration. In order to obtain 10 [BL ], the vector of derivatives of incremental displacements {u0 } needs to be evaluated. Equations (9.7.26) and (9.7.28) can be used again except that 1 ui are replaced by ui and the interpolation equation for ui , Eq. (9.7.17), is applied. The development of the matrix of material stiﬀness, 0 [C 0 ], is discussed next. Here we wish to present it for shell element composed of orthotropic material layers with the principal material coordinates (x1 , x2 , x3 ) oriented arbitrarily with respect to the local coordinate system (x01 , x02 , x03 ) (with x3 = x03 ). For a kth lamina of a laminated composite shell the matrix of material stiﬀnesses is given by ⎡ 0 ⎤ 0 0 C11 C12 C16 0 0 0 0 0 ⎢ C12 C22 C26 0 0 ⎥ ⎢ 0 ⎥ 0 0 0 ⎢ C [C ] = C C 0 0 ⎥ (9.7.30) 0 (k) 26 66 ⎢ 16 ⎥ 0 0 ⎦ ⎣ 0 0 0 C44 C45 0 0 0 0 0 C45 C55 where
0 = m4 Q11 + 2m2 n2 (Q12 + 2Q66 ) + n4 Q22 C11 0 C12 = m2 n2 (Q11 + Q22 − 4Q66 ) + (m4 + n4 )Q12 0 C16 = mn[m2 Q11 − n2 Q22 − (m2 − n2 )(Q12 + 2Q66 )] 0 C22 = n4 Q11 + 2m2 n2 (Q12 + 2Q66 ) + m4 Q22 0 C26 = mn[n2 Q11 − m2 Q22 + (m2 − n2 )(Q12 + 2Q66 )] 0 C66 = m2 n2 (Q11 + Q22 − 2Q12 ) + (m2 − n2 )2 Q66 0 0 C44 = m2 Q44 + n2 Q55 , C45 = mn(Q55 − Q44 ) 0 2 2 C55 = m Q55 + n Q44 m = cos θ(k) , n = sin θ(k)
(9.7.31)
where Qij are the plane stressreduced stiﬀnesses of the kth orthotropic lamina in the material coordinate system. The Qij can be expressed in terms of engineering constants of a lamina Q11 =
E1 ν12 E2 E2 , Q12 = , Q22 = 1 − ν12 ν21 1 − ν12 ν21 1 − ν12 ν21
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
Q44 = G23 , Q55 = G13 , Q66 = G12
377
(9.7.32)
where Ei is the modulus in the xi direction, Gij (i 6= j) are the shear moduli in the xi xj plane, and νij are the associated Poisson’s ratios (see Reddy [10]). To evaluate element matrices in Eqs. (9.7.6a—d), we employ the Gauss quadrature. Since we are dealing with laminated composite structures, integration through the thickness involves individual lamina. One way is to use Gauss quadrature through the thickness direction. Since the constitutive relation 0 [C] is diﬀerent from layer to layer and is not a continuous function in the thickness direction, the integration should be performed separately for each layer. This increases the computational time as the number of layers is increased. An alternative way is to perform explicit integration through the thickness and reduce the problem to a twodimensional one. The Jacobian matrix, in general, is a function of (ξ, η, ζ). The terms in ζ may be neglected provided the thickness to curvature ratios are small. Thus the Jacobian matrix 0 [J] becomes independent of ζ and explicit integration can be employed. If ζ terms are retained in 0 [J], Gauss points through the thickness should be added. In the present study we assume that the Jacobian matrix is independent of ζ in the evaluation of element matrices and the internal nodal force vector. Since the explicit integration is performed through the thickness, the expression for "
#
∂ 1 u0i , ∂x0j
1 0 0 [A ],
{0 u0 },
1
[H],
1 0 0 [B ],
{10 ε0ij }
are now expressed in an explicit form in terms of ζ. Hence, we can use exact integration through the thickness and use the Gauss quadrature to perform numerical integration on the midsurface of the shell element. For thin shell structures, in order to avoid “locking” we use reduced integration scheme to evaluate the stiﬀness coeﬃcients associated with the transverse shear deformation. Hence we split the constitutive matrix 0 [C 0 ] into two parts, one without transverse shear moduli 0 [C 0 ]B , and the other with only transverse shear moduli 0 [C 0 ]S . Full integration is used to evaluate the stiﬀness coeﬃcients containing 0 [C 0 ]B , and reduced integration is used for those containing 0 [C 0 ]S . If a shell element is subjected to a distributed load (such as the weight or pressure), the corresponding load vector 2 {R} from Eq. (9.7.6) is given by 2
{R}5n×1 =
Z
0A
1
[H]T
⎧2 ⎫ ⎨ P1 ⎬
2P d 0A ⎩2 2⎭
(9.7.33)
P3
where 2 Pi is the component of distributed load in the 0 xi direction at time t + ∆t, 0 A is the area of upper or middle or bottom surface of the shell
378
NONLINEAR FINITE ELEMENT ANALYSIS
element depending on the position of the loading and the loading is assumed deformationindependent. Substituting 1 [H] into Eq. (9.7.33) yields ⎡
2
{R}5n×1
... ⎢ ψk ⎢ ⎢ Z 0 ⎢ ⎢ 0 = ⎢ 0A ⎢ 1 ⎢ 2 ζψk hk 1 ek11 ⎢ 1 ⎣ − ζψk hk 1 ek21 2 ...
⎤
... ... ⎥ 0 0 ⎥⎧ ⎥ 2P ⎫ ψk 0 ⎥⎨ 1⎬ ⎥ 2 0 ψk P2 d 0 A ⎥ ⎥ ⎩ 1 1 1 k 1 k 2P ⎭ 3 2 ζψk hk e12 − 2 ζψk hk e13 ⎥ ⎥ 1 1 1 ek 1 ek ⎦ ζψ h − ζψ h k k k k 22 23 2 2 ... ... ⎧ ⎫ ... ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ 2P ⎪ ⎪ ⎪ ⎪ ψ 1 k ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ 2 ⎪ ⎪ ψ P NGP NGP 2 k ⎨ ⎬ X X 2 2 ψP Wξr Wηs  0 J(ξr ,ηs ) (9.7.34) = k P3 ⎪ ⎪ h 3 2 1 k ⎪ 1 r=1 s=1 ⎪ ⎪ ⎪ ⎪ 2 ζψk hk Pi=1 Pi e1i ⎪ ⎪ ⎪ ⎪ ⎪ 3 2 P 1 ek ⎪ ⎪ − 1 ζψ h ⎪ ⎪ ⎪ ⎩ 2 k k i=1 i 2i ⎪ ⎭ ... P
E where h = NP k=1 ψk (ξ, η)hk is the shell thickness at each Gauss point, and W is the weight at each Gauss point, and 0 J is the determinant of the Jacobian matrix in Eq. (9.7.27) at each Gauss point. Here the ζ terms are retained in Jacobian matrix and let ζ equal to 1, −1, or 0, respectively, when the distributed loading is at the top, bottom, or middle surface.
9.7.5 Numerical Examples A number of numerical examples of isotropic and orthotropic plates and shells are presented. Only static bending problems of plates and shells are included. The Riks—Wempner method is employed for tracing the nonlinear load—deßection path (see Appendix 1). For most of the problems the reduced/selective integration scheme is used to evaluate the element stiﬀness coeﬃcients. For additional examples, the reader may refer to [22,24—26]. Simplysupported (BC1) orthotropic plate under uniform load Figure 9.7.2(a) shows the plate and material properties used. A quarter of the plate with the boundary and symmetry conditions shown in the Þgure is modeled with four ninenode shell elements. The results are shown in Figure 9.7.2(b) along with the experimental results of Zaghloul and Kennedy [29]. For this simplysupported plate, the Þnite element results are in good agreement with the experimental results of Zaghloul and Kennedy [29].
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
379
E1 = 3 × 10 6 psi, E 2 = 1.28 × 10 6 psi, G12 = G13 = G23 = 0.37 × 10 6 psi, ν 12 = 0.32 h = 0.138 in.
y
(a)
•
•
•
•
•
•
• • • •
• •
• •
• •
• •
• • • •
b = 6 in.•
• •
x
a = 6 in.
2.0
(b)
Load intensity (psi)
1.5
1.2
Linear solution
0.8
!
0.4
Experiment [29] Present
0.0 0.0
0.1
0.2
0.3
0.4
0.5
Center deflection, u3 (in)
Figure 9.7.2 Geometrically nonlinear response of an orthotropic plate. (a) Geometry and mesh. (b) Loaddeßection curves. Fourlayer (0/90/90/0) clamped plate under uniform load Figure 9.7.3(a) shows a clamped, symmetrically laminated, square plate under uniform load. The material properties of a typical layer and Þnite element mesh are also shown in the Þgure. A quarter of the plate is modeled using four ninenode elements. The present results along with the experimental
380
NONLINEAR FINITE ELEMENT ANALYSIS
results of Zaghloul and Kennedy [29] are shown in Figure 9.7.3(b). The two results are not in good agreement in this case. The diﬀerence between the theoretical and experimental results is attributed to possible diﬀerence in the support conditions used in the Þnite element analysis and those used in the experiment (i.e. the exact nature of clamped boundary conditions used in the test may not be the ones used in the Þnite element analysis), because the present results are veriÞed against independent Þnite element study by Putcha and Reddy [30]. E1 = 1.8282 × 10 6 psi, E 2 = 1.8315 × 10 6 psi, G12 = G13 = G23 = 0.3125 × 10 6 psi, ν 12 = 0.2395 h = 0.096 in.
y
(a)
•
•
•
•
•
• b = 6 in.• • •
• • • •
• •
• •
• •
• •
• • • •
a = 6 in.
x
2.0
(b)
Load intensity (psi)
1.5
1.2
Linear solution
0.8
0.4
!
Experiment [29] Present (close to [30])
0.0 0.0
0.1
0.2
0.3
0.4
0.5
Center deflection, u3 (in)
Figure 9.7.3 Nonlinear response of a clamped crossply laminated plate. (a) Geometry and mesh. (b) Load—deßection curves.
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
381
Cylindrical shell roof under selfweight Here we consider the linear analysis of a cylindrical panel under its own weight [see Figure 9.7.4(a)]. This is often used as a benchmark problem [27] in the literature to validate numerical methods (i.e. assessment of the performance of shell Þnite elements). It was shown in [28] that with the uniform (i.e. for all stiﬀness coeﬃcients) reduction of integration order the quadratic and cubic shell elements show a more rapid convergence and better accuracy than with the reduced integration order applied to transverse shear terms only. The linear results obtained in the present study with the ninenode quadratic element and uniform reduced integration order are shown in Figures 9.7.4(b) and 9.7.4(c). Even with one element the results show good agreement with the exact solution [27], and for further mesh reÞnement the results are close to the exact one. Simplysupported spherical shell under point load A simply supported isotropic spherical shell panel under central point load [see Figure 9.7.5(a)] is analyzed for its large displacement response using a meshes of sixteen fournode elements and four ninenode elements in a quarter of the shell. Figure 9.7.5(b) shows the responses calculated, including the postbuckling range, with the modiÞed Riks—Wempner method. The Þgure also includes the results of Bathe and Ho [17]. Shallow cylindrical shell under point load An isotropic shallow cylindrical shell hinged along the longitudinal edges and free at the curved boundaries and subjected to a point load is analyzed [see Figure 9.7.6(a)]. Onequarter of the shell is modeled with four ninenode shell elements. The structure exhibits snapthrough as well as snapback phenomena, as shown in 9.7.6(b). The solution obtained by CrisÞeld [18] is also shown in Figure 9.7.6(b) to be compared with the present results.
Problems 9.1 Consider the uniform deformation of a square of side 2 units initially centered at X = (0, 0). The deformation is given by the mapping [3] 1
x1 = (18 + 4X1 + 6X2 ), 4
1
x2 = (14 + 6X2 ) 4
(a) Sketch the deformed conÞguration of the body. (b) Compute the components of the deformation gradient tensor F and its inverse (display them in matrix form). (c) Compute the components of the right and left Cauchy—Green deformation tensors (display them in matrix form). (d) Compute the Green’s and Almansi’s strain tensor components (display them in matrix form).
382
NONLINEAR FINITE ELEMENT ANALYSIS
Symmetry line u1 = φ1 = 0
x3, u3
Simply supported rigid diaphram u1 = u3 = φ1 = 0
• • • •• •• •• • •• •• •• • • h = 3 in. • • • •25 ft. u2 = φ2 = 0 • • • Free edge 
Free edge

(a)
x2, u2
25 ft. 40°
E1 = 3×106 psi., ν = 0.0 ρgh = 0.625 lb/in2.

R = 25 ft.
x1, u1
u3
° ♦ •
(b) Mesh 1×1
•
♦
10°
Integration rule 2 × 2
•
40° 20°
°• ♦
2×2
♦
3×3
•
− 0.1
•
°
♦
− 0.2
• ♦ •
°
u2 (c)
•° ♦
•
♦
10°
•
♦ •
θ
30°
°
•
20°
30°
30°
− 0.3
40°
♦
θ
• − 0.005
° ♦•
− 0.01
Figure 9.7.4 A cylindrical shell roof under selfweight. (a) Geometry and the Þnite element mesh. (b) Vertical displacement on midsection. (c) Axial displacement at support.
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
383
x3 Symmetry line: u1 = φ1 = 0
E = 104 psi, ν = 0.3 R = 100 in., a = 30.9017 in.
•••• • • • • • •••• • • •• • • • • •••• 2a
Simply supported: u1 = u2 = u3 = φ1 = 0
P
h = 3.9154 in.


2a R

x2

Simply supported: u1 = u2 = u3 = φ2 = 0
Symmetry line: u2 = φ2 = 0
x1
(a)
20.0 17.5

Bathe and Ho [17] 9node elements 4node elements Present
15.0
Load, P (kips)
12.5

10.0 7.5

5.0 2.5
2.0
2.5
3.0

1.5

1.0

0.5

0.0



0.0
3.5
Transverse deflection, u3/h (b)
Figure 9.7.5 A simply supported spherical shell panel. (a) Geometry and Þnite element mesh. (b) Load—deßection curves.
384
NONLINEAR FINITE ELEMENT ANALYSIS
Symmetry line: u1 = φ1 = 0 x3, u3
Simply supported: u1 = u2 = u3 = φ2 = 0
Free edge ••••• • ••••••••••••• •• •• •L Simply supported: • u1 = u2 = u3 = φ2 = 0
P


