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The Finite Element Method: Its

Basis and Fundamentals

Sixth edition

Professor O.C. Zienkiewicz, CBE, FRS, FREng is Professor Emeritus at the Civil andComputational Engineering Centre, University of Wales Swansea and previously Directorof the Institute for Numerical Methods in Engineering at the University of Wales Swansea,UK. He holds the UNESCO Chair of Numerical Methods in Engineering at the TechnicalUniversity of Catalunya, Barcelona, Spain. He was the head of the Civil EngineeringDepartment at the University of Wales Swansea between 1961 and 1989. He establishedthat department as one of the primary centres of finite element research. In 1968 he becamethe Founder Editor of the International Journal for Numerical Methods in Engineeringwhich still remains today the major journal in this field. The recipient of 27 honorarydegrees and many medals, Professor Zienkiewicz is also a member of five academies anhonour he has received for his many contributions to the fundamental developments of thefinite element method. In 1978, he became a Fellow of the Royal Society and the RoyalAcademy of Engineering. This was followed by his election as a foreign member to theU.S. Academy of Engineering (1981), the Polish Academy of Science (1985), the ChineseAcademy of Sciences (1998), and the National Academy of Science, Italy (Academia deiLincei) (1999). He published the first edition of this book in 1967 and it remained the onlybook on the subject until 1971.

Professor R.L. Taylor has more than 40 years experience in the modelling and simulationof structures and solid continua including two years in industry. He is Professor in theGraduate School and the Emeritus T.Y. and Margaret Lin Professor of Engineering at theUniversity of California at Berkeley. In 1991 he was elected to membership in the USNational Academy of Engineering in recognition of his educational and research contri-butions to the field of computational mechanics. Professor Taylor is a Fellow of the USAssociation of Computational Mechanics USACM (1996) and a Fellow of the Interna-tionalAssociation of Computational Mechanics IACM (1998). He has received numerousawards including the Berkeley Citation, the highest honour awarded by the University ofCalifornia at Berkeley, the USACM John von Neumann Medal, the IACM GaussNewtonCongress Medal and a Dr.-Ingenieur ehrenhalber awarded by the Technical University ofHannover, Germany. Professor Taylor has written several computer programs for finiteelement analysis of structural and non-structural systems, one of which, FEAP, is usedworld-wide in education and research environments. A personal version, FEAPpv, avail-able from the publishers website, is incorporated into the book.

Dr J.Z. Zhu has more than 20 years experience in the development of finite elementmethods. During the last 12 years he has worked in industry where he has been developingcommercial finite element software to solve multi-physics problems. Dr Zhu read for hisBachelor of Science degree at Harbin Engineering University and his Master of Scienceat Tianjin University, both in China. He was awarded his doctoral degree in 1987 fromthe University of Wales Swansea, working under the supervision of Professor Zienkiewicz.Dr Zhu is the author of more than 40 technical papers on finite element methods includingseveral on error estimation and adaptive automatic mesh generation. These have resultedin his being named in 2000 as one of the highly cited researchers for engineering in theworld and in 2001 as one of the top 20 most highly cited researchers for engineering in theUnited Kingdom.

The Finite ElementMethod: Its Basis and

Fundamentals

Sixth edition

O.C. Zienkiewicz, CBE, FRSUNESCO Professor of Numerical Methods in Engineering

International Centre for Numerical Methods in Engineering, BarcelonaPreviously Director of the Institute for Numerical Methods in Engineering

University of Wales, Swansea

R.L. Taylor J.Z. ZhuProfessor in the Graduate School Senior Scientist

Department of Civil and Environmental Engineering ESI US R & D Inc.University of California at Berkeley 5850 Waterloo Road, Suite 140

Berkeley, California Columbia, Maryland

AMSTERDAM BOSTON HEIDELBERG LONDON NEW YORK OXFORDPARIS SAN DIEGO SAN FRANCISCO SINGAPORE SYDNEY TOKYO

Elsevier Butterworth-HeinemannLinacre House, Jordan Hill, Oxford OX2 8DP30 Corporate Drive, Burlington, MA 01803

First published in 1967 by McGraw-HillFifth edition published by Butterworth-Heinemann 2000

Reprinted 2002Sixth edition 2005

Copyright c 2000, 2005. O.C. Zienkiewicz, R.L. Taylor and J.Z. Zhu. All rights reserved

The rights of O.C. Zienkiewicz, R.L. Taylor and J.Z. Zhu to be identified as the authors of this workhave been asserted in accordance with the Copyright, Designs and Patents Act 1988

No part of this publication may be reproduced in any material form (includingphotocopying or storing in any medium by electronic means and whether or nottransiently or incidentally to some other use of this publication) without the writtenpermission of the copyright holder except in accordance with the provisions of theCopyright, Designs and Patents Act 1988 or under the terms of a licence issued bythe Copyright Licensing Agency Ltd, 90, Tottenham Court Road, London,England W1T 4LP. Applications for the copyright holders written permissionto reproduce any part of this publication should be addressed to the publisher.