h = 6.35 mm
(a)
x2, u2
E = 3103 Ν/mm2, ν = 0.3 R = 2540 mm, L = 254 mm.
0.1

L
R
x1, u1
Symmetry line: u2 = φ2 = 0
0.7 0.6 0.5 0.4
Load, P (kN)
0.3
(b)
0.2 0.1 0.0
• Crisfield [18]
−0.1
!
Present
−0.2 −0.3 −0.4 −0.5
0
5
10
15
20
25
30
Center deflection, u3 (mm)
Figure 9.7.6 Geometrically nonlinear response of a shallow cylindrical shell. (a) Geometry and Þnite element mesh. (b) Load— deßection curves. 9.2 Consider the deformation given by the mapping [3] x1 =
1 4
[4X1 + (9 − 3X1 − 5X2 − X1 X2 ) t] ,
x2 =
1 4
[4X2 + (16 + 8X1 ) t]
(a) For X = (0, 0) and t = 1 determine the deformation gradient tensor F and right Cauchy—Green tensor C.
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
385
(b) Find the eigenvalues (stretches) λ1 and λ2 and the associated eigenvectors N1 and ˆ 1 = {0.8385 0.5449}T . N2 . Partial answer: λ1 = 2.2714, N (c) Use the polar decomposition to determine the symmetric stretch tensor U and rotation tensor R. 9.3 Determine the displacements and strains in the (x1 , x2 ) system for the bodies shown in Figure P9.3. 9.4 Determine the displacements and strains in the (x1 , x2 ) system for the bodies shown in Figure P9.4.
x2
x2
X2
X2
e0
C
e0 C
e0 C D
D D
b A
e0
C b
A
a
B
x1, X1
B
A
D
x1=k x22 A
a
B
Figure P9.3
B
x1, X1
Figure P9.4
9.5 Consider the inÞnitesimal tetrahedron in Cartesian coordinates shown in Figure P9.5. If −t1 , −t2 , −t3 , and t denote the stress vectors in the outward directions on the faces of the inÞnitesimal tetrahedron whose areas are ∆s1 , ∆s2 , ∆s3 , and ∆s, respectively, we have by Newton’s second law for the mass inside the tetrahedron, t∆s − t1 ∆s1 − t2 ∆s2 − t3 ∆s3 + ρ∆vf = ρ∆va
(a)
where ∆v is the volume of the tetrahedron, ρ the density, f the body force per unit mass, and a the acceleration. Establish the Cauchy formula by writing ˆ1 + σi2 e ˆ2 + σi3 e ˆ3 = σij e ˆj ti = σi1 e ˆ1 )∆s, n·e ∆s1 = (ˆ
ˆ2 )∆s, ∆s2 = (ˆ n·e
ˆ3 )∆s ∆s3 = (ˆ n·e
∆h ∆s 3 where ∆h is the perpendicular distance from the origin to the slant face. ∆v =
x3 t1 nˆ
t2
tnˆ x2
x1
t3 Figure P9.5
(b) (c) (d)
386
NONLINEAR FINITE ELEMENT ANALYSIS
9.6 Establish the following relationships between area elements and volume elements of reference conÞguration and current conÞguration: ˆ dA. ˆ da = F−T · NJ (a) Nanson’s formula: n (b) dv = J dV . where dA and dV are area and volume elements, respectively, in the reference conÞguration and da and dv are area and volume elements, respectively, in the deformed conÞguration. 9.7 Let an arbitrary region in a continuous medium be denoted by v and the bounding closed surface of this region be continuous and denoted by s. Let each point on the bounding surface move with the velocity vs . It can be shown that the time derivative of the volume integral over some continuous function Q(x, t) is given by D Dt
Z
v
Q(x, t) dv ≡
Z
v
∂Q dv + ∂t
I
s
ˆ ds Qvs · n
(a)
This expression for the diﬀerentiation of a volume integral with variable limits is sometimes known as the threedimensional Leibniz rule. The material derivative operator D/Dt corresponds to changes with respect to a Þxed mass, that is, ρdv is constant with respect to this operator. Show formally by means of Leibniz’s rule, the divergence theorem, and conservation of mass that D Dt
Z
v
ρφ dv ≡
Z
v
ρ
Dφ dv Dt
(b)
9.8 Newton’s second law of motion applied to a continuum states that the rate of change of momentum following a material region of Þxed mass is equal to the sum of all the forces on the region. When the forces are divided into surface forces and body forces, Newton’s second law reads: Z I Z ↔ D ˆ · σ ds + ρb dv ρv dv = n (a) Dt v s v ↔
where σ is the surface stress tensor, b is the body force per unit mass, ρ is the mass density, and v is the material velocity. Since the material particle mass ρdv is constant with respect to the material time derivative D/Dt, make use of the divergence theorem and obtain the diﬀerential form of Newton’s second law of motion for a continuum (see Chapter 7): ↔ Dv ρ (b) = div σ + ρb Dt 9.9 Let e denote the thermodynamic internal energy per unit mass of a material. Then the equation of change for total energy of a material region can be written (see Chapter 7): D Dt
Z
v
µ
ρ e+
v2 2
¶
dv =
I
↔
s
n ˆ · σ · v ds +
Z
v
ρb · v dv −
I
s
q·n ˆ ds
(a)
The Þrst two terms on the righthand side describe the rate of work done on the material region by the surface stresses and the body forces. The third integral describes the net outßow of heat from the region, causing a decrease of energy inside the region. The heatßux vector q describes the magnitude and direction of the ßow of heat energy per unit time and per unit area.
FINITE ELEMENT FORMULATIONS OF SOLID CONTINUA
387
By suitable operations obtain the diﬀerential form of the energy equation: ρ
D Dt
µ
e+
v2 2
¶
= div
³↔
´
σ · v + ρb · v − div q
(b)
Subtract the contribution from kinetic energy and obtain ρ
³↔
De = div Dt
´
↔
σ · v − v · div σ − div q
(c)
This is called the thermodynamic form of the energy equation for a continuum. 9.10 Determine the twodimensional displacement Þeld and the inÞnitesimal strain Þeld for the simple shear of a rectangular block shown in the Þgure below. Determine the normal and shear strain in the diagonal line element of the rectangular block shown in Figure P9.10.
x2
X2 e0
C
e0 D D
C b A
a
B
x1, X1
Figure P9.10
References 1. Malvern, L. E., Introduction to the Mechanics of a Continuous Medium, Prentice—Hall, Englewood Cliﬀs, NJ (1969). 2. Hinton, E. (ed), NAFEMS Introduction to Nonlinear Finite Element Analysis, NAFEMS, Glasgow, UK (1992). 3. Bonet, J. and Wood, R. D., Nonlinear Continuum Mechanics for Finite Element Analysis, Cambridge University Press, New York (1997). 4. Bathe, K. J. , Finite Element Procedures, Prentice—Hall, Englewood Cliﬀs, NJ (1996). 5. Reddy, J. N., Energy Principles and Variational Methods in Applied Mechanics, Second Edition, John Wiley, New York (2002). 6. Reddy, J. N. and Rasmussen, M. L., Advanced Engineering Analysis, John Wiley, New York, 1982; reprinted by Krieger, Melbourne, FL (1990). 7. Heyliger, P. R. and Reddy, J. N., “A Mixed Updated Lagrangian Formulation for Plane Elastic Bodies,” Journal of Composites Technology & Research, 9(4), 131—140 (1987). 8. Heyliger, P. R. and Reddy, J. N., “On a Mixed Finite Element Model for Large Deformation Analysis of Elastic Solids,” International Journal of NonLinear Mechanics, 23(2), 131—145 (1988). 9. Roy, S. and Reddy, J. N., “NonLinear Analysis of Adhesively Bonded Joints,” International Journal of NonLinear Mechanics, 23(2), 97—112 (1988). 10. Reddy, J. N., Mechanics of Laminated Composite Plates and Shells: Theory and Analysis, 2nd edn, CRC Press, Boca Raton, FL (2004).
388
NONLINEAR FINITE ELEMENT ANALYSIS
11. Horrigmoe, G. and Bergan, P. G., “Incremental Variational Principle and Finite Element Models for Nonlinear Problems,” Computer Methods in Applied Mechanics and Engineering, 7, 201—217 (1976). 12. Wunderlich, W., “Incremental Formulations for Geometrically Nonlinear Problems,” in Formulations and Algorithm in Finite Element Analysis, K.J. Bathe, J.T. Oden, and W. Wunderlich (eds), 193—239, MIT Press, Boston, MA (1977). 13. Ramm, E., “A Plate/Shell Element for Large Deßections and Rotations,” in Formulations and Computational Algorithms in Finite Element Analysis, K.J. Bathe, J. T. Oden, and W. Wunderlich (eds), MIT Press, Boston, MA (1977). 14. Bergan, P. G. and Soreide, T. H., “Solution of Large Displacement and Instability Problems Using the Current Stiﬀness Parameter,” Finite Elements in Nonlinear Mechanics, 647—649, Tapir Press (1978). 15. Rao, K. P., “A Rectangular Laminated Anisotropic Shallow Thin Shell Finite Element,” Computer Methods in Applied Mechanics and Engineering, 15, 13—33 (1978). 16. Bathe, K. J. and Bolourchi, S., “A Geometric and Material Nonlinear Plate and Shell Element,” Computers and Structures, 11, 23—48 (1980). 17. Bathe, K. J. and Ho, L. W., “A Simple and Eﬀective Element for Analysis of General Shell Structures,” Computers and Structures, 13, 673—681 (1981). 18. CrisÞeld, M. A., “A Fast Incremental/Iterative Solution Procedure That Handles SnapThrough,” Computers and Structures, 13, 55—62 (1981). 19. Chang, T. Y. and Sawamiphakdi, K., “Large Deformation Analysis of Laminated Shells By Finite Element Method,” Computers and Structures, 13, 331—340 (1981). 20. KanokNukulchai, W., Taylor, R. L., and Hughes, T. J. R., “A Large Deformation Formulation for Shell Analysis by the Finite Element Method,” Computers and Structures, 13, 19—27 (1981). 21. Surana, K. S.,“Geometrically Nonlinear Formulation for the Three Dimensional SolidShell Transition Finite Elements,” Computers and Structures, 15, 549—566 (1982). 22. Chao, W. C. and Reddy, J. N., “Analysis of Laminated Composite Shells Using a Degenerated 3D Element,” International Journal for Numerical Methods in Engineering, 20, 1991—2007 (1984). 23. Stanley, G. M. and Felippa, C. A., “Computational Procedures for Postbuckling for Composite Shells,” in Finite Element Methods for Nonlinear Problems, P. G. Bergan, K. J. Bathe, and W. Wunderlich (eds.), 359—385, Springer—Verlag, Berlin (1986). 24. Liao, C. L., Reddy, J. N., and Engelstad, S. P., “A SolidShell Transition Element for Geometrically Nonlinear Analysis of Laminated Composite Structures,” International Journal for Numerical Methods in Engineering, 26, 1843—1854 (1988). 25. Liao, C. L. and Reddy, J. N., “A ContinuumBased Stiﬀened Composite Shell Element for Geometrically Nonlinear Analysis,” AIAA Journal, 27(1), 95—101 (1989). 26. Liao, C. L. and Reddy, J. N., “Analysis of Anisotropic, Stiﬀened Composite Laminates Using a Continuum Shell Element,” Computers and Structures, 34(6), 805—815 (1990). 27. Scordelis, A. C. and Lo, K. S., “Computer Analysis of Cylindrical Shells,” ACI Journal, 61, 539—561 (1964). 28. Zienkiewicz, O. C., Taylor, R. L., and Too, J. M., “Reduced Integration Techniques in General Analysis of Plates and Shells,” International Journal for Numerical Methods in Engineering, 3, 275—290 (1971). 29. Zaghloul, S. A. and Kennedy, J. B., “Nonlinear Behavior of Symmetrically Laminated Plates,” Journal of Applied Mechanics, 42, 234—236 (1975). 30. Putcha, N. S. and Reddy, J. N., “A ReÞned Mixed Shear Flexible Finite Element for the Nonlinear Analysis of Laminated Plates,” Computers and Structures, 22, 529—538 (1986).
10 Material Nonlinearities and Coupled Problems
10.1 Introduction Recall that nonlinearities arise from two independent sources. (1) Nonlinearity due to changes in the geometry or position of the material particles of a continuum, which is called the geometric nonlinearity. (2) Nonlinearity due to the nonlinear material behavior, which is called material nonlinearity. In solid mechanics, the geometric nonlinearity arises from large strains and/or large rotations, and these enter the formulation through the strain—displacement relations as well as the equations of motion. In ßuid mechanics and coupled ßuid ßow and heat transfer, the geometric nonlinearity arises as a result of the spatial (or Eulerian) description of motion, and they enter the equations of motion through material time derivative term. Material nonlinearity in all disciplines of engineering arise from nonlinear relationship between the kinetic and kinematic variables, for example, stress—strain relations, heat ßux— temperature gradient relations, and so on. In general, material nonlinearities arise due to the material parameters (e.g. moduli, viscosity, conductivity, etc.) being functions of strains (or their rates), temperature, and other basic variables. The Þnite element formulations presented in the previous chapters were largely based on geometric nonlinearity. However, the nonlinearity in the oneand twodimensional Þeld problems discussed in Chapters 3 and 5 could have come from either sources. In this chapter, material nonlinear formulations are given attention. This Þeld is very broad and special books are devoted to various types of nonlinearities (e.g. plasticity, viscoelasticity, and nonNewtonian materials). The objective here is to brießy discuss nonlinear elastic and elastic—plastic material models for solids and powerlaw model for viscous incompressible ßuids.
390
NONLINEAR FINITE ELEMENT ANALYSIS
10.2 Nonlinear Elastic Problems Materials for which the constitutive behavior is only a function of the current state of deformation are known as elastic. In the special case in which the work done by the stresses during a deformation is dependent only on the initial state and the current conÞguration, the material is called hyperelastic, that is, there exists a strain energy density function U0 (Eij ) such that Sij =
∂U0 ∂Eij
(10.2.1)
where Sij ard Eij are the components of the second Piola—Kirchhoﬀ stress tensor and Green—Lagrange strain tensor, respectively. When U0 is a nonlinear function of the strains, the body is said to be nonlinearly elastic. A nonlinearly elastic material has the following features: (a) U0 is a nonlinear function of strains, (b) all of the deformation is recoverable on removal of loads causing the deformation, and (c) there is no loss of energy (i.e. loading and unloading is along the same stressstrain path; see Figure 10.2.1). Here we consider a onedimensional problem to discuss the Þnite element formulation of a nonlinear elastic material for the case of kinematically inÞnitesimal strains. Consider the nonlinear uniaxial stress—strain relation σxx = E F(εxx )
(10.2.2)
where εxx is the inÞnitesimal strain, E is a material constant, and F is a nonlinear function of the strain. The virtual work expression for the axial deformation of a bar made of a nonlinear elastic material is 0= =
Z Z xb A x Z xb a xa
σxx δεxx dxdA −
Z xb xa
f δu dx − P1e δu(xa ) − P2e δu(xb )
[EA F(εxx )δεxx − f δu] dx − P1e δu(xa ) − P2e δu(xb )
(10.2.3)
The residual vector for the Þnite element model is Rie
=
Z xb ∙ xa
¸
dψ e EA F(εxx ) i − f ψie dx − Pie dx
(10.2.4)
and the tangent stiﬀness matrix is e = Kij
∂Rie = EA ∂uej
Z xb ∂F ∂εxx dψie
¶ Z xb µ ∂F dψie dψje
dx dx dx (10.2.5) where small strain assumption is used in arriving at the last step. xa
∂εxx ∂uej dx
dx = EA
xa
∂εxx
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
391
Linear elastic Loading and unloading Nonlinear elastic
Stress Softening type
Hardening type Strain
Figure 10.2.1 A nonlinear elastic stress—strain curve. An example of the nonlinear elastic response is provided by Romberg— Osgood model "F F(!xx ) = (!xx )n , = n (!xx )n−1 (10.2.6) "!xx where n > 0 is a material parameter. The value of n = 1 yields the linear elastic case. This discussion can be extended to multiaxial case, where F = F(!ij ).
10.3 Small Deformation Theory of Plasticity 10.3.1 Introduction Plasticity refers to nonrecoverable deformation and nonunique stress paths in contrast to nonlinear elasticity, where the entire load—deßection path is unique and the strains are recovered on load removal. The mathematical theory of plasticity is of a phenomenological nature on the macroscopic scale, and the objective of the theory is to provide a theoretical description of the relationship between stress and strain for a material that exhibits an elastoplastic response. The plastic behavior is characterized by irreversibility of stress paths and the development of permanent (i.e. nonrecoverable) deformation (or strain), known as yielding (or plastic ßow). If uniaxial behavior of a material is considered, a nonlinear stressstrain relationship on loading alone does not determine if nonlinear elastic or plastic behavior is exhibited. Unloading part of the curve determines if it is elastic or plastic [see Figure 10.3.1(a) and (b)]; the elastic material follows the same path in loading and unloading, while the plastic material shows a historydependent path unloading.
392
NONLINEAR FINITE ELEMENT ANALYSIS
Stress Loading
Unloading Strain (a)
Stress
Loading Unloading
Plastic strain Strain (b)
Figure 10.3.1 Stress—strain behavior of (a) ideal plasticity, and (b) strain— hardening plasticity. The theory of plasticity deals with an analytical description of the stress— strain relations of a deformed body after a part or all of the body has yielded. The stress—strain relations must contain: 1. The elastic stressstrain relations. 2. The stress condition (or yield criterion) which indicates onset of yielding. 3. The stress—strain or stress—strain increment relations after the onset of plastic ßow.
10.3.2 Ideal Plasticity Many materials exhibit an ideal plastic (or elastic—perfectly—plastic) behavior, as shown in Figure 10.3.1(a). In this case, there exists a limiting stress, called yield stress, denoted by #Y , at which the strains are indeterminate. For all
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
393
stresses below the yield stress, a linear (or nonlinear) stress—strain relation is assumed: σij < σY linear elastic behavior σij ≥ σY plastic deformation (not recoverable)
(10.3.1)
10.3.3 Strain Hardening Plasticity A hardening plastic material model provides a reÞnement of the ideal plastic material model. In this model, it is assumed that the yield stress depends on some parameter κ (e.g. plastic strain εp ), called the hardening parameter. The general yield criterion is expressed in the form F (σij , κ) = 0
(10.3.2)
This yield criterion can be viewed as a surface in the stress space, with the position of the surface dependent on the instantaneous value of the hardening parameter κ. Since any yield criterion should be independent of the orientation of the coordinate system used, F should be a function of the stress invariants only. Experimental observations indicate that plastic deformation in metals is independent of hydrostatic pressure. Therefore, F must be a function of the stress invariants of the deviatoric stress tensor σ 0 : 1
1
2
3
0 0 0 0 0 F (J20 , J30 , κ) = 0, J20 = σij σij , J30 = σij σjk σki
(10.3.3)
Two of the most commonly used yield criteria are given next. The Tresca yield criterion F = 2¯ σ cos θ − Y (κ) = 0,
σ ¯=
q
J20
(10.3.4)
The Huber—von Mises yield criterion F =
q
3J20 − Y (κ) = 0
(10.3.5)
where Y is the yield stress from uniaxial tests, θ is the q angle between the line of pure shear and the principal stress σ1 , and σ ¯ = J20 is called the eﬀective stress. After initial yielding, the stress level at which further plastic deformation occurs may be dependent on the current degree of plastic straining, known as strain hardening. Thus, the yield surface will vary (i.e. expand) at each stage of plastic deformation. When the yield surface is independent of the degree of plasticity, the material is said to be ideally (or perfectly) plastic.
394
NONLINEAR FINITE ELEMENT ANALYSIS
If the subsequent yield surfaces are a uniform expansion of the original yield surface, the hardening model is said to be isotropic. On the other hand, if the subsequent yield surfaces preserve their shape and orientation but translate in the stress space, kinematic hardening is said to take place. Consider the uniaxial stress—strain curve shown in Figure 10.3.2. The behavior is initially linear elastic with slope E (Young’s modulus) until onset of yielding at the uniaxial yield stress #Y . Thereafter, the material response is elasto—plastic with the local tangent to the curve, ET , called the elastoplastic tangent modulus, continually changing. At some stress level # in the plastic range, if the load is increased to induce a stress of d#, it results in a corresponding strain d!. This increment of strain contains two parts: elastic d!e (recoverable) and plastic d!p (nonrecoverable): d# d# , = ET E d! The strainhardening parameter, H, is deÞned by d! = d!e + d!p ,
H=
d!e =
dσ d# ET dε = e = dε p d! 1 − dε 1 − EET
(10.3.6)
(10.3.7)
The element stiﬀness for the linear elastic portion is, say [K e ]: [K e ] =
Z
xb
[B]T [De ][B] dx
(10.3.8)
xa
where [De ] is the linear elasticity matrix (De = E for the uniaxial case). When the element deforms plastically, [De ] reßects the decreased stiﬀness. This is computed, for uniaxial material behavior, by the following procedure: The Stress Slope ET  Elastic−plastic tangent modulus
dεp
dσp dεe
dε
σY E Strain
Figure 10.3.2 A strain hardening plastic behavior for the uniaxial case.
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
395
increment in load dF causes an incremental displacement du du = he dεxx = he (dεe + dεp ) , dF = Adσ = Ae Hdεp
(10.3.9)
where he is the length and Ae the area of crosssection of the element. The eﬀective stiﬀness is E
ep
∙
dF E Ae Hdεp EAe = 1− = = e p du he (dε + dε ) he (E + H)
¸
(10.3.10)
The element stiﬀness for plastic range becomes, [K ep ] =
Z xb
[B]T [Dep ][B]dx
(10.3.11)
xa
where [Dep ] is the material stiﬀness in the plastic range. For uniaxial case Dep = E ep . Equation (10.3.8) is valid when σ < σY and Eq. (10.3.11) is valid for σ > σY . Note that dσ = σ − σY when σ > σY .
10.3.4 Elastic—Plastic Analysis of a Bar Here we present a detailed computational procedure for the analysis of an elasto—plastic problem. The procedure is described via a onedimensional elasto—plastic bar problem. We shall consider a linear strainhardening material subjected to an increasing uniaxial load. Update of stresses At a loadstep number r where the deformation is elastic, the stress in a typical element with the strain increment ∆εr can be readily updated as (r−1)
σir = σi
+ Ei ∆εr
(10.3.12)
where Ei is the elastic modulus of element i. This linear elastic behavior will continue up till a point where the resulting strain increment will initiate plastic yielding of the material. Now the updating of the stress in the element is not as straighforward as given in Eq. (10.3.12), and it can get complicated when the deformation is partly elastic and partly elastoplastic, as shown from Points A to B in the stress—strain curve of Figure 10.3.3. To update the stress state from Points A to B, one can Þrst assume that the deformation is elastic and compute the corresponding elastic stress, commonly referred to as the elastic stress predictor. Using Eq. (10.3.12), the elastic stress predictor σe can be calculated as (r−1)
σei = σi
+ Ei ∆εri
(10.3.13)
396
NONLINEAR FINITE ELEMENT ANALYSIS
Computing the elastic stress predictor brings the stress state from Point A to A0 . A correction is made to transfer the stress state back to the elastoplastic state at Point B. We introduce a correction factor R (see Figure 10.3.3) #ei − #y
R=
(10.3.14)
(r−1)
#ei − #i
so that the stress at point B can be written as (r−1)
#ir = #i
+ [(1 − R)Ei + RET ] ∆!ri
(10.3.15)
Here ET denotes the elastic—plastic tangent modulus, which is related to the elastic modulus E and strainhardening parameter H by Eq. (10.3.7). In the case where the element has already yielded in previous load steps, as illustrated by point C in Figure 10.3.3, the approach of determining the elastic stress predictor and making correction to the stress state at point D still applies with R = 1 in Eq. (10.3.15): (r−1)
#ir = #i
+ ET ∆!ri
(10.3.16)
Update of plastic strain The extent of plastic ßow in a deformed material can be readily characterized by the measure of plastic strain. To determine the plastic strain in an element at point B of Figure 10.3.3, it will be useful to rewrite Eq. (10.3.15) as #ir = #Y + ET (R ∆!ri ) ≡ #Y + ∆#ir
σ σ ei
σ
C′
A′
C
r i
D
B
σY σ ir −1
(10.3.17)
A
R 1
∆ε ir
ε
Figure 10.3.3 Transition of elastic to elasto—plastic behavior.
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
397
Equation (10.3.17) can be interpreted as that adjusts the stress state at point A to the yield stress before predicting the elastic stress and its correction. This will allow one to isolate the stress component ∆#ir and strain R!ri that are involved in the plastic ßow. With Eq. (10.3.6), the plastic strain increment is µ ¶ ∆#ir ET ∆!rpi = R∆!ri − = 1− R ∆!ri (10.3.18) Ei Ei Equation (10.3.18) can also be used for elements that have already yielded in previous load steps by setting R = 1. Update of yield stress limit Besides assessing the extent of plastic deformation, the measure of the plastic strain will become especially crucial for strainhardening materials where the yield limit is a function of the plastic strain. A plot of yield limit against the plastic strain for a typical linear strainhardening material is shown in Figure 10.3.4. Once the plastic strain occurs, the yield limit will be modiÞed and updated as r #yi = #Y + H ∆!rpi (10.3.19) IdentiÞcation of deformation modes The updated yield limit will come in handy when one is to check the type of deformation an element is undergoing. Once the correct type of deformation is identiÞed, the stress and strain values can then be updated according to Eqs. (10.3.l5) and (10.3.18). There are four types of deformation:
σ yir
H σY
1
εp
Figure 10.3.4 Stress—strain behavior of a strain—hardening material.
398
NONLINEAR FINITE ELEMENT ANALYSIS
(a) Elastic Loading: (an element that has not yielded previously continues to deform elastically) r−1 r−1 #ir−1  < #yi  and #ei  < #yi 
(10.3.20a)
(b) ElasticPlastic Loading: (an element that has not yielded previously will deform elastoplastically) r−1 r−1 #ir−1  < #yi  and #ei  > #yi 
(10.3.20b)
(c) Plastic Loading: (an element that previously yielded will continue to deform plastically) r−1  and #ei  > #ir−1  #ir−1  > #yi
(10.3.20c)
(d) Elastic Unloading: (an element previously yielded is now unloading elastically) r−1 #ir−1  > #yi  and #ei  < #ir−1 
(10.3.20d)
Force equilibrium Since the displacement Þnite element model is based on the principle of virtual displacements, the solution satisÞes the equilibrium equations, provided the deformation is linearly elastic. However, in the Þnite element analysis of elasto—plastic problems equilibrium equations may not be satisÞed during the period when stresses are adjusted to account for plastic strains. Adjustments must be made to achieve equilibrium at each step by redistributing the forces neighboring elements. For example, consider a node N at the interface of element i that has yielded and the adjacent element i + 1 that is still elastic (see Figure 10.3.5). At this node, the force equilibrium will be violated during the analysis because the force in element i is limited such that the stress in the element does not exceed the yield stress. The diﬀerence between the force calculated using the elastic analysis and the plastic force must now be taken up by all other elastic elements in the mesh. Thus to restore equilibrium of forces, a force correction must be made at the node N : ∆F = Fi − Fi+1 − FN
(10.3.21)
∆F FN Fi
i
i+1
Fi+1
N
Figure 10.3.5 Force equilibrium at node N.
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
399
However, to preserve the Þnite element equations of the original problem (to retain the same forces in other unaﬀected elements), the force correction cannot be imposed as a nodal force. Instead, the force correction may be applied as a nodal displacement ∆ucN =
∆F Li ET Ai
(10.3.22)
where Li and Ai are the length and crosssectional area of element i. This correction procedure will continue until force equilibrium at all nodes is restored, within an acceptable error of tolerance. Figure 10.3.6 contains the ßow chart of various steps in the elasto—plastic analysis of a typical problem. A subroutine that updates stresses and plastic strains is also given in Box 10.3.1. A numerical example Consider a bar of length 5 m that is Þxed at one end and is subjected to a uniform body force f. The material properties of the bar are taken as E = 104 N/m2 ,
A = 1 m2 ,
σy = 5 N/m2 ,
H = 103 N/m2
The bar is discretized using a mesh of Þve linear elements. The elasto—plastic iterative scheme discussed in this section was implemented and the results are presented in Tables 10.3.1 and 10.3.2. At the start of the analysis when elements are still elastic, a nominal body force of 0.005 N/m was imposed to Þnd the maximum stress induced in the elements. The critical load Fcr for the Þrst element to yield was computed from this maximum element stress and is imposed in the next loadstep: fcr =
σY f maxσi 
(10.3.23)
Here i is the element number and N is the total number of unyielded elements. In the new loadstep where f is 1.1111 N/m, the computed results reveal that the Þrst element (element 1 in this example) had just yielded; up to this point, the analysis is still elastic. Then another nominal body force of 0.005 N/m is added to calculate the critical load for the next element to yield. The stiﬀness of the yielded Element 1 is reduced in this loadstep and the results in Table 10.3.2 indicate a violation of force equilibrium at the nodes connecting the yielded element, except for the node that is Þxed. Corrections to the nodal displacements were made until equilibrium was satisÞed at all nodes. Only then, the critical load for the next element to yield could be calculated and the same procedure is repeated until all elements yield. Reaction forces at the Þxed end of the bar against the freeend displacements are plotted in Figure 10.3.7, together with the results from
400
NONLINEAR FINITE ELEMENT ANALYSIS
♦ Load increment
f r = f r −1 + ∆ f
START
♦ Solve
[K ]{u}r = {F }
Compute element strain increment
∆εir
(assume elastic deformation) Compute element elastic stress predictor and yield stress limit
⎛ σ ei = σ ir −1 + E∆ε i ⎞ ⎜ ⎟ ⎜ σ r −1 = σ + Hε r −1 ⎟ yi Y pi ⎝ ⎠ (Element has not yielded in the previous loadstep) NO
r −1 σ ei > σ yi
σ ir −1
(Element is deforming elastically) NO
>
r −1 σ yi
(Element yielded in the previous loadstep) YES
(Element is unloading
R=0
elastically) NO
YES (Element has just yielded)
σ ei > σ ir −1
(Element continues YES to yield)
Update element stress and plastic strain
R=
σ ei − σ Y σ ei − σ ir −1
Modify nodal displacements
uir = uir + ∆uiC ∆ui = uir − uir −1
σ ir = σ ir −1 + [(1 − R )Ei + R ⋅ ET ]∆ε ir ε rpi
=
NO
ε rpi−1
⎛ E ⎞ + ⎜⎜1 − T ⎟⎟ R ⋅ ∆ε ir Ei ⎠ ⎝
Is equilibrium YES r satisfied at each node? (Go to the next loadstep)
R =1
= r +1
Figure 10.3.6 Flow chart for elasto—plastic analysis of a bar.
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
401
Box 10.3.1 Fortran statements for the subroutine to calculate the stress and plastic strain of a bar element. SUBROUTINE STRESS(N,MXELM,NDF,NPE,IELEM,ELSIG,PSTRAIN) C _____________________________________________________________________________ C The subroutine evaluates the stress at the center of the element and then update C the stress and plastic strain against the yield criteria. C C C C C
SIGMA … Updated elastic stress YOUNG … Elastic modulus TYIELD … Current yield limit TYOUNG… Elasto−plastic tangent modulus ELSIG(N) … Elastic stress of element N YSTRESS… Yield stress PSTRAIN(N)… Plastic strain of element N HP … Strain hardening parameter _____________________________________________________________________________ . . Definition of global and local variables and common blocks . C Compute the total strain increment at the center of element H = ELX(NPE) − ELX(1) CALL SHP1D (H,IELEM,NPE,0.D0) UX=0.D0 DO K=1,NPE UX=UX+EPV((K−1)*NDF+1)*GDSF(K) ENDDO C Update the elastic stress and current yield limit SIGMA = ELSIG(N)+YOUNG*UX IF(ELSIG(N).GT.0.0)TYIELD=YSTRESS+HP*PSTRAIN(N) IF(ELSIG(N).LE.0.0)TYIELD=−YSTRESS+HP*PSTRAIN(N) C Check the updated elastic stress for yielding IF (ABS(ELSIG(N)) .GT. ABS(TYIELD)) THEN IF (ABS(SIGMA) .GT. ABS(ELSIG(N))) THEN R = 1.D0 ELSE R = 0.D0 ENDIF ELSE IF (ABS(SIGMA) .GT. ABS(TYIELD)) THEN R = (ABS(SIGMA)ABS(TYIELD))/(ABS(SIGMA)ABS(ELSIG(N))) ELSE R = 0.D0 ENDIF ENDIF
C Correct the elastic stress and calculate the plastic strain ELSIG(N)= ELSIG(N)+((1.D0R)*YOUNG+R*TYOUNG)*UX PSTRAIN(N)= PSTRAIN(N)+(R/(1.D0+HP/YOUNG))*UX RETURN END
402
NONLINEAR FINITE ELEMENT ANALYSIS
Table 10.3.1 Nodal displacements for various load steps. Body force f
Node no.
Nodal displacement*
0.0050
2 3 4 5 6
2.2500×10−6 4.0000×10−6 5.2500×10−6 6.0000×10−6 6.2500×10−6
2 3 4 5 6
5.0000×10−4 8.8889×10−4 1.1667×10−3 1.3333×10−3 1.3889×10−3
2 3 4 5 6
5.2475×10−4
(5.2475×10−4 )
2 3 4 5 6
4.9525×10−2 8.8044×10−2 1.1556×10−1 1.3207×10−1 1.3257×10−1
(4.4525×10−2 ) (7.8044×10−2 ) (1.0056×10−1 ) (1.1207×10−1 ) (1.1257×10−1 )
1.1111
1.1161
10.0050
9.1539×10−4 1.1944×10−3 1.3618×10−3 1.4176×10−3
R 1 2 3
f
4 5
(9.1539×10−4 ) (1.1944×10−3 ) (1.3618×10−3 ) (1.4176×10−3 )
* Values in parenthesis are corrected to satisfy force equilibrium at each node.
• • • 90 Present results (5 elements) • • ABAQUS (5 elements) • 80 • • 70 R • • 60 • • 50 • • 40 • E = 10,000, • 30 f A = 1, L = 5, • H = 1,000, σ = 5 y • 20 • 10 •• • 0•
Reaction force, R
100
0.20