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ISBN 0 7506 6320 0

Published with the cooperation of CIMNE,the International Centre for Numerical Methods in Engineering,

Barcelona, Spain (www.cimne.upc.es)

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Dedication

This book is dedicated to our wives Helen, Mary Lou andSong and our families for their support and patience duringthe preparation of this book, and also to all of our studentsand colleagues who over the years have contributed to ourknowledge of the finite element method. In particular wewould like to mention Professor Eugenio Onate and his

group at CIMNE for their help, encouragement and supportduring the preparation process.

Contents

Preface xiii

1 The standard discrete system and origins of the finite element method 11.1 Introduction 11.2 The structural element and the structural system 31.3 Assembly and analysis of a structure 51.4 The boundary conditions 61.5 Electrical and fluid networks 71.6 The general pattern 91.7 The standard discrete system 101.8 Transformation of coordinates 111.9 Problems 13

2 A direct physical approach to problems in elasticity: plane stress 192.1 Introduction 192.2 Direct formulation of finite element characteristics 202.3 Generalization to the whole region internal nodal force concept

abandoned 312.4 Displacement approach as a minimization of total potential energy 342.5 Convergence criteria 372.6 Discretization error and convergence rate 382.7 Displacement functions with discontinuity between elements

non-conforming elements and the patch test 392.8 Finite element solution process 402.9 Numerical examples 402.10 Concluding remarks 462.11 Problems 47

3 Generalization of the finite element concepts. Galerkin-weighted residual andvariational approaches 543.1 Introduction 543.2 Integral or weak statements equivalent to the differential equations 573.3 Approximation to integral formulations: the weighted residual-

Galerkin method 60

viii Contents

3.4 Virtual work as the weak form of equilibrium equations for analysisof solids or fluids 69

3.5 Partial discretization 713.6 Convergence 743.7 What are variational principles? 763.8 Natural variational principles and their relation to governing

differential equations 783.9 Establishment of natural variational principles for linear, self-adjoint,

differential equations 813.10 Maximum, minimum, or a saddle point? 833.11 Constrained variational principles. Lagrange multipliers 843.12 Constrained variational principles. Penalty function and perturbed

lagrangian methods 883.13 Least squares approximations 923.14 Concluding remarks finite difference and boundary methods 953.15 Problems 97

4 Standard and hierarchical element shape functions: some general familiesof C0 continuity 1034.1 Introduction 1034.2 Standard and hierarchical concepts 1044.3 Rectangular elements some preliminary considerations 1074.4 Completeness of polynomials 1094.5 Rectangular elements Lagrange family 1104.6 Rectangular elements serendipity family 1124.7 Triangular element family 1164.8 Line elements 1194.9 Rectangular prisms Lagrange family 1204.10 Rectangular prisms serendipity family 1214.11 Tetrahedral elements 1224.12 Other simple three-dimensional elements 1254.13 Hierarchic polynomials in one dimension 1254.14 Two- and three-dimensional, hierarchical elements of the rectangle

or brick type 1284.15 Triangle and tetrahedron family 1284.16 Improvement of conditioning with hierarchical forms 1304.17 Global and local finite element approximation 1314.18 Elimination of internal parameters before assembly substructures 1324.19 Concluding remarks 1344.20 Problems 134

5 Mapped elements and numerical integration infinite andsingularity elements 1385.1 Introduction 1385.2 Use of shape functions in the establishment of coordinate

transformations 1395.3 Geometrical conformity of elements 1435.4 Variation of the unknown function within distorted, curvilinear

elements. Continuity requirements 143

Contents ix

5.5 Evaluation of element matrices. Transformation in , , coordinates 1455.6 Evaluation of element matrices. Transformation in area and volume

coordinates 1485.7 Order of convergence for mapped elements 1515.8 Shape functions by degeneration 1535.9 Numerical integration one dimensional 1605.10 Numerical integration rectangular (2D) or brick regions (3D) 1625.11 Numerical integration triangular or tetrahedral regions 1645.12 Required order of numerical integration 1645.13 Generation of finite element meshes by mapping. Blending functions 1695.14 Infinite domains and infinite elements 1705.15 Singular elements by mapping use in fracture mechanics, etc. 1765.16 Computational advantage of numerically integrated finite elements 1775.17 Problems 178

6 Problems in linear elasticity 1876.1 Introduction 1876.2 Governing equations 1886.3 Finite element approximation 2016.4 Reporting of results: displacements, strains and stresses 2076.5 Numerical examples 2096.6 Problems 217

7 Field problems heat conduction, electric and magnetic potentialand fluid flow 2297.1 Introduction 2297.2 General quasi-harmonic equation 2307.3 Finite element solution process 2337.4 Partial discretization transient problems 2377.5 Numerical examples an assessment of accuracy 2397.6 Concluding remarks 2537.7 Problems 253

8 Automatic mesh generation 2648.1 Introduction 2648.2 Two-dimensional mesh generation advancing front method 2668.3 Surface mesh generation 2868.4 Three-dimensional mesh generation Delaunay triangulation 3038.5 Concluding remarks 3238.6 Problems 323

9 The patch test, reduced integration, and non-conforming elements 3299.1 Introduction 3299.2 Convergence requirements 3309.3 The simple patch test (tests A and B) a necessary condition for

convergence 3329.4 Generalized patch test (test C) and the single-element test 3349.5 The generality of a numerical patch test 3369.6 Higher order patch tests 336

x Contents

9.7 Application of the patch test to plane elasticity elements withstandard and reduced quadrature 337

9.8 Application of the patch test to an incompatible element 3439.9 Higher order patch test assessment of robustness 3479.10 Concluding remarks 3479.11 Problems 350