0.15

0.10

0.05


0.00
0.25
Nodal displacement at the bar end
Figure 10.3.7 Reaction forces versus nodal displacements at bar end.
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
403
Table 10.3.2 Element stresses and strains for various load steps. Body force f
Element number
Total strain ε
0.0050
1 2 3 4 5
2.2500×10−6 1.7500×10−6 1.2500×10−6 0.7500×10−6 0.2500×10−6
0 0 0 0 0
0.0225 0.0175 0.0125 0.0075 0.0025
1.1111
1* 2 3 4 5
5.0000×10−4 3.8889×10−4 2.7778×10−4 1.6667×10−4 0.5556×10−4
0 0 0 0 0
5.0000 3.8889 2.7778 1.6667 0.5556
1.1161
1 2 3 4 5
5.5248×10−4 3.9064×10−4 2.7903×10−4 1.6742×10−4 0.5581×10−4
4.5680×10−3 0 0 0 0
9.5680 3.9064 2.7903 1.6742 0.5581
Corrected to satisfy force equilibrium
1 2 3 4 5 ... 1 2 3 4 5
5.2475×10−4 3.9064×10−4 2.7903×10−4 1.6742×10−4 0.5581×10−4 ... 4.9525×10−2 3.8519×10−2 2.7514×10−2 1.6508×10−2 0.5503×10−2
0.2250×10−4 0 0 0 0 ... 4.4568×10−2 3.4563×10−2 2.4558×10−2 1.4553×10−2 0.4548×10−2
5.0225 3.9064 2.7903 1.6742 0.5581 ... 49.5680 39.5630 29.5580 19.5530 9.5480
1 2 3 4 5
4.4525×10−2 3.3519×10−2 2.2514×10−2 1.1508×10−2 5.0275×10−4
4.0023×10−2 3.0018×10−2 2.0013×10−2 1.0008×10−2 0.0025×10−4
45.0225 35.0175 25.0125 15.0075 5.0025
... 10.0050
Corrected to satisfy force equilibrium
Plastic strain εp
Bar force F  = σA
* Element just yielded at that load step.
the commercial Þnite element software ABAQUS. There is a very good agreement with the solutions generated by the iterative scheme discussed and those obtained with ABAQUS. A 20element mesh also produced results identical to those in Figure 10.3.7; the results are also veriÞed using ABAQUS. Figure 10.3.8 contains the true stress—strain diagram of Element 1, where one may note that the elasto—plastic material curve is recovered.
404
NONLINEAR FINITE ELEMENT ANALYSIS
50
•
Stress, σ
45 40
•
35
Element 1
R
30 909.08
25 20
•
15 10 5 0
•• 0.0
•
1
1
2
E = 10,000, A = 1, L = 5, H = 1,000, σy = 5
4
3
5
0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040 0.045 0.050
Total strain, ε
Figure 10.3.8 True stress—strain curve of Element 1.
10.4 NonNewtonian Fluids 10.4.1 Introduction Some of the most challenging and important areas currently under investigation in computational ßuid mechanics concern ßows of nonNewtonian ßuids. A nonNewtonian ßuid is deÞned to be the one whose constitutive behavior is nonlinear (i.e., stresses are nonlinear functions of strain rates). Such ßuids may or may not have memory of past deformation (i.e. viscoelastic or not). Practical examples of such ßuids are multigrade oils, liquid detergents, paints, and printing inks. Polymer solutions and polymer melts also fall within this category. All such ßows are extremely important in forming processes of various kinds applied to metals, plastics, or glass. Numerical approaches used for analyzing ßows of nonNewtonian ßuids diﬀer vary little from the ones used for Newtonian ßuids. When the shear viscosity is a function of the rate of deformation tensor (i.e., for socalled powerlaw ßuids), the equation of motion can still be written explicitly in terms of velocity components. For such ßuids, the same formulations as those of the Newtonian ßuids can be used. For example, the pressure—velocity and penalty Þnite element models discussed in Chapter 7 can be used for powerlaw ßuids. The constitutive equations of a viscoelastic ßuid can be described in terms of the extra stress components and are given in terms of either diﬀerential equations or as integral equations. In a diﬀerential constitutive model, the extra stress components and their derivatives are related to the
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
405
velocity components and their derivatives [6—9]. The computational models of viscoelastic ßuids obeying a diﬀerential model require us to treat stress components as independent variables along with the velocity components and pressure. Most existing works in the Þniteelement analysis of viscoelastic ßuids have considered diﬀerent types of Maxwell/Oldroyd models, in which both viscosity and relaxation time are constants. The analyses were based on mixed formulations in which the velocities, pressure, and stresses are treated as nodal variables. An important class of ßuids, which are characterized by shear viscosity and elasticity require additional study from numerical simulation point of view. The viscoelastic eﬀects along with memory eﬀects of such ßuids can be studied using the White—Metzner model [7]. The objective of this section is to study penalty Þnite element models of powerlaw and viscoelastic White—Metzner type ßuids. As a part of this discussion, we present Þnite element models of equations governing ßows of viscous incompressible ßuids through axisymmetric geometries. Therefore, Þrst we rewrite the conservation of mass and momentum in the cylindrical coordinate system.
10.4.2 Governing Equations in Cylindrical Coordinates Equations (7.3.1)—(7.3.3) can be expressed in a cylindrical coordinate system, (r, θ, z), by writing all vectors and tensors, including the del operator, in terms of components in a cylindrical coordinate system. For example, the del operator and the material time derivative operators in the cylindrical coordinate system are given by ∂ 1 ∂ ∂ +ˆ eθ +ˆ ez ∂r r ∂θ ∂z ∂ vθ ∂ ∂ ∂ D = + vr + + vz Dt ∂t ∂r r ∂θ ∂z ∇=ˆ er
(10.4.1) (10.4.2)
where (ˆ er , ˆ eθ , ˆ ez ) are the unit basis vectors and (vr , vθ , vz ) are the velocity components in the r, θ, and z directions, respectively. Note that the basis vector ˆ ez is constant while the vectors ˆ er and ˆ eθ depend on the angular coordinate θ. Thus the derivatives of the basis vectors ˆ er and ˆ eθ with respect to the coordinates r and z are zero, and the derivatives with respect to θ are given by ∂ˆ er ∂ˆ eθ (10.4.3) =ˆ eθ ; = −ˆ er ∂θ ∂θ The components of the strain rate tensor D in Eq. (7.2.13) and constitutive relations (7.2.11) and (7.2.15) take the form Drr =
∂vr ; ∂r
Dθθ =
1 ∂vθ vr + r ∂θ r
406
NONLINEAR FINITE ELEMENT ANALYSIS
∂vz ; 2Drθ = ∂z 1 ∂vz ∂vθ = + ; 2Dzr = r ∂θ ∂z
Dzz = 2Dθz
∂vθ vθ 1 ∂vr − + ∂r r r ∂θ ∂vr ∂vz + ∂z ∂r
(10.4.4)
σrr = −P + 2µDrr ; σθθ = −P + 2µDθθ ; σzz = −P + 2µDzz σrθ = 2µDrθ ; σθz = 2µDθz ; σzr = 2µDzr (10.4.5) The governing equations (7.3.1)—(7.3.3) expressed in the cylindrical coordinate system are 1 ∂ 1 ∂vθ ∂vz (rvr ) + + =0 r ∂r r ∂θ ∂z Ã
Dvr vθ2 − ρ Dt r
ρ
µ
!
(10.4.6)
∙
¸
= ρfr +
1 ∂(rσrr ) ∂σrθ ∂(rσzr ) σθθ − + + (10.4.7a) r ∂r ∂θ ∂z r
Dvθ vr vθ = ρfθ + + Dt r µ ¶ Dvz = ρfz + ρ Dt
1 ∂(rσrθ ) ∂σθθ ∂(rσθz ) σrθ + + + (10.4.7b) r ∂r ∂θ ∂z r ∙ ¸ 1 ∂(rσzr ) ∂σzθ ∂(rσzz ) (10.4.7c) + + r ∂r ∂θ ∂z
ρCs
¶
µ
∙
¸
¶
µ
DT ∂T ∂T 1 ∂ 1 ∂ rkrr + 2 kθθ = Dt r ∂r ∂r r ∂θ ∂θ +Φ+Q
¶
+
µ
∂T ∂ kzz ∂z ∂z
¶
(10.4.8)
where the stress components are known in terms of the velocity components via equations (10.4.4) and (10.4.5). The viscous dissipation Φ is given by ³
´
³
2 2 2 2 2 2 Φ = 2µ Drr + Dθθ + Dzz + 4µ Drθ + Drz + Dzθ
´
(10.4.9)
and the material time derivative becomes ∂ ∂ ∂ vθ ∂ D = + vr + + vz (10.4.10) Dt ∂t ∂r r ∂θ ∂z Note that the time derivative of a vector (or tensor) in a rotating reference frame is given by (see Reddy and Rasmussen [5], pp. 69—74) ∙
D(·) Dt
¸
nonrot
∙
D(·) = Dt
¸
rot
+ ω × (·)
(10.4.11)
where ω = dθ ez = vrθ ˆ ez is the angular velocity vector of the rotating frame of dt ˆ reference. Therefore, we have µ
Dv Dt
¶
nonrot
µ
¶
Dv vθ = +( ˆ ez ) × v Dt rot r µ ¶ µ ¶ Dvr vθ Dvθ vθ Dvz =ˆ er +ˆ eθ +ˆ ez − vθ + vr (10.4.12) Dt r Dt r Dt
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
407
Equation (10.4.11) could be used in conjunction with (10.4.7a—c) to provide a description of ßuid motion in a rotating cylindrical coordinate system. For axisymmetric conditions (i.e. all variables are independent of θ, and fθ = vθ = 0), the governing equations are simplied to ∂vz 1 ∂ (rvr ) + =0 r ∂r ∂z µ
¶
(10.4.13)
∂vr ∂vr ∂vr = ρfr + + vr + vz ∂t ∂r ∂z µ ¶ ∂vz ∂vz ∂vz = ρfz + + vr + vz ρ ∂t ∂r ∂z ρ
ρCs
µ
∂T ∂T ∂T + vr + vz ∂t ∂r ∂z
¶
=
1 ∂(rσrr ) ∂σzr σθθ + − (10.4.14a) r ∂r ∂z r 1 ∂(rσzr ) ∂σzz + (10.4.14b) r ∂r ∂z
µ
∂T 1 ∂ rkrr r ∂r ∂r +Φ+Q
¶
+
µ
∂T ∂ kzz ∂z ∂z
¶
(10.4.15)
10.4.3 PowerLaw Fluids Many ßuids used in industrial applications are characterized by socalled powerlaw constitutive behavior. Powerlaw ßuids exhibit nonlinear material behavior according to the relation σ = τ − P I, µ = µ0 (I2 ) 1
τ = 2µ(I2 )D
(n−1) 2
I2 = Dij Dij (sum on i and j) 2
(10.4.16) (10.4.17a) (10.4.17b)
where τ is the viscous part of the stress tensor σ, I is the unit tensor, I2 is the second invariant of D, and parameters µ0 and n characterize the ßuid (determined experimentally). For many nonNewtonian ßuids, the viscosity µ decreases with increasing shear rate. These are called shear thinning ßuids and have the power law index n < 1. Fluids with power law index n > 1 are called shearthickening ßuids. For such ßuids, µ increases with increasing shear rate. For n = 1, Eq. (10.4.17a) gives the Newtonian relation (µ = µ0 , constant). The powerlaw constitutive relation in (10.4.16) makes the problem nonlinear for n 6= 1, even for the case where the convective term v · ∇v is negligible.
To illustrate how the powerlaw constitutive equation aﬀects the Þnite element equations, we consider steady, isothermal, axisymmetric ßows of viscous incompressible ßuids. Assuming negligible viscous dissipation and body forces, The governing equations (10.4.13)—(10.4.15) can be expressed
408
NONLINEAR FINITE ELEMENT ANALYSIS
in terms of the velocity components (vr , vz ) as
ρ vr
µ
∂vr ∂r
µ
∂vz ∂r
ρ vr
1 ∂ ∂vz (rvr ) + =0 (10.4.18) ∂z ¶ ¶ r ∂r µ ∙ µ ¶¸ ∂vr ∂vr ∂vz 1 ∂ ∂vr ∂ + vz + = 2µr + µ ∂z r ∂r ∂r ∂z ∂z ∂r vr ∂P − 2µ 2 − (10.4.19) r∙ µ ∂r ¶ ¶¸ µ ¶ ∂vz ∂vr ∂vz 1 ∂ ∂ ∂vz = µr + 2µ + vz + ∂z r ∂r ∂z ∂r ∂z ∂z ∂P − (10.4.20) ∂z
The penalty—Þnite element model of Eqs. (10.4.18)—(10.4.20) is straightforward. After replacing P in Eqs. (10.4.19) and (10.4.20) with P = −γp
µ
∂vr vr ∂vz + + ∂r r ∂z
¶
(10.4.21)
one may construct their week forms and go on to develop the Þnite element model [note that Eq. (10.4.18) is no longer used]. The value of the penalty parameter γp that is most suitable for this class problems must be determined by conducting numerical experiments with some benchmark problems. The Þnite element model has the form ˆ [C(v) + K(µ) + Kp ]{v} = {F} or [K(v)]{v} = {F}
(10.4.22)
where [C], [K], and [Kp ] are the convective, diﬀusive, and penalty contributions to the coeﬃcient matrix. Note that [C] and [K] both depend on the unknown velocity Þeld. The direct iteration scheme for this case is given by (r−1) ˆ [K({v} )]{v}r = {F}
(10.4.23)
ˆ is evaluated using the viscosity µ = µ(Dij ) computed according to where [K] Eq. (10.4.17a) at the rth iteration. It would be more appropriate to compute ˆ ij , µ at the Gauss points of the element and use it in the evaluation of K than to assume µ is elementwise constant. For example, consider the element 11 in the diﬀusion/viscous portion of the coeﬃcient matrix: coeﬃcients Kij 11 Kij =
Z
Ωe
µ
2µ
∂ψi ∂ψj ∂ψi ∂ψj +µ ∂x ∂x ∂y ∂y
¶
dxdy
(10.4.24)
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
409
11 we use the Gauss quadrature rule For numerical evaluation of Kij
11 Kij
=
NGP X
I,J=1
AM U (I, J) ∗ [2.0 ∗ GDSF (1, i) ∗ GDSF (1, j)
+ GDSF (2, i) ∗ GDSF (2, j) ] ∗ CON ST
(10.4.25)
where AM U (I, J) is the value of µ at the (I, J)th Gauss point. Obviously, AM U (I, J) must be evaluated using Eq. (10.4.17a) prior to using it in Eq. (10.4.25).
10.4.4 White—Metzner Fluids The general constitutive equation for ßuids dominated by shear viscosity is given by (see [7—9]) τ + λ{
∂τ + v · ∇τ − [(∇v)T · τ + τ · (∇v)]} = 2ηD ∂t
(10.4.26)
where λ is the relaxation time, η is the shear viscosity of the ßuid, and D is the rate of deformation tensor. To characterize the White—Metzner ßuid, it is necessary to know the viscosity curve as a function of shear rate and the Þrst normal stress diﬀerence. The extra stress tensor τ is separated into purely viscous part τ 2 and viscoelastic part τ 1 : τ = τ1 + τ2 (10.4.27) 1
τ +λ
(
)
h i ∂τ 1 + v · ∇τ 1 − (∇v)T · τ 1 + τ 1 · (∇v) ∂t
τ 2 = 2η2 D
= 2η1 D
(10.4.28) (10.4.29)
and η1 and η2 can be deÞned as the fractions of the shear viscosity η. The ßuid which obeys Eq. (10.4.27) is characterized by η1 , η2 , and λ, which are the functions of rate of deformation tensor, D. Here, the relaxation time λ is assumed to depend on the shear rate γ according to λ(γ) = a + b(log γ) + c(log γ)2 ,
γ=
p
4I2
(10.4.30)
All constants in Eq. (10.4.30) are evaluated by curve Þtting data of polymeric melts (see [16]). Also, η is assumed to be of the particular form η = η(I2 ),
1 I2 = Dij Dij 2
(10.4.31)
410
NONLINEAR FINITE ELEMENT ANALYSIS
Although Eq. (10.4.31) gives the general functional form for the viscosity function, experimental observation and a theoretical basis must be used to provide a speciÞc model for nonNewtonian viscosities. A variety of models can be used to calculate viscosity. Here, the power law model in Eq. (10.,4.17a) is used (replace µ with η). In view of Eq. (10.4.27), the equation of motion (in the absence of body forces) for the White—Metzner viscoelastic ßuids can be written as ∇ · (τ 1 + 2η2 D) − ∇P = ρ
µ
∂v + v · ∇v ∂t
¶
(10.4.32)
For simplicity, we drop the superscript 1 from τ 1 in Eq. (10.4.32) as well as in Eq. (10.4.28). Equations (10.4.32) and (10.4.28), together with the continuity equation ∇ · v = 0, represent the system of governing equations for the WhiteMetzner ßuids. The viscosity η1 is calculated using Eq. (10.4.28) and λ from Eq. (10.4.30). Viscosity η2 is often taken as a function of η1 . Note that the powerlaw constitutive equation can be obtained as a special case from Eq. (10.4.28). For axisymmetric ßows of White—Metzner ßuids, the governing equations are given by ∂u u ∂w + + =0 ∂r ¶ r µ ∂z µ ¶ ∂ ∂u ∂u u 1 τrr + 2η2 + τrr + 2η2 − τθθ − 2η2 ∂r ∂r r ∂r r ∙ µ ¶¸ µ ¶ ∂u ∂u ∂u ∂w ∂u ∂ ∂P + =ρ +u +w + τrz + η2 − ∂r ∂z ∂r ∂r ∂t ∂r ∂z ∙ µ ¶¸ ∙ µ ¶¸ ∂u ∂w ∂u ∂w 1 ∂ τrz + η2 + τrs + η2 + + ∂r ∂z ∂r r ∂z ∂r µ ¶ µ ¶ ∂w ∂w ∂w ∂w ∂ ∂P =ρ +u +w + τzz + 2η2 − ∂z ∂z ∂z ∂t ∂r ∂z ∙
µ
(10.4.33)
(10.4.34)
(10.4.35)
¶¸
∂τrr ∂u ∂u ∂u ∂τrr ∂τrr +u +w −2 τrr + τrz = 2η1 (10.4.36a) τrr + λ ∂t ∂r ∂z ∂r ∂z ∂r ∙ µ ¶¸ ∂τzz ∂τzz ∂τzz ∂w ∂w ∂w +u +w −2 τzz + τrz = 2η1 (10.4.36b) τzz + λ ∂t ∂r ∂z ∂z ∂r ∂z ∙ µ ¶¸ ∂τrz ∂u ∂w ∂u ∂w ∂τrz ∂τrz τrz + λ +u +w −2 τrz + τrz + τzz + τrr ∂t ∂r ∂z ∂r ∂z ∂z ∂r µ ¶ ∂w ∂u + = η1 (10.4.36c) ∂r ∂z ∙ ¸ ∂τθθ u ∂τθθ ∂τθθ u +u +w − 2 τθθ = 2η1 (10.4.36d) τθθ + λ ∂t ∂r ∂z r r where vr = u and vz = w denote the velocity components in the radial and axial directions, respectively.
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
411
The (mixed) penalty—Þnite element model of the above equations can be developed by constructing the weak forms of Eqs. (10.4.34)—(10.4.36) [after replacing the pressure with Eq. (10.4.21)] and interpolating velocities (u, w) and extra stress components (τrr , τθθ , τzz , τrz ) as follows (with equal interpolation of all variables): u(r, z, t) =
N X
uj (t)ψj (r, z),
w(r, z, t) =
j=1
τrr (r, z, t) =
L X
τrz (r, z, t) =
wj (t)ψj (r, z)
j=1 j τrr (t)ψj (r, z),
τzz (r, z, t) =
j=1 L X
N X
L X
j τzz (t)ψj (r, z) (10.4.37)
j=1 j τrz (t)ψj (r, z),
τθθ (r, z, t) =
j=1
L X j
τθθ (t)ψj (r, z)
j=1
j , τ j , τ j , τ j ) are the nodal extra where, (uj , wj ) are the nodal velocities, (τrr zz rz θθ stress components, and ψj are the Lagrange interpolation functions. The Þnite element model has the general form
ú e } + [K e ]{∆e } = {F e } [M e ]{∆ where
{∆e } =
⎧ ⎫ {u} ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ {w} ⎪ ⎪ ⎪ ⎪ ⎪ ⎨ ⎬
{τrr }
⎪ {τzz } ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎩ {τrz } ⎪ ⎭
(10.4.38)
(10.4.39)
{τθθ }
The nonzero coeﬃcient matrices are listed below.
12 Kij 13 Kij 21 Kij 22 Kij
Z
∙
µ
¶
µ
¶¸
∂ψj ∂ψj ∂ψi ∂ψj ∂ψi ∂ψj ψi ψj + η2 2 r dr dz +w + + 2 ∂r ∂z ∂r ∂r ∂z ∂z r Ωe ¶µ ¶ Z µ ∂ψi 1 ∂ψj 1 + ψi + ψj r dr dz + γp ∂r r ∂r r Ωe ¶ ¶ Z µ Z µ ∂ψi ∂ψj ∂ψi 1 ∂ψj = η2 r dr dz + γp + ψi r dr dz ∂z ∂r ∂r r ∂z Ωe Ωe Z Z ∂ψi ∂ψj 31 = = −2 η2 ψi ψj r dr dz, Kij r dr dz ∂r Ωe ∂r Ωe ¶ ¶ Z µ Z µ ∂ψi ∂ψj ∂ψj ∂ψi 1 = η2 r dr dz + γp + ψj r dr dz ∂r ∂z ∂r r ∂z Ωe Ωe µ ¶ µ ¶¸ Z ∙ ∂ψi ∂ψj ∂ψj ∂ψj ∂ψi ∂ψj = ρψi u + η2 r dr dz +w +2 ∂r ∂z ∂r ∂r ∂z ∂z Ωe Z ∂ψi ∂ψj r dr dz + γp Ωe ∂z ∂z
11 = Kij
ρψi u
412 24 Kij 33 Kij 35 Kij 44 Kij 45 Kij 52 Kij 54 Kij 55 Kij 66 Kij
Mij11
NONLINEAR FINITE ELEMENT ANALYSIS
=
Z
=
Z
Z
∂ψi ψj r dr dz ∂r ( ∙ µ ¶ µ ¶¸) Z ∂u ∂ψj ∂ψj = ψi ψj + λ ψi u − 2ψi r dr dz +w ψj ∂r ∂z ∂r Ωe Z Z ∂ψj ∂u 42 =2 λ ψi ψj r dr dz, Kij = −2 η2 ψi r dr dz ∂z Ωe ∂z Ωe ( ) ∙ µ ¶ ¸ Z ∂ψj ∂ψj ∂w +w ψi ψj r dr dz = ψi ψj + λ ψi u −2 ∂r ∂z ∂z Ωe Z Z ∂ψi ∂w 51 = −2 λ = −2 η1 ψi ψi ψj r dr dz, Kij r dr dz ∂z Ωe ∂r Ωe Z Z ∂ψj ∂w 53 = −2 η1 ψi = −2 λ r dr dz, Kij ψi ψj r dr dz ∂r Ωe Ωe ∂r Z Z ψj ∂u 61 = −2 λ ψi ψj r dr dz, Kij = −2 η1 ψi r dr dz r Ωe ∂z Ωe ( ) µ ¶ µ ¶ Z ∂u ∂w ψj ∂ψj = 2 ψi ψj + λ u + w − ψi ψj r dr dz + ∂r ∂z ∂r ∂z Ωe
=
Mij33 = Mij55 =
Z
Ωe
Ωe
e
ZΩ Z
Ωe Ωe
∂ψi ψj r dr dz, ∂z
(
∙
ψi ψj + λ ψi
25 Kij
=
µ
Ωe
∂ψj ∂ψj +w u ∂r ∂z
ρψi ψj r dr dz,
Mij22
λψi ψj r dr dz,
Mij44 =
λψi ψj r dr dz,
Mij66 =
=
Z
e
ZΩ
e ZΩ
Ωe
¶
u − 2 ψi ψj r
¸)
r dr dz
ρψi ψj r dr dz λψi ψj r dr dz λψi ψj r dr dz
(10.4.40)
Bar over u and w indicate that they are to be calculated using values from the previous iteration. For viscoelastic ßuids, speciÞcation of velocities is insuﬃcient on account of ßuid memory. If the boundary of the domain contains an entry region, then fully developed ßow conditions may be assumed. All extra stress components must be speciÞed as essential boundary conditions along the entry region. Failing to do so may lead to the propagation of errors throughout the ßow domain when relaxation time λ becomes large.
10.4.5 Numerical Examples Consider the steady ßow of a powerlaw ßuid in a uniform pipe (R = 1, L = 6, w0 = 10, n = 0.2, µ0 = 1.0, and γp = 108 ). The geometry and boundary conditions of the computational domain are shown in Figure 10.4.1. Three diﬀerent meshes shown in Figure 10.4.2 are used. The Þnite element results obtained using the velocity Þnite element model in Eq. (10.4.22) along with
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
413
the analytical results for the axial velocities as a function of the radial distance are presented in Table 10.4.1.
R z
r
L u= 0
u= 0 w = w0
z u= 0
r
u=w=0
Figure 10.4.1 Geometry and boundary conditions for ßow through a pipe.
Mesh 1: 3×5 (uniform)
Mesh 2: 6×10 (uniform)
Mesh 3: 12×15 (nonuniform)
Figure 10.4.2 Three diﬀerent meshes used for the pipe ßow.
414
NONLINEAR FINITE ELEMENT ANALYSIS
Table 10.4.1 A comparison of the velocity Þeld w(r, 6) for the ßow of a powerlaw ßuid in a pipe (R = 1, L = 6, w0 = 10, n = 0.2, µ0 = 1.0, and γp = 108 ). Velocity, w(r, 6) r 0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
* w(r, 6) = w0
Mesh 1
Mesh2
Mesh 3
Analytical*
11.894
12.833
13.402
− − −
−
−
13.333 13.333 13.333 13.333 13.332 13.330 13.324 13.309 13.279 13.223 13.125 12.964 12.711 12.328 11.765 10.960 9.838 8.305 6.248 3.532 0.000
12.832
11.865
12.826
− − −
12.795
−
13.360
−
12.686
− − −
12.401
−
13.243
−
−
12.940 12.679 12.305 11.787 11.086 10.156 8.946 7.396 5.451 2.993 0.000
−
10.852
11.783
− − −
10.610
− − −
5.133
− −
7.916
8.563 − −
0.000
n+1
−
13.395
−
11.721
¡ 3n+1 ¢ h
13.401
−
0.000
1−
¡ r ¢1+ n1 i R
.
Next, the same problem (with R = 1 and L = 5) is studied with the mixed Þnite element model in Eq. (10.4.38). Both powerlaw (n = 0.25, η0 = 104 , η2 = 0) and White—Metzner (n = 0.25, η0 = 104 , η2 = 0, a = 0.435, b = −0.453, c = 0.1388) ßuids are analyzed. A uniform mesh of 10 × 6 bilinear elements is used. In addition to the velocity boundary conditions shown in Figure 10.4.1, the stresses are speciÞed to be zero at the entrance. The penalty parameter is taken to be γp = 108 η0 . Figures 10.4.3 and 10.4.4 contain plots of the axial velocity proÞles at z = 2.0 and z = 5.0, respectively. We note that the velocity proÞles obtained with the velocity model (VM) in Eq. (10.4.22) and mixed model (MM) in Eq. (10.4.38) are essentially the same when stress boundary conditions are not imposed. The stress boundary conditions (which can be imposed pointwise only in the mixed model) do have an eﬀect: speciÞcation of the stresses at the inlet increases the centerline velocity for both powerlaw ßuids as well as
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
°
Linear (VM, MM) without SBC Nonlinear (VM) SBC = Stress boundary conditions
r 1.0
w(r, 0)
Nonlinear (MM) without SBC Linear (MM) with SBC Nonlinear (MM) with SBC
r
r
1.0
1.0
0.8
0.8
0.6
0.6
0.6
0.4
0.4
0.4
0.2
0.2
0.2
0.0 0 0.0 0.5 1.0 0
0.8
w(r, 2)
415
w(r, 5)
0.0 0.0 0.5 1.0 1.5 2.0
0.5 1.0 1.5 2.0 Axial velocity, w(r,z)
Figure 10.4.3 Axial velocity proÞles for the ßow of powerlaw ßuid through a pipe. Linear without SBC Linear with SBC
r 1.0
1.0
0.8
0.8
0.6
0.6
0.4
0.4 w(r, 2)
0.2 0.0
0.0
0.5
1.0
1.5
Axial velocity, w(r,z)
°
Nonlinear with SBC SBC = Stress Boundary Conditions
r
w(r, 5)
0.2
2.0
2.5
0.0
0.0
0.5
1.0 1.5 2.0 Axial velocity, w(r,z)
2.5
Figure 10.4.4 Axial velocity proÞles for the ßow of White—Metzner ßuid through a pipe.
416
NONLINEAR FINITE ELEMENT ANALYSIS
White—Metzner ßuids. If the stress boundary conditions are not imposed, the mixed model for the White—Metzner ßuid does not yield converged solution. Figure 10.4.5 shows the eﬀect of the penalty parameter on the velocity Þeld. The results are selfexplanatory. Table 10.4.2 Eﬀect of the convective terms on the velocity Þeld for the ßow of a powerlaw ßuid in a pipe (R = 1, L = 5, w0 = 10, n = 0.2, and 10 × 6Q4 mesh). Velocity, w(r, 5) NonNewtonian r 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 * Re =
Newtonian
Re = 0
Re = 102
Re = 103
1.8358 1.8001 1.7398 1.6461 1.5186 1.3578 1.1651 0.9397 0.6712 0.3079 0.0000
1.3357 1.3355 1.3338 1.3271 1.3081 1.2657 1.1849 1.0468 0.8273 0.4795 0.0000
1.1312 1.1302 1.1289 1.1270 1.1241 1.1197 1.1097 1.0799 0.9809 0.6709 0.0000
1.0267 1.0265 1.0266 1.0268 1.0271 1.0275 1.0274 1.0265 1.0209 0.8967 0.0000
ρw0 R . η0
r
γp= 102
γp= 100
1.0
γp =
0.8
101
γp= 1012
γp= 104 γp= 108 − 1012
γp= 102
0.6 γp= 103
0.4
Newtonian NonNewtonian
0.2 0.0 0
5
10 15 Axial velocity, w(r,6)
20
25
Figure 10.4.5 The eﬀect of the penalty parameter on the ßow of a powerlaw ßuid.
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
417
The eﬀect of the convective terms on the velocity Þeld is also investigated for the powerlaw ßuid (n = 0.25, η0 = 104 , η2 = 0, R = 1, L = 5, and w0 = 1) using the velocity model in Eq. (10.4.22). The results are presented in Table 10.4.2. The eﬀect is to ßatten the velocity proÞle from a parabolic one. Figure 10.4.6 shows the axial velocity proÞle for ßow through a plane channel (a = 6, b = 1, n = 0.2, η0 = η1 = 1). The results are in good agreement with those in [9]. x 1.0
Newtonian fluid (Re = 0) NonNewtonian (Re = 0) ° NonNewtonian (Re = 12.5) • NonNewtonian (Re = 25.0)
0.8 0.6 0.4 0.2 0.0 0
2
4
6
8 10 12 14 16 Axial velocity, v(x,6)
18 20
Figure 10.4.6 Axial velocity proÞles for ßow of powerlaw ßuid through a plane channel.
10.5 Coupled Fluid Flow and Heat Transfer 10.5.1 Finite Element Models There exist many engineering systems where the ßuid ßow is aﬀected by the heat transfer to or from the system and vice versa. Convective cooling of a heated body (like in an internal combustion engine) is a generic example of the coupling. In such cases, we solve the governing equations of ßuid ßow (i.e. the Navier—Stokes equations and continuity equation) as well as the heat transfer (i.e. the energy equation), as discussed in Section 7.3. The energy equation is given by [see Eq. (7.3.3)] ρC
∙µ
u
∂T ∂T +v ∂x ∂y
¶
+
∂T ∂t
¸
=
µ
∂ ∂T k11 ∂x ∂x
¶
+
µ
∂ ∂T k22 ∂y ∂y
¶
+ Φ + q (10.5.1)
where C is speciÞc heat, Φ is the dissipation energy Φ = σij εij and q is internal heat generation.
(10.5.2)
418
NONLINEAR FINITE ELEMENT ANALYSIS
The Þnite element model of the energy equation is given by [C e ]{Tú e } + [H e ]{T e } = {Qe }
(10.5.3)
where e Hij
= ρC
Z
Ωe
⎛
Z
e Cij
⎛
(2) (2) ⎝ ∂ψj ψi u
(2)
⎞
∂ψj ⎠ dxdy +v ∂x ∂y (2)
(2)
⎞
(2) (2) ∂ψi ∂ψj ∂ψi ∂ψj ⎠ ⎝ + k22 k11 dxdy + ∂x ∂x ∂y ∂y Ωe
= ρC
Qei =
Z
Z
Ωe
(2) (2)
Ωe
ψi ψj dxdy
(2)
ψi (q + Φ)dxdy +
I
(2)
Γe
qn ψi ds
(10.5.4)
u and v being the velocity components (that couple to the ßuid ßow (2) problem), and ψi the Lagrange interpolation functions used to interpolate the temperature Þeld. Equation (10.5.3) must be solved along with the ßow equations ú e } + [K e ]{∆e } = {F e } [M e ]{∆
(10.5.5)
developed earlier (see Chapters 7 and 8). Both equations, Eqs. (10.5.3) and (10.5.5), must be solved iteratively, using the latest velocity Þeld and temperature Þeld to compute the coeﬃcient matrices. It should be noted that ßuid ßow equations are coupled to the heat transfer equation through the body force terms fx and fy in the Navier—Stokes equations. For buoyancydriven ßows, the body force in the direction of the gravity is a function of the temperature, that is, fy = −ρgβ(T − T0 ) if the yaxis is taken vertically up. Thus, the momentum equations are fully coupled with the energy equation. The following strategy is found suitable for the convective heat transfer problems. Solve the energy equation (10.5.3) with an assumed velocity Þeld (say, zero). Then use the assumed velocity and temperature Þelds in (10.5.5) and solve for the new velocity Þeld. The initial guess of the velocity Þeld can be either the linear (Stokes) solution (or the solution of a problem at lower Rayleigh number when solving for high Rayleigh number ßows). The Rayleigh number Ra and Prandtl number P r (charactristic numbers used in convective heat transfer) are deÞned by Ra =
β g L3 ∆T ν µ k , Pr = , ν= , κ= κν ρ ρcp κ
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
419
where g is acceleration due to gravity, β is thermal expansion coeﬃcient, L the characteristic dimension of the ßow region, ∆T the temperature diﬀerence between hot and cold walls, ρ the density, µ the viscosity, k the conductivity, and cp is the speciÞc heat at constant pressure.
10.5.2 Numerical Examples Here we include couple of sample problems, taken from Reddy and Gartling [15], to illustrate the ideas presented above. Heated cavity Consider a closed square cavity Þlled with a viscous incompressible ßuid. The top and bottom faces of the cavity are assumed to be insulated while the vertical faces subjected to diﬀerent temperatures, as shown in Figure 10.5.1(a). A typical 16 × 16 mesh of eightnode quadratic elements is shown in Figure 10.5.1(b). This problem was solved using the earliest versions of NACHOS code [24], which is based on velocity—pressure (mixed) formulation and used the direct iteration (Picard) scheme. More recent solutions have been obtained using Newton’s method for the combined equation set equivalent to (10.5.3) and (10.5.5). The streamline and isotherm plots are shown in Figures 10.5.2 and 10.5.3 for Rayleigh numbers of Ra = 104 and 106 , respectively (P r = ν/κ = 0.71). For the lower Rayleigh number, the ßow is relatively weak and the thermal Þeld is only slightly perturbed from a conduction solution. At the higher Rayleigh number, the ßow Þeld develops a considerable structure while the thermal Þeld becomes vertically stratiÞed in the core of the cavity with high heat ßux regions along the vertical boundaries [24].
y
u=v=0
T = TH u=v=0
T = TC u=v=0 L
••• •••••
Insulated
(a)
(b) L
x u=v=0
Figure 10.5.1 (a) Geometry of a heated square cavity. (b) A typical Þnite element mesh.
420
NONLINEAR FINITE ELEMENT ANALYSIS
Solar receiver Figure 10.5.4 shows a cross section of an annular solar receiver tube surrounded by an eccentrically located glass envelope. The inner tube carries a heat transfer ßuid that is heated by a ßux that varies with position around the tube. The incident ßux is due to solar energy being concentrated on the tube by a parabolic trough collector. The glass envelope provides a shield to reduce the forced convection (wind) heat loss from the collector tube [15, 24].
Streamlines
Isotherms
Figure 10.5.2 Streamlines and isotherms for natural convection in a square cavity Þlled with viscous ßuid (Ra = 104 , P r = 0.71).
Streamlines
Isotherms
Figure 10.5.3 Streamlines and isotherms for natural convection in a square cavity Þlled with viscous ßuid (Ra = 106 , P r = 0.71).
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
421
Figures 10.5.5—10.5.7 contain streamline and isotherm plots for an airÞlled annulus for various temperature and geometric conÞgurations. The ßow pattern and heat ßux distribution are quite sensitive to variations in these parameters even though the Rayleigh number is the same for all cases [15, 24].
T = constant u=v=0
T = constant u=v=0
+
Receiver tube
Glass envelop
Figure 10.5.4 Mesh and boundary conditions for the annular solar receiver.
Streamlines
Isotherms
−4
(×10 cm2/sec)
Streamlines
Isotherms
−4
(×10 cm2/sec)
528 472 48 101 153 470 417 365 312 259 206
444
556º K 500º K 583º K
417 389 361 333
(a)
593º K 636º K 679º K −402 −357 −312 −266 −221 −176 −132 −86 −41
549 506 463 420
377
(b)
Figure 10.5.5 Plots of streamlines and isotherms for the solar receiver. (a) uniform wall temperature, Ra = 1.2 × 104 . (b) asymmetric wall temperature, hot on top (Ra = 1.2 × 104 ).
422
NONLINEAR FINITE ELEMENT ANALYSIS
677.8
Streamlines
755.6 833.2 911.1 908.9
−2.33 −1.25 −0.38 −5.49×10−3 4.52 3.54 2.56 1.58 0.60