10 Mixed formulation and constraints complete field methods 35610.1 Introduction 35610.2 Discretization of mixed forms some general remarks 35810.3 Stability of mixed approximation. The patch test 36010.4 Two-field mixed formulation in elasticity 36310.5 Three-field mixed formulations in elasticity 37010.6 Complementary forms with direct constraint 37510.7 Concluding remarks mixed formulation or a test of element

robustness 37910.8 Problems 379

11 Incompressible problems, mixed methods and other procedures of solution 38311.1 Introduction 38311.2 Deviatoric stress and strain, pressure and volume change 38311.3 Two-field incompressible elasticity (up form) 38411.4 Three-field nearly incompressible elasticity (upv form) 39311.5 Reduced and selective integration and its equivalence to penalized

mixed problems 39811.6 A simple iterative solution process for mixed problems: Uzawa

method 40411.7 Stabilized methods for some mixed elements failing the

incompressibility patch test 40711.8 Concluding remarks 42111.9 Problems 422

12 Multidomain mixed approximations domain decomposition and framemethods 42912.1 Introduction 42912.2 Linking of two or more subdomains by Lagrange multipliers 43012.3 Linking of two or more subdomains by perturbed lagrangian and

penalty methods 43612.4 Interface displacement frame 44212.5 Linking of boundary (or Trefftz)-type solution by the frame of

specified displacements 44512.6 Subdomains with standard elements and global functions 45112.7 Concluding remarks 45112.8 Problems 451

13 Errors, recovery processes and error estimates 45613.1 Definition of errors 45613.2 Superconvergence and optimal sampling points 45913.3 Recovery of gradients and stresses 465

Contents xi

13.4 Superconvergent patch recovery SPR 46713.5 Recovery by equilibration of patches REP 47413.6 Error estimates by recovery 47613.7 Residual-based methods 47813.8 Asymptotic behaviour and robustness of error estimators the

Babuska patch test 48813.9 Bounds on quantities of interest 49013.10 Which errors should concern us? 49413.11 Problems 495

14 Adaptive finite element refinement 50014.1 Introduction 50014.2 Adaptive h-refinement 50314.3 p-refinement and hp-refinement 51414.4 Concluding remarks 51814.5 Problems 520

15 Point-based and partition of unity approximations. Extended finiteelement methods 52515.1 Introduction 52515.2 Function approximation 52715.3 Moving least squares approximations restoration of continuity of

approximation 53315.4 Hierarchical enhancement of moving least squares expansions 53815.5 Point collocation finite point methods 54015.6 Galerkin weighting and finite volume methods 54615.7 Use of hierarchic and special functions based on standard finite

elements satisfying the partition of unity requirement 54915.8 Concluding remarks 55815.9 Problems 558

16 The time dimension semi-discretization of field and dynamic problemsand analytical solution procedures 56316.1 Introduction 56316.2 Direct formulation of time-dependent problems with spatial finite

element subdivision 56316.3 General classification 57016.4 Free response eigenvalues for second-order problems and dynamic

vibration 57116.5 Free response eigenvalues for first-order problems and heat

conduction, etc. 57616.6 Free response damped dynamic eigenvalues 57816.7 Forced periodic response 57916.8 Transient response by analytical procedures 57916.9 Symmetry and repeatability 58316.10 Problems 584

17 The time dimension discrete approximation in time 58917.1 Introduction 589

xii Contents

17.2 Simple time-step algorithms for the first-order equation 59017.3 General single-step algorithms for first- and second-order equations 60017.4 Stability of general algorithms 60917.5 Multistep recurrence algorithms 61517.6 Some remarks on general performance of numerical algorithms 61817.7 Time discontinuous Galerkin approximation 61917.8 Concluding remarks 62417.9 Problems 626

18 Coupled systems 63118.1 Coupled problems definition and classification 63118.2 Fluidstructure interaction (Class I problems) 63418.3 Soilpore fluid interaction (Class II problems) 64518.4 Partitioned single-phase systems implicitexplicit partitions

(Class I problems) 65318.5 Staggered solution processes 65518.6 Concluding remarks 660

19 Computer procedures for finite element analysis 66419.1 Introduction 66419.2 Pre-processing module: mesh creation 66419.3 Solution module 66619.4 Post-processor module 66619.5 User modules 667

Appendix A: Matrix algebra 668

Appendix B: Tensor-indicial notation in the approximation of elasticity problems 674

Appendix C: Solution of simultaneous linear algebraic equations 683

Appendix D: Some integration formulae for a triangle 692

Appendix E: Some integration formulae for a tetrahedron 693

Appendix F: Some vector algebra 694

Appendix G: Integration by parts in two or three dimensions (Greens theorem) 699

Appendix H: Solutions exact at nodes 701

Appendix I: Matrix diagonalization or lumping 704

Author index 711

Subject index 719

Preface

It is thirty-eight years since the The Finite Element Method in Structural and ContinuumMechanics was first published. This book, which was the first dealing with the finiteelement method, provided the basis from which many further developments occurred. Theexpanding research and field of application of finite elements led to the second edition in1971, the third in 1977, the fourth as two volumes in 1989 and 1991 and the fifth as threevolumes in 2000. The size of each of these editions expanded geometrically (from 272pages in 1967 to the fifth edition of 1482 pages). This was necessary to do justice to arapidly expanding field of professional application and research. Even so, much filteringof the contents was necessary to keep these editions within reasonable bounds.