Isotherms
1066.7 1144.4 1222.2
Figure 10.5.6 Plots of streamlines and isotherms for the solar receiver; uniform wall temperature, hot on bottom (Ra = 1.2 × 104 ).
936.3
1010.5
862.1
Streamlines −3
(×10 in2/sec)
787.9 11.91 10.58 713.7 9.44 8.21 6.98 5.75 4.52 3.29 2.05 0.82
Isotherms 639.5 565.4 491.2 417
Figure 10.5.7 Plots of streamlines and isotherms for the solar receiver; uniform wall temperature, eccentric geometry (Ra = 1.2 × 104 ). Additional details on the formulation as well as applications of coupled heat transfer and ßuid ßow can be found in [17—24] (in particular, see [23,24] and Chapter 5 of the book by Reddy and Gartling [15] and reference therein).
MATERIAL NONLINEARITIES AND COUPLED PROBLEMS
423
References 1. Hill, R., The Mathematical Theory of Plasticity, Oxford University Press, Oxford, U.K., 1950. 2. Johnson, W. and Mellor, P. B., Engineering Plasticity, John Wiley, New York (1983). 3. Owen, D. R. J. and Hinton, E., Finite Elements in Plasticity: Theory and Practice, Pineridge Press, UK (1980) (see pp. 2629 for onedimensional bar problems; pp. 129148 for onedimensional Timoshenko beams; pp. 215281 for twodimensional problems). 4. Zienkiewicz, O. C. and Taylor, R. L., The Finite Element Method, 4th edn, Vol. 2: Solid and Fluid Mechanics, Dynamics and NonLinearity, McGraw—Hill, London, UK (1989) (see Chapter 7). 5. Reddy, J. N. and Rasmussen, M. L., Advanced Engineering Analysis, John Wiley, New York (1982); reprinted by Krieger Publishing, Melbourne, FL (1991). 6. Phan Thien, N. and Tanner, R. I., “A New Constitutive Equation Derived From Network Theory,” Journal of NonNewtonian Fluid Mechanics, 2, 353—365 (1977). 7. White, J. L. and Metzner, A. B., “Development of Constitutive Equations for Polymeric Melts and Solutions,” Journal of Applied Polymer Science, 7, 1867—1889 (1963). 8. Crochet, M. J. and Walters, K., “Numerical Methods in NonNewtonian Fluid Mechanics,” Annual Review of Fluid Mechanics, 15, 241—260 (1983). 9. Crochet, M. J., Davies, A. R., and Walters, K., Numerical Simulation of NonNewtonian Flow, Elsevier, Amsterdam, The Netherlands (1984). 10. Gartling, D. K. and Phan Thien, N., “A Numerical Simulation of a Plastic Fluid in a ParallelPlate Plastometer,” Journal of NonNewtonian Fluid Mechanics, 14, 347—360 (1984). 11. Reddy, J. N. and Padhye, V. A., “A Penalty Finite Element Model for Axisymmetric Flows of NonNewtonian Fluids,” Numerical Methods for Partial Diﬀerential Equations, 4, 33—56 (1988). 12. Iga, M. and Reddy, J. N., “Penalty Finite Element Analysis of Free Surface Flows of PowerLaw Fluids,” International Journal of NonLinear Mechanics, 24(5), 383—399 (1989). 13. Reddy, M. P. and Reddy, J. N., “Finite Element Analysis of Flows of NonNewtonian Fluids in ThreeDimensional Enclosures,” International Journal of NonLinear Mechanics, 27(1), 9—26 (1992). 14. Reddy, M. P. and Reddy, J. N., “Numerical Simulation of Forming Processes Using a Coupled Fluid Flow and Heat Transfer Model,” International Journal for Numerical Methods in Engineering, 35, 807—833 (1992). 15. Reddy, J. N. and Gartling, D. K., The Finite Element Method in Heat Transfer and Fluid Dynamics, Second Edition, CRC Press, Boca Raton, FL (2001). 16. Coleman, C. J., “A Finite Element Routine for Analyzing NonNewtonian Fluids, Part 1,” Journal of NonNewtonian Fluid Mechanics, 7, 289—301 (1980). 17. Gartling, D. K., “Convective Heat Transfer Analysis by the Finite Element Method,” Computer Methods in Applied Mechanics and Engineering, 12, 365—382 (1977). 18. Marshall, R. S., Heinrich, J. C., and Zienkiewicz, O. C., “Natural Convection in a Square Enclosure by a Finite Element, Penalty Function Method, Using Primitive Fluid Variables,” Numerical Heat Transfer, 1, 315—330 (1978). 19. Reddy, J. N. and Satake, A., “A Comparison of Various Finite Element Models of Natural Convection in Enclosures,” Journal of Heat Transfer, 102, 659—666 (1980).
424
NONLINEAR FINITE ELEMENT ANALYSIS
20. Reddy, J. N., “PenaltyFiniteElement Analysis of 3D NavierStokes Equations,” Computer Methods in Applied Mechanics and Engineering, 35, 87—106 (1982). 21. de Vahl Davis, G. and Jones, I. P., “Natural Convection in a Square Cavity: A Comparison Exercise,” International Journal for Numerical Methods in Fluids, 3, 227— 248 (1983). 22. Dhaubhadel, M., Telionis, D., and Reddy, J. N., “ Finite Element Analysis of Fluid Flow and Heat Transfer for Staggered Bundle of Cylinders in Cross Flow,” International Journal for Numerical Methods in Fluids, 7, 1325—1342 (1987). 23. Pelletier, D. H., Reddy, J. N., and Schetz, J. A., “Some Recent Developments and Trends in Finite Element Computational Natural Convection,” in Annual Review of Numerical Fluid Mechanics and Heat Transfer, Vol. 2, C. L. Tien and T. C. Chawla (eds.), Hemisphere, New York, 39—85 (1989). 24. Gartling, D. K., “Convective Heat Transfer Analysis by the Finite Element Method,” Computer Methods in Applied Mechanics and Engineering, 12, 365—382 (1977).
Appendix A1 Solution Procedures for Linear Algebraic Equations
A1.1 Introduction All Þnite element equations, after assembly and imposition of boundary (and initial) conditions, can be expressed as a set of linear algebraic equations that must be solved. In general, the Þnite element equations are of the form [A]{X} = {B}
(A1.1.1)
where [A] is the coeﬃenct matrix resulting from the assembly of element matrices, {X} column of unknowns (typically nodal values), and {B} is the known column vector. In the Þnite element method, [A] is a banded sparse matrix that may be either symmetric or unsymmetric depending on the characteristics of the governing diﬀerential equation(s) describing the physical problem, and possibly the nonlinear solution method used (see Appendix A2). A banded matrix is one in which all elements beyond a diagonal parallel to the main diagonal are zero. The maximum number of nonzero diagonals above or below the main diagonal plus 1 (to account for the main diagonal) is called half bandwidth (NHBW) of the matrix [see Figure A1.1.1(a)]. When the matrix is symmetric, it is suﬃcient to store elements in upper or lower half bandwidth of the matrix [A] [see Figure A1.1.1(b)] and write solvers to take note of the fact aij = aji for all rows i and columns j of the matrix. When the matrix is not symmetric, one must store all elements in the full bandwidth (2*NHBW1) of the matrix. Here we include the Fortran subroutines of a symmetric banded equation solver (SYMSOLVR) and unsymmetric banded equation solver (UNSYMSLV). A linear set of algebraic equations can be solved by either a direct or iterative method. Direct methods, like the Gauss elimination method, provide the solution after a Þxed number of steps and are less sensitive to the
426
NONLINEAR FINITE ELEMENT ANALYSIS
conditioning of the matrix (the condition number of a matrix is the ratio of the largest to smallest eigenvalue associated with the matrix). Apart from the roundoﬀ errors introduced in the computer, direct methods yield the “exact” solution to the equations. However, the main deÞciency of the direct solvers is that they require the coeﬃcient matrix be stored in an ordered format to carry out the elementary operations (i.e. multiplication, division, etc.) on the coeﬃcients of the augmented matrix [AB] and obtain the solution. In recent years, the direct solvers have been reÞned to reduce this deÞciency through innovative data management techniques (e.g. frontal solvers, skyline solvers, and others; see Carey and Oden [1]). These improvements enable users to solve moderately large systems of equations eﬃciently. However, they have been found to be unsuitable for solving very large systems of equations (especially in threedimensional problems) because they demand outofcore storage of the equations and hence require large data transfers. In addition, direct methods are diﬃcult to organize for eﬃcient use on multiprocessor, parallel computers and are therefore seeing reduced utilization.
All zeros
NHB1
a11 a12 . . . a1k 0 0 0 . . . 0 a21 a22 . . . a2k a2k+1 0 0 . . . 0 a31 a32 . . . a3k a3k+1 a3k+2 0 0 . .
(a) NHB2
[A] = ..
.
.
.
.
.
.
.
NEQ = n A parallel diagonal
.
All zeros
Main diagonal 0 . . . 0
ank
. . .
ank . . . ann
NHBW= max {NHB1, NHB2}
(b)
NHBW
NHBW
a11 a12 . . . a1k 0 0 0 . . . 0 a21 a22 . . . a2k a2k+1 0 0 . . . 0 a31 a32 . . . a3k a3k+1 a3k+2 0 0 . .
[A] = .
(symm) .
.
.
0 . . . 0
.
ank
.
. . .
.
.
.
.
ank . . . ann
[A] = (half banded form)
a11 a12 a22 a23 a33 a34
... ... ...
a1k a2k+1 a3k+2
. . . . . . .
NEQ = n
an1, n1 an1, n...0 ann 0 . . . 0
Main diagonal Last diagonal beyond which all coefficients are zero
Figure A1.1.1 Banded symmetric and unsymmetric matrices.
SOLUTION PROCEDURES FOR LINEAR ALGEBRAIC EQUATIONS
427
The limitations on CPU time and storage requirements preclude the use of direct solvers for complex problems with more than a quarter million equations, and iterative methods are found to be more eﬃcient in that they require less storage and CPU time. Iterative methods are approximate and only converged solutions will be close to the true solution. In iterative methods, the global matrix formation may be avoided. The major operation in iterative methods is the matrixvector multiplication as compared to the matrix reduction (by elementary operations) in direct methods. A signiÞcant advantage of iterative methods is that a given set of equations can be divided into as many subsets of equations as there are processors and calculations can be performed in parallel on the array of processors. However, convergence characteristics of iterative methods depend on the condition number of the system of linear equations, and a suitable preconditioner is a must to achieve convergence. Following this introduction, a brief review of some methods for the solution of linear algebraic equations is presented. For additional details, the reader may consult [2—4].
A1.2 Direct Methods A1.2.1 Preliminary Comments Direct methods are those in which simultaneous linear algebraic equations are solved “exactly” (within the computational roundoﬀ error) by successive elimination of variables and back substitution. The Gauss elimination method is a Þxedstep technique [2—8], and frontal [5] and skyline [6] solution methods are examples of direct solution methods that use the Gauss elimination technique eﬃciently. The direct methods are the most commonly used techniques when the number of equations involved is not too large. The number of elementary operations for Gauss elimination is of the order n3 /3 + O(n2 ), where n denotes the number of unknowns. The frontal solution procedure [5] is faster than most direct solvers; it requires less core space as long as active variables can be kept in the core, and it allows for partial pivoting. An additional advantage is that no speciÞc node numbering scheme is needed, though a judicious element numbering helps to minimize the front width. Due to the fact that the approximation functions are deÞned only within an element, the coeﬃcient matrix in the Þnite element method is banded; that is, aij = 0 for j > i + nb , where nb is the half bandwidth of the matrix [A]. This greatly reduces the number of operations in solving the equations, if we make note of the fact that elements outside the bandwidth are zero. Of course, the bandwidth size depends on the global node numbering. The skyline technique is one in which bandedness of the Þnite element equations is exploited by storing the row number mj of the Þrst nonzero element in column j. The
428
NONLINEAR FINITE ELEMENT ANALYSIS
variables mi , i = 1, 2, · · · , n, deÞne the skyline of the matrix. For additional details see Bathe [6]. Here, we include the Fortran subroutines of a symmetric banded equation solver (SYMSOLVR) and unsymmetric banded equation solver (UNSYMSLV) that are used in most of the calculations presented in this book [7].
A1.2.2 Symmetric Solver In the case of symmetric solver the global coeﬃcient matrix [A] ≡ [GLK] is stored in symmetric banded form (NEQ × N HBW ), where NEQ is the number of global equations and N HBW is the half bandwidth. Fortran statements for the assembly of element coeﬃcient matrices [ELK] and element source vector {ELF } for the symmetric case are included in Box A1.2.1. Note that the global source vector is stored in array {GLF }. After Gauss elimination is completed in SYMSOLVR (see Box A1.2.2), the solution is returned in array {GF }. Box A1.2.1 Fortran statements for the calculation of the half bandwidth and assembly of element coeﬃcient matrices.
110
120 130 140
DO 140 N=1,NEM DO 110 I=1,NPE NI=NOD(N,I) ELU(I)=GAMA*GP2(NI)+(1.0GAMA)*GP1(NI) ELX(I)=GLX(NI) CALL ELEKMF(IEL,NPE,NONLIN,F0) DO 130 I=1,NPE GAMA = Acceleration parameter, ρ NR=(NOD(N,I)1)*NDF [see Eq. (3.4.5)] DO 130 II=1,NDF GP2 = Solution from the(r − 2) iteration NR=NR+1 GP1 = Solution from (r − 1) iteration L=(I1)*NDF+II GLF(NR)=GLF(NR)+ELF(L) DO 120 J=1,NPE NCL=(NOD(N,J)1)*NDF DO 120 JJ=1,NDF M=(J1)*NDF+JJ NC=NCLNR+JJ+1 IF(NC.GT.0)THEN GLK(NR,NC)=GLK(NR,NC)+ELK(L,M) ENDIF CONTINUE CONTINUE CONTINUE
SOLUTION PROCEDURES FOR LINEAR ALGEBRAIC EQUATIONS
429
Box A1.2.2 Subroutine for solution of banded symmetric equations. SUBROUTINE SYMSOLVR(NRM,NCM,NEQNS,NBW,BAND,RHS,IRES) C C C C C C C
10 20 30 40
50 60 C C C
The subroutine solves a banded, symmetric, system of algebraic equations [BAND]{U} = {RHS} using the Gauss elimination method: The coefficient matrix is input as BAND(NEQNS,NBW) and the column vector is input as RHS(NEQNS), where NEQNS is the actual number of equations and NBW is the half band width. The true dimensions of the matrix [BAND] in the calling program, are NRM by NCM. When IRES is greater than zero, the right hand elimination is skipped. IMPLICIT REAL*8(AH,OZ) DIMENSION BAND(NRM,NCM),RHS(NRM) MEQNS=NEQNS1 IF(IRES.LE.0) THEN DO 30 NPIV=1,MEQNS NPIVOT=NPIV+1 LSTSUB=NPIV+NBW1 IF(LSTSUB.GT.NEQNS) THEN LSTSUB=NEQNS ENDIF DO 20 NROW=NPIVOT,LSTSUB NCOL=NROWNPIV+1 FACTOR=BAND(NPIV,NCOL)/BAND(NPIV,1) DO 10 NCOL=NROW,LSTSUB ICOL=NCOLNROW+1 JCOL=NCOLNPIV+1 BAND(NROW,ICOL)=BAND(NROW,ICOL)FACTOR*BAND(NPIV,JCOL) RHS(NROW)=RHS(NROW)FACTOR*RHS(NPIV) CONTINUE ELSE DO 60 NPIV=1,MEQNS NPIVOT=NPIV+1 LSTSUB=NPIV+NBW1 IF(LSTSUB.GT.NEQNS) THEN LSTSUB=NEQNS ENDIF DO 50 NROW=NPIVOT,LSTSUB NCOL=NROWNPIV+1 FACTOR=BAND(NPIV,NCOL)/BAND(NPIV,1) RHS(NROW)=RHS(NROW)FACTOR*RHS(NPIV) CONTINUE NPIVOT=NPIV1 ENDIF DO 80 JKI=LSTSUB,NPIVOT NROW=NPIVOTJKI+LSTSUB Back substitution NCOL=NPIVNROW+1 FACTOR=BAND(NROW,NCOL) DO 90 IJK=2,NEQNS 80 RHS(NROW)=RHS(NROW)NPIV=NEQNSIJK+2 FACTOR*RHS(NPIV) RHS(NPIV)=RHS(NPIV)/BAND(NPIV,1) 90 CONTINUE LSTSUB=NPIVNBW+1 RHS(1)=RHS(1)/BAND(1,1) IF(LSTSUB.LT.1) THEN Continued in the inset RETURN LSTSUB=1 END ENDIF
430
NONLINEAR FINITE ELEMENT ANALYSIS
A1.2.3 Unsymmetric Solver In the case of unsymmetric solver the matrix [GLK] is stored in full bandwidth form (N EQ × N BW ), where NEQ is the number of global equations and N BW = 2 ∗ N HBW is twice the half bandwidth. Since there is an additional column in N BW , we use it to store the global source vector; that is, GLF (I) → GLK(I, N BW ). Fortran statements for the assembly of element matrices for the unsymmetric case are included in Box A1.2.3. After Gauss elimination is completed in UNSYMSLV (see Box A1.2.4), the solution is returned in the last column of the coeﬃcient matrix {GLK(I, N BW )}. Box A1.2.3 Fortran statements for assembly of the element matrices and source vector into a banded unsymmetric global matrix.
110
120 130 140
DO 140 N=1,NEM DO 110 I=1,NPE NI=NOD(N,I) ELU(I)=GAMA*GP2(NI)+(1.0GAMA)*GP1(NI) ELX(I)=GLX(NI) CALL ELEKMF(IEL,NPE,NONLIN,F0) DO 130 I=1,NPE NR=(NOD(N,I)1)*NDF DO 130 II=1,NDF NR=NR+1 L=(I1)*NDF+II GLK(NR,NBW)=GLK(NR,NBW)+ELF(L) DO 120 J=1,NPE NCL=(NOD(N,J)1)*NDF DO 120 JJ=1,NDF M=(J1)*NDF+JJ NC=NCLNR+JJ+NHBW IF(NC.GT.0)THEN GLK(NR,NC)=GLK(NR,NC)+ELK(L,M) ENDIF CONTINUE CONTINUE CONTINUE
A1.3 Iterative Methods A1.3.1 General Comments Among the various iterative methods that are available in the literature, the Conjugate Gradient (CG) method [9] is most widely used because it is a Þnite step method (i.e., apart from roundoﬀ errors, the solution is achieved in a Þxed number of iterations) and it can be used to determine the inverse. However, the number of iterations required depends on the condition number of the coeﬃcient matrix. The convergence of the conjugate gradient method, and iterative methods in general, can be improved by preconditioning and/or scaling the equations [10—12].
SOLUTION PROCEDURES FOR LINEAR ALGEBRAIC EQUATIONS
431
Box A1.2.4 Subroutine for solution of banded unsymmetric equations. SUBROUTINE UNSYMSLV(A,NRMAX,NCMAX,N,ITERM) C ________________________________________________________________ C C Solver for BANDED UNSYMMETRIC system of algebraic equations C ________________________________________________________________ C IMPLICIT REAL*8 (AH,OZ) DIMENSION A(NRMAX,NCMAX) CERO=1.0D15 PARE=CERO**2 NBND=2*ITERM NBM=NBND1 C C Begin elimination of the lower left C DO 80 I=1,N IF (DABS(A(I,ITERM)).LT.CERO) GO TO 10 GO TO 20 10 IF (DABS(A(I,ITERM)).LT.PARE) GO TO 110 20 JLAST=MIN0(I+ITERM1,N) L=ITERM+1 DO 40 J=I,JLAST L=L1 IF (DABS(A(J,L)).LT.PARE) GO TO 40 B=A(J,L) DO 30 K=L,NBND 30 A(J,K)=A(J,K)/B IF (I.EQ.N) GO TO 90 40 CONTINUE L=0 JFIRST=I+1 IF (JLAST.LE.I) GO TO 80 DO 70 J=JFIRST,JLAST L=L+1 IF (DABS(A(J,ITERML)).LT.PARE) GO TO 70 DO 50 K=ITERM,NBM 50 A(J,KL)=A(JL,K)A(J,KL) A(J,NBND)=A(JL,NBND)A(J,NBND) IF (I.GE.NITERM+1) GO TO 70 DO 60 K=1,L 60 A(J,NBNDK)=A(J,NBNDK) 70 CONTINUE 80 CONTINUE 90 L=ITERM1 DO 100 I=2,N DO 100 J=1,L IF (N+1I+J.GT.N) GO TO 100 A(N+1I,NBND)=A(N+1I,NBND)A(N+1I+J,NBND)*A(N+1I,ITERM+J) 100 CONTINUE RETURN 110 WRITE (6,140) I,A(I,ITERM) STOP 140 FORMAT (/,2X,'Computation stopped in UNSYMSLV because zero appears * on the main diagonal *** Eqn no. and value:',I5,E12.4) END
432
NONLINEAR FINITE ELEMENT ANALYSIS
The limitations on storage can be overcome by solving the equations at the element level, that is, use the Gauss—Seidel iteration idea for the set of variables associated with the element. This approach avoids assembly of element matrices to form the global coeﬃcient matrix. This idea of using the elementbyelement data structure of the coeﬃcient matrix was Þrst pointed out by Fox and Stanton [13] and Fried [14—16]. The phrase elementbyelement refers to a particular data structure for Þnite element techniques wherein information is stored and maintained at the element level rather than assembled into a global data structure. In this method the matrix—vector multiplications are carried out at the element level and the assembly is carried out on the resultant vector. This idea proves to be very attractive when solving large problems, because the matrix—vector multiplication can be done in parallel on a series of processors. Another advantage of this method is that the resultant savings in storage, compared to direct solvers, allows solution of large problems on small computers. For iterative solution methods, the advantages of the elementbyelement data structure over assembling the global coeﬃcient matrix are 1. the need for formation and storage of a global matrix is eliminated, and therefore the total storage and computational costs are low, 2. the amount of storage is independent of the node numbering and mesh topology and depends on the number and type of elements in the mesh, and 3. the elementbyelement solution algorithms can be vectorized for eﬃcient use on supercomputers. The major disadvantage of the elementbyelement data structure is the limited number of preconditioners that can be formulated from the unassembled matrices. This becomes of critical importance when the linear system is not wellconditioned as in the mixed method, incompressible ßow model. A review of the literature on elementbyelement algorithms can be found in [17], and the methods have been investigated by numerous investigators [18—32].
A1.3.2 Solution Algorithms In this section, we review three iterative solvers from [17] that are applicable to nonsymmetric, positive deÞnite equation systems that are typical of isothermal ßow algorithms. The three iterative solution schemes used here are the Biorthogonal Conjugate Gradient method (BCG) [11], the Lanczos ORTHORES [11], and the GMRES [31]. The conjugate gradient method for solving a system of equations can be interpreted as the search for the minimum of the energy E of the system. The energy of the system is a minimum when the residual vector
SOLUTION PROCEDURES FOR LINEAR ALGEBRAIC EQUATIONS
433
˜ − KU ˜ ∗ vanishes. The algorithm for the biorthogonal conjugate r = F gradient method (also known as twoterm form of the steepest descent method, Lanczos/ORTHOMIN) for unsymmetric systems of equations [11,17] is given in Table A1.3.1, and the steps involved in the Lanczos ORTHORES solution algorithm [11,17] are given in Table A1.3.2. The third iterative solver uses the GMRES solution algorithm. For an ¯ 0 + z, where U ¯ 0 is the initial guess approximate solution of the form U vector and z is a member of the Krylov space K of dimension k, the ˜ − K( ˜ U ¯ 0 + z) k GMRES algorithm determines the vector z such that k F The Krylov space is given is minimized,nwhere k · k denotes the L2 −norm. o 2 k−1 ¯ 0, K ¯ 0, · · · K ¯ 0 . Therefore, when solving large ˜U ¯ 0, K ˜ U ˜ U by K = span U systems of equations, as the value of k increases, the amount of storage required also increases. This drawback can be overcome by employing the GMRES algorithm iteratively by using a smaller value for k and restarting the algorithm after every k steps. The restart version of the GMRES algorithm [31,17] is explained in Table A1.3.3. The presence of a penalty matrix in the global coeﬃcient matrix of the penalty model spoils the condition number. This results in slow convergence when using iterative solvers. However, the convergence of the iterative solvers can be improved by preconditioning the system. In [17], the system of equations is transformed using diagonal scaling matrix (Jacobi/diagonal preconditioning). Accordingly, ¯U ¯ =F ¯ ˜ ∗=F ˜ becomes K KU ¯ = W−1/2 KW ¯ = W1/2 U∗ ; F ¯ = W−1/2 F ˜ −1/2 ; U ˜ K
(A1.3.1) (A1.3.2)
˜ −1 is a diagonal matrix. During the matrix multiplication, where Wii = K ii the elementbyelement data structure is exploited and the multiplications are carried out at the element level and the residuals are then assembled to form the global vector. The diagonal terms are always positive because of the viscous and penalty terms.
References 1. Carey, G. F. and Oden, J. T., Finite Elements: Computational Aspects, Prentice Hall, Englewood Cliﬀs, NJ (1984). 2. Atkinson, K. E., An Introduction to Numerical Analysis, John Wiley, New York (1978). 3. Carnahan, B., Luther, H. A., and Wilkes, J. O., Applied Numerical Methods, John Wiley, New York (1969). 4. Fadeev, D. K. and Fadeeva, V. N., Computational Methods of Linear Algebra, Freeman, San Francisco, CA (1963). 5. Hood, P., “Frontal Solution Program for Unsymmetric Matrices,” International Journal for Numerical Methods in Engineering, 10, 379—399 (1976); also see 10, 1055 (1976) for a correction.
434
NONLINEAR FINITE ELEMENT ANALYSIS
Table A1.3.1 Steps involved in using biorthogonal conjugate gradient method (Lanczos/ORTHOMIN solver). Repeat the following steps for each nonlinear iteration: I. Initial Calculations ˜ e and force vectorF ˜ e. (1) Form the element stiﬀness matrix K ˜ e and F ˜ e. (2) Apply essential and/or natural boundary conditions, and modify K ¯ (whose dimensions are nem, neleq, neleq)*. (3) Store the element matrices in A (4) Store inverse of the diagonal terms of the global system in W (Wii = Pnemthe ˜ −1 ). K e=1
ii
(5) Assemble the global force vector.
II. Preconditioning Form the preconditioned system of equations ¯U ¯ = F; ¯ K
¯ = W−1/2 KW ˜ −1/2 , K
¯ = W1/2 U∗ , U
˜ ¯ = W−1/2 F F
III. Lanczos ORTHOMIN Algorithm ¯ 0 , compute: (1) For known initial solution vector U ¯U ¯0 , ¯ − K r0 = F α0 = 0 ,
P0 = r0 ,
¡
¢
r0 ,˜ r0 ¢ , λ0 = ¡ ¯ 0 ,˜ KP r0
˜ 0 = r0 , ˜ r0 = P ¯1 = U ¯ 0 + λ0 P0 U
(2) For each ORTHOMIN iteration m = 1, 2, 3, . . ., compute [(a, b) = λm = ¡
rm ) (rm ,˜ ¢ , ¯ m ,˜ KP rm
Pm = rm + αm Pm−1 , ¯ m, rm+1 = rm − λm KP
αm =
rm ) (rm ,˜ rm−1 ) (rm−1 ,˜
,
P
ai bi ]:
˜ m−1 , ˜m = ˜ P r + αm P ¯TP ˜ m−1 , ˜ rm+1 = ˜ rm − λm K
¯ m+1 = U ¯ m + λm Pm U ¯ m+1 k/kr0 k ≤ 10−6 (3) Convergence criterion kU ¯ ∗ = W−1/2 U ¯ m+1 (4) If convergence criterion is satisÞed U * nem = number of elements in the Þnite element mesh, neleq = number of element equations.
SOLUTION PROCEDURES FOR LINEAR ALGEBRAIC EQUATIONS
435
Table A1.3.2 Steps involved in using Lanczos/ORTHORES solver. Repeat the following steps for each nonlinear iteration: I. Initial Calculations ˜ e and force vector F ˜ e. (1) Form the element stiﬀness matrix K ˜e. ˜ e and F (2) Apply the boundary conditions, and modify K ¯ (whose dimensions are nem, neleq, neleq). (3) Store the element matrices in A (4) Store inverse of the diagonal terms of the global system in W Wii =
nem X
˜ −1 K ii
e=1
(5) Assemble the global force vector. II. Preconditioning: Form the preconditioned system of equations ¯U ¯ =F ¯ , K
¯ = W−1/2 KW ˜ −1/2 , K
˜ , ¯ = W1/2 U U
¯ = W−1/2 F ˜ F
III. Lanczos ORTHORES Algorithm ¯ 0 , compute: (1) For known initial solution vector U ¯U ¯0 ¯ − K r0 = F (2) For each ORTHORES iteration m = 0, 1, 2, . . ., compute (˜ r = r0 , λ0 = 0): rm ) (rm ,˜ ¢ λm+1 = ¡ m ¯ Kr ,˜ rm
β m+1 =
(
h
1−
1
rm ) (rm ,˜ λm+1 1 λm (rm−1 ,˜ rm−1 ) βm
¡
¢
¡
i−1
; if m = 0 ; if m ≥ 1
¢
¯ m + 1 − β m+1 rm−1 rm+1 = β m+1 rm − λm+1 Kr
¡
¢
¡
¢
¯ rm + 1 − β m+1 ˜ ˜ rm+1 = β m+1 ˜ rm − λm+1 K˜ rm−1
¡
¢
¡
¢
¯ m+1 = β m+1 U ¯ m + λm+1 rm + 1 − β m+1 U ¯ m−1 U (3) Check convergence
¯ m+1 k/kr0 k ≤ 10−6 kU (4) If convergence criterion is satisÞed, set ¯ m+1 U∗ = W−1/2 U
436
NONLINEAR FINITE ELEMENT ANALYSIS
Table A1.3.3 Steps involved in using the GMRES solver. Repeat the following steps for each nonlinear iteration: I. Initial Calculations ˜ e. ˜ e and force vector F (1) Form the element stiﬀness matrix K ˜ e and F ˜ e. (2) Apply the boundary conditions, and modify K ¯ (whose dimensions are nem, neleq, neleq). (3) Store the element matrices in A (4) Store inverse of the diagonal terms of the global system in W: Wii = (5) Assemble the global force vector.
Pnem ˜ −1 K e=1
ii
II. Preconditioning: Form the preconditioned system of equations ¯U ¯ =F ¯ , K
¯ = W−1/2 KW ˜ −1/2 , K
˜ , ¯ = W1/2 U U
¯ = W−1/2 F ˜ F
III. GMRES Algorithm ¯ 0 and compute (1) Start Choose U 0 ¯U ¯ 0 , and ¯ −K r =F ¯0 k ¯ 0 /k U v1 = U (2) Iterate: For j = 1, 2..., k do: ¯ j , vi ), i = 1, 2, ..., j, hi,j = (Kv P ¯ j − j hi,j vi , vˆj +1 = Kv i=1 hj +1,j =k vˆj +1 k, and vj +1 = vˆj +1 /hj +1,j (3) Form the approximate solution ¯k = U ¯ 0 + Vy, where y minimizes U ¯ k e − Hy k, y ∈ Rk . (4) Restart ¯U ¯ k; ¯ −K Compute rk = F check convergence; if satisÞed stop; otherwise, compute ¯ 0 := U ¯ k , and go to step 2. ¯ k , v1 := U ¯ k /U U ¯ m+1 k/kr0 k ≤ 10−6 (5) Convergence criterion kU ¯ (6) If convergence criterion is satisÞed U∗ = W−1/2 U where V is a N × k matrix whose columns are ¯ is the ¯ 0 k, 0, . . . , 0}, and H {v1 , v2 , . . . , vk }, e = {k U whose entries are the scalars hi,j . When using the algorithm, the total number of iteration m can be restarts and the dimension of k.
l—2 orthonormal basis vectors upper k × k Hessenberg matrix restart version of the GMRES computed from the number of
SOLUTION PROCEDURES FOR LINEAR ALGEBRAIC EQUATIONS
437
6. Bathe, K. J., Finite Element Procedures, Prentice Hall, Englewood Cliﬀs, New Jersey (1998). 7. Reddy, J. N., An Introduction to the Finite Element Method, 2nd edn, McGraw—Hill, New York (1993). 8. Reddy, J. N. and Gartling, D. K., The Finite Element Method in Heat Transfer and Fluid Dynamics, CRC Press, Boca Raton, FL (1994); 2nd edn (2000). 9. Hestenes, M. R. and Stiefel, E. L., “Methods of Conjugate Gradients for Solving Linear Systems,” National Bureau of Standards Journal of Research, 49, 409—436 (1952). 10. Golub, G. H. and Van Loan, C. F., Matrix Computations, 2nd edn, The Johns Hopkins University Press, Baltimore, MD (1989). 11. Jea, K. C. and Young, D. M., “On the SimpliÞcation of Generalized Conjugate—Gradient Methods for Nonsymmetrizable Linear Systems,” Linear Algebra Applications, 52, 399—417 (1983). 12. Evans, D. J., “Use of Preconditioning in Iterative Methods for Solving Linear Equations with Symmetric Positive DeÞnite Matrices,” Computer Journal, 4, 73—78 (1961). 13. Fox, R. L. and Stanton, E. L., “Developments in Structural Analysis by Direct Energy Minimization,” AIAA Journal, 6, 1036—1042 (1968). 14. Fried, I., “Gradient Methods for Finite Element Eigenproblems,” AIAA Journal, 7, 739—741 (1969). 15. Fried, I., “More on Generalized Iterative Methods in Finite Element Analysis,” AIAA Journal, 7, 565—567 (1969). 16. Fried, I., “A Gradient Computational Procedure for the Solution of Large Problems Arising from the Finite Element Discretization Method,” International Journal for Numerical Methods in Engineering, 2, 477—494 (1970). 17. Reddy, M. P., Reddy, J. N., and Akay, H. U., “Penalty Finite Element Analysis of Incompressible Flows Using Element by Element Solution Algorithms,” Computer Methods in Applied Mechanics and Engineering, 100, 169—205 (1992). 18. Wathen, A. J., “An Analysis of Some ElementbyElement Techniques,” Computer Methods in Applied Mechanics and Engineering, 74, 271—287 (1989). 19. Hughes, T. J. R., Levit, I., and Winget, J., “An ElementbyElement Implicit Algorithm for Heat Conduction,” Journal of the Engineering Mechanics Division, ASCE, 109(2), 576—585 (1983). 20. Winget, J. M. and Hughes, T. J. R., “Solution Algorithms for Nonlinear Transient Heat Conduction Analysis Employing ElementbyElement Iterative Strategies,” Computer Methods in Applied Mechanics and Engineering, 52, 711—815 (1985). 21. Carey, G. F. and Jiang, B., “ElementbyElement Linear and Nonlinear Solution Schemes,” Communications in Applied Numerical Methods, 2, 145—153 (1986). 22. Hayes, J. L. and Devloo, P., “An ElementbyElement Block Iterative Method for Large NonLinear Problems,” in Innovative Methods for Nonlinear Behavior, W. K. Liu et al. (eds.), Pineridge Press, Swansea, UK, 51—62 (1985). 23. Buratynski, E. K.,“An ElementbyElement Method for Heat Conduction CAE Including Composite Problems,” International Journal for Numerical Methods in Engineering, 26, 199—215 (1988). 24. Levit, I., “Element By Element Solvers of Order N,” Computers & Structures, 27(3), 357—360 (1987). 25. de La Bourdinnaye, A., “The ElementbyElement Method as a Preconditioner for Linear Systems Coming from Finite Element Models,” International Journal of Supercomputer Applications, 3, 60—68 (1989).