In the present edition we have decided not to pursue the course of having three contiguousvolumes but rather we treat the whole work as an assembly of three separate works, eachone capable of being used without the others and each one appealing perhaps to a differentaudience. Though naturally we recommend the use of the whole ensemble to people wishingto devote much of their time and study to the finite element method.

In particular the first volume which was entitled The Finite Element Method: The Basisis now renamed The Finite Element Method: Its Basis and Fundamentals. This volumehas been considerably reorganized from the previous one and is now, we believe, bettersuited for teaching fundamentals of the finite element method. The sequence of chaptershas been somewhat altered and several examples of worked problems have been added tothe text. A set of problems to be worked out by students has also been provided.

In addition to its previous content this book has been considerably enlarged by includingmore emphasis on use of higher order shape functions in formulation of problems and anew chapter devoted to the subject of automatic mesh generation. A beginner in the finiteelement field will find very rapidly that much of the work of solving problems consists ofpreparing a suitable mesh to deal with the whole problem and as the size of computers hasseemed to increase without limits the size of problems capable of being dealt with is alsoincreasing. Thus, meshes containing sometimes more than several million nodes have to beprepared with details of the material interfaces, boundaries and loads being well specified.There are many books devoted exclusively to the subject of mesh generation but we feelthat the essence of dealing with this difficult problem should be included here for thosewho wish to have a complete encyclopedic knowledge of the subject.

xiv Preface

The chapter on computational methods is much reduced by transferring the computersource program and user instructions to a web site. This has the very substantial advantageof not only eliminating errors in program and manual but also in ensuring that the readershave the benefit of the most recent version of the program available at all times.

The two further volumes form again separate books and here we feel that a completelydifferent audience will use them. The first of these is entitled The Finite Element Methodin Solid and Structural Mechanics and the second is a text entitled The Finite ElementMethod in Fluid Dynamics. Each of these two volumes is a standalone text which providesthe full knowledge of the subject for those who have acquired an introduction to the finiteelement method through other texts. Of course the viewpoint of the authors introduced inthis volume will be continued but it is possible to start at a different point.

We emphasize here the fact that all three books stress the importance of considering thefinite element method as a unique and whole basis of approach and that it contains many ofthe other numerical analysis methods as special cases. Thus, imagination and knowledgeshould be combined by the readers in their endeavours.

The authors are particularly indebted to the International Center of Numerical Methods inEngineering (CIMNE) in Barcelona who have allowed their pre- and post-processing code(GiD) to be accessed from the web site. This allows such difficult tasks as mesh generationand graphic output to be dealt with efficiently. The authors are also grateful to ProfessorsEric Kasper and Jose Luis Perez-Aparicio for their careful scrutiny of the entire text andDrs Joaquim Peiro and C.K. Lee for their review of the new chapter on mesh generation.

Resources to accompany this bookWorked solutions to selected problems in this book are available online for teachers andlecturers who either adopt or recommend the text. Please visit http://books.elsevier.com/manuals and follow the registration and log in instructions on screen.

OCZ, RLT and JZZ

Complete source code and user manual for program FEAPpv may be obtained at no cost from the publishersweb page: http://books.elsevier.com/companions or from the authors web page: http://www.ce.berkeley.edu/rlt

1

The standard discrete system andorigins of the finite element

method

1.1 Introduction

The limitations of the human mind are such that it cannot grasp the behaviour of its complexsurroundings and creations in one operation. Thus the process of subdividing all systemsinto their individual components or elements, whose behaviour is readily understood, andthen rebuilding the original system from such components to study its behaviour is a naturalway in which the engineer, the scientist, or even the economist proceeds.

In many situations an adequate model is obtained using a finite number of well-definedcomponents. We shall term such problems discrete. In others the subdivision is continuedindefinitely and the problem can only be defined using the mathematical fiction of aninfinitesimal. This leads to differential equations or equivalent statements which imply aninfinite number of elements. We shall term such systems continuous.

With the advent of digital computers, discrete problems can generally be solved readilyeven if the number of elements is very large. As the capacity of all computers is finite,continuous problems can only be solved exactly by mathematical manipulation. Theavailable mathematical techniques for exact solutions usually limit the possibilities to over-simplified situations.

To overcome the intractability of realistic types of continuous problems (continuum),various methods of discretization have from time to time been proposed by engineers, sci-entists and mathematicians. All involve an approximation which, hopefully, approaches inthe limit the true continuum solution as the number of discrete variables increases.