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NONLINEAR FINITE ELEMENT ANALYSIS
26. Prakhya, K. V. G., “Some Conjugate Gradient Methods for Symmetric and Nonsymmetric Systems,” Communications in Applied Numerical Methods, 4, 531—539 (1988). 27. Carey, G. F., Barragy, E., R. Mclay, and Sharma, M., “ElementbyElement Vector and Parallel Computations,” Communications in Applied Numerical Methods, 4, 299— 307 (1988). 28. Hayes, L. J. and Devloo, P., “A Vectorized Version of a Sparse MatrixVector Multiplication,” International Journal for Numerical Methods in Engineering, 23, 1043—1056 (1986). 29. Shakib, F., Hughes, T. J. R., and Johan, Z., “A MultiElement Group Preconditioned GMRES Algorithm for Nonsymmetric Systems Arising in Finite Element Analysis,” Computer Methods in Applied Mechanics and Engineering, 75, 415—456 (1989). 30. Tezduyar, T. E. and Liou, J., “Grouped ElementByElement Iteration Schemes for Incompressible Flow Computations,” Computational Physics Communications, 53, 441—453 (1989). 31. Saad, Y. and Schultz, M. H., “GMRES: A Generalized Minimal Residual Algorithm for Solving Nonsymmetric Linear Systems,” SIAM Journal of ScientiÞc and Statistical Computations, 7(3), 856—869 (1986). 32. Mitchell, J. A. and Reddy, J. N., “A Multilevel Hierarchical Preconditioner for Thin Elastic Solids,” International Journal for Numerical Methods in Engineering, 43, 1383—1400 (1998).
Appendix A2 Solution Procedures for Nonlinear Algebraic Equations
A2.1 Introduction Finite element formulations of nonlinear diﬀerential equations lead to nonlinear algebraic equations for each element of the Þnite element mesh. The element equation is of the form, [K e ({ue })]{ue } = {F e }
(A2.1.1)
where [K e ] − element coeﬃcient matrix (or “stiﬀness” matrix), which depends on the solution vector{ue }, (A2.1.2) {ue } − column vector of element nodal values, and e {F } − column vector of element nodal “forces”. When [K e ] is independent of {ue }, the matrix coeﬃcients can be evaluated for all elements, and the assembled equations can be solved for the global nodal values {u} after imposing boundary conditions. When [K e ] depends on the unknown solution vector {ue }, the matrix coeﬃcients cannot be evaluated. If we can Þnd an approximation to {ue }, say {ue }1 , then [K e ({ue }1 )] can be evaluated and assembled. This amounts to linearizing the nonlinear equations, Eq. (A2.1.1). Then a next approximation to the solution can be obtained by solving the assembled equations, {u}2 = [K({u}1 )]−1 {F }
(A2.1.3)
This procedure can be repeated until the approximate solution comes close to the actual solution in some measure. Such a procedure is called an iterative procedure.
440
NONLINEAR FINITE ELEMENT ANALYSIS
Here we discuss the following commonly used iterative procedures: 1. The Picard Iteration (or Direct Iteration) Method 2. The Newton—Raphson Iteration Method 3. The Riks Method The details of these methods are discussed next, with the aid of a single nonlinear equation. For a more complete presentation of these methods, the reader may consult the references at the end of this appendix. Consider the nonlinear equation, K(u) · u = F
or R(u) = 0
(A2.1.4)
where u is the solution to be determined, K(u) is a known function of u, F is the known ‘force’, and R is the residual R(u) = K(u) · u − F
(A2.1.5)
A plot of the equilibrium path, R(u, F ) = 0, is shown in Figure A2.1.1. For any value u1 , K(u1 ) denotes the secant of the curve at u = u1 , and ( ∂R ∂u )u1 denotes the tangent of the curve at u = u1 .
A2.2 Picard Iteration Method In the Picard iteration, also known as the direct iteration method, we begin with an initial guess for u, say u(0) , (u(0) = 0 in Figure A2.1.1) and determine a Þrst approximation to u by solving the equation ³
Load, F
u(1) = K(u(0) )
F1
Linear, K(0)
´−1
F
Tangent to the path Equilibrium path R(u,F) = 0
° secant
u1
Displacement, u
Figure A2.1.1 Typical force—displacement curve.
(A2.2.1)
SOLUTION PROCEDURES FOR NONLINEAR ALGEBRAIC EQUATIONS
441
u(1) 6= u, and a second approximation for u is sought by using the last approximation to evaluate K ³
´−1
u(2) = K(u(1) )
F
(A2.2.2)
This procedure is continued until the diﬀerence between two consecutive approximations of u diﬀer by a preselected value. Thus, the algorithm and criterion for convergence may be written as Algorithm
³
´−1
u(r) = K(u(r−1) )
Convergence Criterion
s
F
(u(r) − u(r−1) )2 C'onfigiiration. :i28. 337. 338. 339 Conforming element. IG1 CoTiju^file gradient method, 274. 277. 280. 130 Consistent m;iss m a t r i x , 292 Consistent, penally model. 215. 24(> Const ant averapjeaceelerat ion method. 29:!. 309, 312, 371 Constitutive relations, 40!) Continuity equation. 231. 242 Continuum formulation. 87 Convergence criterion, 4 4 1 , 413
460
NONLINEAR FINITE ELEMENT ANALYSIS
Convergence tolerance, 441 CPT, see: Classical plate theory CrankNicolson scheme, 291, 307, 31 Cylindrical coordinate system, 405 Cylindrical shell, 209, 212 Deformation gradient tensor, 330 Direct iteration, 65, 77, 98, 107, 12 131, 408, 440 Direct methods, 427 Displacement finite element model, 157 Doublycurved shell, 196, 21 displacement field, 204 equations of motion, 205 finite element model of, 206, 207 strains, 204 stress resultants, 203 Eigenvalue problem, 289 Eigenvalues, 334 Elasticity tensor, 346 Elasticperfectlyplastic, 392 Elastoplastic tangent modulus, 394 Elementbyelement, 432 Energeticallyconjugate, 336 Engineering constants, 376 Equations of equilibrium, EulerBernoulli beam theory, 91 Timoshenko beam theory, 113 Essential boundary condition, 76 see Boundary conditions Euler strain tensor, 332, 343 EulerBernoulli beam theory, 88 displacement field of, 88 EulerBernoulli hypotheses, 88 EulerLagrange equations, 112, 176 Eulerian description, 230 Euler's explicit method, 4 Explicit scheme, 288 Extensional stiffnesses, 94, 153 Finger tensor, 332 Finite difference method, 5 Finite element, 5, 13 Finite element mesh, 25 Finite element method, 5, 290
Finite element model of: beams (EBT), 95 beams (TBT), 113 heat transfer, 297, 418 isotropic, Newtonian, viscous, incompressible fluids, 235 plates (CPT), 153 plates (FSDT), 177 power law fluids, 407, 411 Firstorder shear deformation theory: displacement field, 173 equations of motion, 177 finite element model of, 177, 17 generalized displacement, 173 strains, 174 First law of thermodynamics, 232 First PiolaKirchhoff stress, 335 Fixed edge, 151 Flow over a backwardfacing step, 277 Flow past a circular cylinder, 278 Fluid mechanics, 229 Force boundary condition, 92 Forward difference scheme, 4, 290, 291 Fourier's heat conduction law, 233 Freebody diagram, 17 Free edge, 151 Frontal solution, 427 FSDT, see: Firstorder shear deformation theory Full integration, 104, 116, 118, 120, 1 Galerkin's method, 15, 291, 293 Gauss elimination method, 427 Gauss points, 45, 135, 185, 251 Gauss quadrature, 45, 102, 116, 118, 135 Gauss weights, 45, 135, 251 GaussLegendre quadrature, 44 Generalized nodal displacements, 90 Generalized nodal forces, 89 Geometric nonlinearity, 7, 389 Global coordinates, 30, 130, 133 GMRES, 432 Governing Equations: of Newtonian, viscous, incompressible fluids, 235 of nonNewtonian fluids, 406 GreenLagrange incremental strain, 341
SUBJECT INDEX
GreenLagrange strain tensor, 201, 331, 333, 340, 390 /irefinement, 218 Half bandwidth, 425 Hamilton's principle, 369 Hardening, 8 Heated cavity, 419 Heat flux, 233 Hermite cubic interpolation functions, 95 Hinged, 119, 219 Homogeneous motion, 331 Hydrostatic pressure, 233 Hyperelastic material, 346, 390 Ideal plastic, 392 Imbalance force, 445 Implicit scheme, 288 Incompressibility condition, 231 Incompressible flow, 229 Infinitesimal strain tensor, 341, 343 Initial conditions, 288, 292 Initialvalue problem, 3 Interpolation property, 19, 30 Inviscid fluid, 229 Isoparametric formulation, 42 Iterative method, 274, 430 Jacobian, 44, 134, 331, 340 Jacobian matrix, 43, 134, 251 Kinetic hardening, 394 Kirchhoff assumptions, 141 Kirchhoff freeedge condition, 150 Kirchhoff stress increment tensor, 344 Kovasznay flow, 275 LadyzhenskayaBabuskaBrezzi (LBB) condition, 248 Lagrange interpolation, 30, 32, 38, 114, 130, 155, 179, 207, 411, 418 Higherorder, 20 Linear, 19, 95 Quadratic, 20 Lagrange multiplier method, 241 Lagrangian description, 230, 328
461
Lame constants, 199, 233 Laminar flow, 230 Lanczos orthores, 433 Least squares finite element model, 267, 269, 272, 299, 315 Least squares functional, 271, 272, 275, 277, 299, 318 Linear acceleration method, 293 Linear element, 23 Linear form, 17, 28, 241, 270 Linear rectangular element, 32 Linear triangular element, 30 Linearly independent, 15 Local coordinates, 133 Locking, 274 membrane, 102, 117 shear, 115, 184 Load increments, 100, 106, 108, 119, 172, 361 Master element, 36, 41 Material coordinates, 328 Material description, 230, 328 Material nonlinearity, 7, 389 Material stiffnesses, 376 Material strain rate tensor, 336 Mathematical model, 1 Matrix, coefficient, 23 stiffness, 23, 358 tangent stiffness, 358 Membrane locking, see Locking Metric, 197 Mindlin plate theory, 181 Mixed finite element model, 237, 246, 254, 274 Moment, 91 Natural boundary condition, 16, 79, 92
NavierStrokes equation, 230, 232 Newmark's integration scheme, 293, 296, 371 NewtonRaphson iteration scheme, 68, 78, 98, 107, 121, 131, 157, 189, 439, 444 modified, 70, 445
462
NONLINEAR FINITE ELEMENT ANALYSIS
Newtonian fluid, 229, 233 Newton's iteration procedure, 68 Nodal degrees of freedom, 23 Nodes, 6, 26 global, 33 Nonconforming element, 161 Nonlinear analysis of: EulerBernoulli beams, 88 Timoshenko beams, 110 NonNewtonian fluid, 229, 404 Numerical integration, 40 Numerical simulation, 3 prefinement, 21 Parabolic equations, 289 Penalty finite element model, 237, 241, 246, 405 Penalty parameters, 243, 247, 256, 261 Picard iteration, 65, 440 Pinned, 107, 121 Plane stress, 365 Plane stressreduced stiffnesses, 152, 376 Plastic flow, 391 Plasticity, 391 Powerlaw fluid, 404, 407 Primary variables, 16, 27, 62, 112, 150, 177, 206, 235 Primitive variables, 235 Principal radii of curvature, 197 Principle of: conservation of angular momentum, 232 conservation of energy, 232 conservation of linear momentum, 232 conservation of mass, 231, 340 minimum total potential energy, 18 virtual displacements, 89, 111, 145, 174, 347, 369 virtual work, 349, 352 Quadratic element, 23 Rate of deformation gradient tensor, 336 Rectangular elements, 38
Reduced integration, 102, 104, 118, 120, 168, 212, 214, 249, 254, 274, 377 Reduced integration penalty model, 244 Relaxation, 443 Residual, 15 Riks, 440, 448 RiksWempner method, 378 modified, 381 RombergOsgood model, 391 Rotation tensor, 334 Sanders' shell theory, 196, 205 Second PiolaKirchhofT stress, 335, 390 Secondary variables, 16, 27, 62, 112, 150, 177, 206, 235 Serendipity elements, 39 Shallow cylindrical panel, 217, 219 Shear correction coefficient, 111, 175 Shear correction factors, 203 Shear force, 112, 203 Shear locking, see Locking Shear thickening fluids, 407 Shear thinning fluids, 407 Simply supported, 151, 167, 169, 185 189, 210, 312, 378, 381 Skyline technique, 427 Softening, 9 Solar receiver, 420 Spacetime coupled formulation, 321 Spatial approximation, 287 Spatial description, 230, 329 Specific heat, 236 Stability, 295, 296 Stiffnesses: bending, 94, 153 extensional, 94, 153 extensionalbending, 94 Strokes flow, 230, 241, 269 Straindisplacement relations, 143 nonlinear, 88 Strain energy, 89, 174 Strain energy density, 390 Strain hardening, 393, 394 Strain rate tensor, 232, 405 Stream function, 237
SUBJECT INDEX
Stress, Cauchy, 334 deviatoric, 393 invariants, 393 tensor. 232 vector. 334 Stretch tensor, 334 Subparametric formulation, 42 Superconvergent element, 95 Superparametric formulation, 42 Tangent matrix, 69, 131, 445 Tangent stiffness matrix, 98, 117, 157 182, 390 Theorem of Rodrigues, 197 Thermal coefficients of expansion, 153 Thermal stress resultant, 153 Timoshenko beam theory, 110 displacement field of, 110 finite element model of, 113 Tolerance, 65, 108, 119, 169, 189, 1 Total Lagrangian description, 338 Total Lagrangian formultaion, 348, 353 Transverse force, 2, 175 Tresca yield criterion, 393 Triangular elements, 37 Turbulent flow, 230 Unconditionally stable, 295 Updated GreenLagrange strain tensor, 342 Updated Kirchhoff stress increment tensor, 344 Updated Kirchhoff stress tensor, 344 Updated Lagrangian description, 338 Updated Lagrangian formulation, 350, 360
463
Validation, 12 Variational problem, 17, 241, 242, 270 Vectors, 23 Velocity gradient tensor, 335 VelocityPressure model, 237 VelocityPressureVelocity model, 299 VelocityPressureVorticity model, 299 Verification, 11 Viscoelastic fluid, 404 Viscosity, 229 Viscous incompressible fluid, 407 Viscous stress tensor, 233 von Karman nonlinearity, 300 von Karman strains, 89, 144, 174 von Mises yield criterion, 393 Vorticity, 237, 271 Weak form, 16, 26, 62, 128, 239 Weak forms for: Doublycurved shell, 206 EulerBernoulli beam, 89 heat transfer, 297 plates (CPT), 147 plates (FSDT), 174, 300 steady Stokes flow, 241 Timoshenko beam, 111 Weight functions, 15 Weightedintegral, 15 WeingartenGauss relations, 197 WhiteMetzner model, 409 Yield criterion, 392 Yield stress, 392 Yielding, 391