The discretization of continuous problems has been approached differently by mathemati-cians and engineers. Mathematicians have developed general techniques applicable directlyto differential equations governing the problem, such as finite difference approximations,13

various weighted residual procedures,4, 5 or approximate techniques for determining thestationarity of properly defined functionals.6 The engineer, on the other hand, often ap-proaches the problem more intuitively by creating an analogy between real discrete elementsand finite portions of a continuum domain. For instance, in the field of solid mechanicsMcHenry,7 Hrenikoff,8 Newmark,9 and Southwell2 in the 1940s, showed that reasonablygood solutions to an elastic continuum problem can be obtained by replacing small por-tions of the continuum by an arrangement of simple elastic bars. Later, in the same context,Turner et al.10 showed that a more direct, but no less intuitive, substitution of properties

2 The standard discrete system and origins of the finite element method

can be made much more effectively by considering that small portions or elements in acontinuum behave in a simplified manner.

It is from the engineering direct analogy view that the term finite element was born.Clough11 appears to be the first to use this term, which implies in it a direct use of astandard methodology applicable to discrete systems (see also reference 12 for a historyon early developments). Both conceptually and from the computational viewpoint this isof the utmost importance. The first allows an improved understanding to be obtained; thesecond offers a unified approach to the variety of problems and the development of standardcomputational procedures.

Since the early 1960s much progress has been made, and today the purely mathematicaland direct analogyapproaches are fully reconciled. It is the object of this volume to presenta view of the finite element method as a general discretization procedure of continuumproblems posed by mathematically defined statements.

In the analysis of problems of a discrete nature, a standard methodology has beendeveloped over the years. The civil engineer, dealing with structures, first calculates forcedisplacement relationships for each element of the structure and then proceeds to assemblethe whole by following a well-defined procedure of establishing local equilibrium at eachnode or connecting point of the structure. The resulting equations can be solved for theunknown displacements. Similarly, the electrical or hydraulic engineer, dealing with anetwork of electrical components (resistors, capacitances, etc.) or hydraulic conduits, firstestablishes a relationship between currents (fluxes) and potentials for individual elementsand then proceeds to assemble the system by ensuring continuity of flows.

All such analyses follow a standard pattern which is universally adaptable to discretesystems. It is thus possible to define a standard discrete system, and this chapter will beprimarily concerned with establishing the processes applicable to such systems. Much ofwhat is presented here will be known to engineers, but some reiteration at this stage isadvisable. As the treatment of elastic solid structures has been the most developed areaof activity this will be introduced first, followed by examples from other fields, beforeattempting a complete generalization.

The existence of a unified treatment of standard discrete problems leads us to the firstdefinition of the finite element process as a method of approximation to continuum problemssuch that

(a) the continuum is divided into a finite number of parts (elements), the behaviour ofwhich is specified by a finite number of parameters, and

(b) the solution of the complete system as an assembly of its elements follows preciselythe same rules as those applicable to standard discrete problems.

The development of the standard discrete system can be followed most closely throughthe work done in structural engineering during the nineteenth and twentieth centuries. Itappears that the direct stiffness process was first introduced by Navier in the early partof the nineteenth century and brought to its modern form by Clebsch13 and others. In thetwentieth century much use of this has been made and Southwell,14 Cross15 and others haverevolutionized many aspects of structural engineering by introducing a relaxation iterativeprocess. Just before the Second World War matrices began to play a larger part in castingthe equations and it was convenient to restate the procedures in matrix form. The work ofDuncan and Collar,1618 Argyris,19 Kron20 and Turner10 should be noted. A thorough studyof direct stiffness and related methods was recently conducted by Samuelsson.21

The structural element and the structural system 3

It will be found that most classical mathematical approximation procedures as well asthe various direct approximations used in engineering fall into this category. It is thusdifficult to determine the origins of the finite element method and the precise moment ofits invention.

Table 1.1 shows the process of evolution which led to the present-day concepts of finiteelement analysis. A historical development of the subject of finite element methods hasbeen presented by the first author in references 3436. Chapter 3 will give, in more detail,the mathematical basis which emerged from these classical ideas.1, 2227, 29, 30, 32

1.2 The structural element and the structural system

To introduce the reader to the general concept of discrete systems we shall first consider astructural engineering example with linear elastic behaviour.

Figure 1.1 represents a two-dimensional structure assembled from individual componentsand interconnected at the nodes numbered 1 to 6. The joints at the nodes, in this case, arepinned so that moments cannot be transmitted.

As a starting point it will be assumed that by separate calculation, or for that matterfrom the results of an experiment, the characteristics of each element are precisely known.Thus, if a typical element labelled (1) and associated with nodes 1, 2, 3 is examined, theforces acting at the nodes are uniquely defined by the displacements of these nodes, thedistributed loading acting on the element ( p), and its initial strain. The last may be due totemperature, shrinkage, or simply an initial lack of fit. The forces and the corresponding

Table 1.1 History of approximate methods

ENGINEERING MATHEMATICS

Trialfunctions

Finitedifferences

Rayleigh 187022

Ritz 190823Richardson 19101

Liebman 191824

Southwell 19462

Variationalmethods

Weightedresiduals

Rayleigh 187022

Ritz 190823

Gauss 179525

Galerkin 191526

BiezenoKoch 192327

Structuralanalogue

substitution

Piecewisecontinuous

trial functionsHrenikoff 19418

McHenry 194328

Newmark 19499

Courant 194329

PragerSynge 194730

Argyris 195519

Zienkiewicz 196431

Directcontinuumelements

Variationalfinite

differences

Turner et al. 195610 Varga 196232

Wilkins 196433

PRESENT-DAYFINITE ELEMENT METHOD


y

y

x

x

p

p

Y4

X4

v3(V3)

u3(U3)

1 2

3 4

5 6

3

1

2Nodes(1)

(1)

(2)

(3)

(4)

A typical element (1)

Fig. 1.1 A typical structure built up from interconnected elements.

displacements are defined by appropriate components (U , V and u, v) in a common co-ordinate system (x, y).

Listing the forces acting on all the nodes (three in the case illustrated) of the element (1)as a matrix we have

q1 =

q11q12q13

q

11 =

{U1V1

}, etc. (1.1)

and for the corresponding nodal displacements

u1 =

u11u12u13

u

11 =

{u1v1

}, etc. (1.2)

Assuming linear elastic behaviour of the element, the characteristic relationship willalways be of the form

q1 = K1u1 + f1 (1.3)in which f1 represents the nodal forces required to balance any concentrated or distributedloads acting on the element. The first of the terms represents the forces induced by dis-placement of the nodes. The matrix Ke is known as the stiffness matrix for the element (e).

Equation (1.3) is illustrated by an example of an element with three nodes with theinterconnection points capable of transmitting only two components of force. Clearly, the

A limited knowledge of matrix algebra will be assumed throughout this book. This is necessary for reasonableconciseness and forms a convenient book-keeping form. For readers not familiar with the subject a brief appendix(Appendix A) is included in which sufficient principles of matrix algebra are given to follow the developmentintelligently. Matrices and vectors will be distinguished by bold print throughout.

Assembly and analysis of a structure 5

same arguments and definitions will apply generally. An element (2) of the hypotheticalstructure will possess only two points of interconnection; others may have quite a largenumber of such points. Quite generally, therefore,

qe =

qe1qe2...

qem

and ue =

u1u2...

um

(1.4)

with each qea and ua possessing the same number of components or degrees of freedom.The stiffness matrices of the element will clearly always be square and of the form

Ke =

Ke11 Ke12 Ke1m

Ke21. . .

......

......

Kem1 Kemm

(1.5)

in which Ke11, Ke12, etc., are submatrices which are again square and of the size l l,

where l is the number of force and displacement components to be considered at each node.The element properties were assumed to follow a simple linear relationship. In principle,similar relationships could be established for non-linear materials, but discussion of suchproblems will be postponed at this stage. In most cases considered in this volume theelement matrices Ke will be symmetric.

1.3 Assembly and analysis of a structure

Consider again the hypothetical structure of Fig. 1.1. To obtain a complete solution the twoconditions of

(a) displacement compatibility and(b) equilibrium

have to be satisfied throughout.Any system of nodal displacements u:

u =

u1...

un

(1.6)

listed now for the whole structure in which all the elements participate, automaticallysatisfies the first condition.

As the conditions of overall equilibrium have already been satisfied within an element,all that is necessary is to establish equilibrium conditions at the nodes (or assembly points)of the structure. The resulting equations will contain the displacements as unknowns, andonce these have been solved the structural problem is determined. The internal forces inelements, or the stresses, can easily be found by using the characteristics established apriori for each element.


If now the equilibrium conditions of a typical node, a, are to be established, the sum of thecomponent forces contributed by the elements meeting at the node are simply accumulated.Thus, considering all the force components we have

me=1

qea = q1a + q2a + = 0 (1.7)

in which q1a is the force contributed to node a by element 1, q2a by element 2, etc. Clearly,

only the elements which include point a will contribute non-zero forces, but for concisenessin notation all the elements are included in the summation.

Substituting the forces contributing to node a from the definition (1.3) and noting thatnodal variables ua are common (thus omitting the superscript e), we have

( me=1

Kea1

)u1 +

( me=1

Kea2

)u2 + +

me=1

f ei = 0 (1.8)

The summation again only concerns the elements which contribute to node a. If all suchequations are assembled we have simply

Ku + f = 0 (1.9)

in which the submatrices are

Kab =me=1

Keab and fa =me=1

f ea (1.10)

with summations including all elements. This simple rule for assembly is very convenientbecause as soon as a coefficient for a particular element is found it can be put immediatelyinto the appropriate location specified in the computer. This general assembly processcan be found to be the common and fundamental feature of all finite element calculationsand should be well understood by the reader.

If different types of structural elements are used and are to be coupled it must be remem-bered that at any given node the rules of matrix summation permit this to be done only ifthese are of identical size. The individual submatrices to be added have therefore to be builtup of the same number of individual components of force or displacement.

1.4 The boundary conditions

The system of equations resulting from Eq. (1.9) can be solved once the prescribed supportdisplacements have been substituted. In the example of Fig. 1.1, where both componentsof displacement of nodes 1 and 6 are zero, this will mean the substitution of

u1 = u6 ={

00

}

which is equivalent to reducing the number of equilibrium equations (in this instance 12) bydeleting the first and last pairs and thus reducing the total number of unknown displacement

Electrical and fluid networks 7

components to eight. It is, nevertheless, often convenient to assemble the equation accordingto relation (1.9) so as to include all the nodes.

Clearly, without substitution of a minimum number of prescribed displacements to pre-vent rigid body movements of the structure, it is impossible to solve this system, becausethe displacements cannot be uniquely determined by the forces in such a situation. Thisphysically obvious fact will be interpreted mathematically as the matrix K being singular,i.e., not possessing an inverse. The prescription of appropriate displacements after theassembly stage will permit a unique solution to be obtained by deleting appropriate rowsand columns of the various matrices.

If all the equations of a system are assembled, their form is

K11u1 + K12u2 + + f1 = 0K21u1 + K22u2 + + f2 = 0etc.

(1.11)

and it will be noted that if any displacement, such as u1 = u1, is prescribed then thetotal force f1 cannot be simultaneously specified and remains unknown. The first equa-tion could then be deleted and substitution of known values u1 made in the remainingequations.

When all the boundary conditions are inserted the equations of the system can be solvedfor the unknown nodal displacements and the internal forces in each element obtained.

1.5 Electrical and fluid networks

Identical principles of deriving element characteristics and of assembly will be found inmany non-structural fields. Consider, for instance, the assembly of electrical resistancesshown in Fig. 1.2.

If a typical resistance element, ab, is isolated from the system we can write, by Ohmslaw, the relation between the currents (J ) entering the element at the ends and the endvoltages (V ) as

J ea =1

re(Va Vb) and J eb =

1

re(Vb Va) (1.12)

or in matrix form {J eaJ eb

}= 1re

[1 1

1 1]{Va

Vb

}

which in our standard form is simply

Je = KeVe (1.13)

This form clearly corresponds to the stiffness relationship (1.3); indeed if an externalcurrent were supplied along the length of the element the element force terms could alsobe found.

To assemble the whole network the continuity of the voltage (V ) at the nodes is assumedand a current balance imposed there. With no external input of current at node a we must


a

b

Pa

a

bJb,Vb

Ja ,Va

re

Fig. 1.2 A network of electrical resistances.

have, with complete analogy to Eq. (1.8),

nb=1

me=1

KeabVb = 0 (1.14)

where the second summation is over all elements, and once again for all the nodes

KV = 0 (1.15)

in which

Kab =me=1

Keab

Matrix notation in the latter has been dropped since the quantities such as voltage andcurrent, and hence also the coefficients of the stiffness matrix, are scalars.

If the resistances were replaced by fluid-carrying pipes in which a laminar regime per-tained, an identical formulation would once again result, with V standing for the hydraulichead and J for the flow.

For pipe networks that are usually encountered, however, the linear laws are in generalnot valid and non-linear equations must be solved.

Finally it is perhaps of interest to mention the more general form of an electrical net-work subject to an alternating current. It is customary to write the relationships between

The general pattern 9

the current and voltage in complex arithmetic form with the resistance being replaced bycomplex impedance. Once again the standard forms of (1.13)(1.15) will be obtained butwith each quantity divided into real and imaginary parts.

Identical solution procedures can be used if the equality of the real and imaginary quan-tities is considered at each stage. Indeed with modern digital computers it is possible to usestandard programming practice, making use of facilities available for dealing with complexnumbers. Reference to some problems of this class will be made in the sections dealingwith vibration problems in Chapter 15.

1.6 The general pattern

An example will be considered to consolidate the concepts discussed in this chapter. Thisis shown in Fig. 1.3(a) where five discrete elements are interconnected. These may be ofstructural, electrical, or any other linear type. In the solution:

The first step is the determination of element properties from the geometric material andloading data. For each element the stiffness matrix as well as the corresponding nodal

1 2

3 4 5

67

8

12 3

4 5

12 3 4 5

u++++

+ + + +=

a

{ f}[K ]

=

BAND(c)

(b)

(a)

Fig. 1.3 The general pattern.


loads are found in the form of Eq. (1.3). Each element shown in Fig. 1.3(a) has its ownidentifying number and specified nodal connection. For example:

element 1 connection 1 3 42 1 4 23 2 54 3 6 7 45 4 7 8 5

Assuming that properties are found in global coordinates we can enter each stiffnessor force component in its position of the global matrix as shown in Fig. 1.3(b). Eachshaded square represents a single coefficient or a submatrix of type Kab if more than onequantity is being considered at the nodes. Here the separate contribution of each elementis shown and the reader can verify the position of the coefficients. Note that the varioustypes of elements considered here present no difficulty in specification. (All forces,including nodal ones, are here associated with elements for simplicity.)

The second step is the assembly of the final equations of the type given by Eq. (1.9). Thisis accomplished according to the rule of Eq. (1.10) by simple addition of all numbers inthe appropriate space of the global matrix. The result is shown in Fig. 1.3(c) where thenon-zero coefficients are indicated by shading.

If the matrices are symmetric only the half above the diagonal shown needs, in fact,to be found.

All the non-zero coefficients are confined within a band or profile which can be calcu-lated a priori for the nodal connections. Thus in computer programs only the storage ofthe elements within the profile (or sparse structure) is necessary, as shown in Fig. 1.3(c).Indeed, if K is symmetric only the upper (or lower) half need be stored.

The third step is the insertion of prescribed boundary conditions into the final assembledmatrix, as discussed in Sec. 1.3. This is followed by the final step.

The final step solves the resulting equation system. Here many different methods canbe employed, some of which are summarized in Appendix C. The general subject ofequation solving, though extremely important, is in general beyond the scope of thisbook.

The final step discussed above can be followed by substitution to obtain stresses, currents,or other desired output quantities. All operations involved in structural or other networkanalysis are thus of an extremely simple and repetitive kind. We can now define the standarddiscrete system as one in which such conditions prevail.

1.7 The standard discrete system

In the standard discrete system, whether it is structural or of any other kind, we find that:

1. A set of discrete parameters, say ua , can be identified which describes simultaneouslythe behaviour of each element, e, and of the whole system. We shall call these the systemparameters.

Transformation of coordinates 11

2. For each element a set of quantities qea can be computed in terms of the system parametersua . The general function relationship can be non-linear, for example

qea = qea(u) (1.16)but in many cases a linear form exists giving

qea = Kea1u1 + Kea2u2 + + f ea (1.17)3. The final system equations are obtained by a simple addition

ra =me=1

qea = 0 (1.18)

where ra are system quantities (often prescribed as zero). In the linear case this resultsin a system of equations

Ku + f = 0 = 0 (1.19)such that

Kab =me=1

Keab and fa =me=1

f ea (1.20)

from which the solution for the system variables u can be found after imposing necessaryboundary conditions.

The reader will observe that this definition includes the structural, hydraulic, and elec-trical examples already discussed. However, it is broader. In general neither linearity norsymmetry of matrices need exist although in many problems this will arise naturally.Further, the narrowness of interconnections existing in usual elements is not essential.

While much further detail could be discussed (we refer the reader to specific books formore exhaustive studies in the structural context37, 38), we feel that the general expose givenhere should suffice for further study of this book.

Only one further matter relating to the change of discrete parameters need be mentionedhere. The process of so-called transformation of coordinates is vital in many contexts andmust be fully understood.

1.8 Transformation of coordinates

It is often convenient to establish the characteristics of an individual element in a coordinatesystem which is different from that in which the external forces and displacements of theassembled structure or system will be measured. A different coordinate system may, in fact,be used for every element, to ease the computation. It is a simple matter to transform thecoordinates of the displacement and force components of Eq. (1.3) to any other coordinatesystem. Clearly, it is necessary to do so before an assembly of the structure can be attempted.

Let the local coordinate system in which the element properties have been evaluated bedenoted by a prime suffix and the common coordinate system necessary for assembly haveno embellishment. The displacement components can be transformed by a suitable matrixof direction cosines L as

u = Lu (1.21)


As the corresponding force components must perform the same amount of work in eithersystem

qTu = qTu (1.22)On inserting (1.21) we have

qTu = qTLuor

q = LTq (1.23)The set of transformations given by (1.21) and (1.23) is called contravariant.

To transform stiffnesseswhich may be available in local coordinates to global ones notethat if we write

q = Ku (1.24)then by (1.23), (1.24), and (1.21)

q = LTKLuor in global coordinates

K = LTKL (1.25)In many complex problems an external constraint of some kind may be imagined, en-

forcing the requirement (1.21) with the number of degrees of freedom of u and u beingquite different. Even in such instances the relations (1.22) and (1.23) continue to be valid.

An alternative and more general argument can be applied to many other situations ofdiscrete analysis. We wish to replace a set of parameters u in which the system equationshave been written by another one related to it by a transformation matrix T as

u = Tv (1.26)

In the linear case the system equations are of the form

Ku = f (1.27)

and on the substitution we haveKTv = f (1.28)

The new system can be premultiplied simply by TT, yielding

(TTKT)v = TT TTf (1.29)

which will preserve the symmetry of equations if the matrix K is symmetric. However,occasionally the matrix T is not square and expression (1.26) represents in fact an approx-imation in which a larger number of parameters u is constrained. Clearly the system ofequations (1.28) gives more equations than are necessary for a solution of the reduced setof parameters v, and the final expression (1.29) presents a reduced system which in somesense approximates the original one.

We have thus introduced the basic idea of approximation, which will be the subject ofsubsequent chapters where infinite sets of quantities are reduced to finite sets.

With ( )T standing for the transpose of the matrix.

Problems 13

1.9 Problems

1.1 A simple fluid network to transport water is shown in Fig. 1.4. Each element of thenetwork is modelled in terms of the flow, J, and head, V, which are approximated bythe linear relation

Je = KeVewhere Ke is the coefficient array for element (e). The individual terms in the flowvector denote the total amount of flow entering (+) or leaving () each end point. Theproperties of the elements are given by

Ke = ce 3 2 12 4 2

1 2 3

for elements 1 and 4, and for elements 2 and 3 by

Ke = ce[

1 11 1

]

where ce is an element related parameter. The system is operating with a known headof 100 m at node 1 and 30 m at node 6. At node 2, 30 cubic metres of water per hourare being used and at node 4, 10 cubic metres per hour.(a) For all ce = 1, assemble the total matrix from the individual elements to give

J = K VN.B. J contains entries for the specified usage and connection points.

(b) Impose boundary conditions by modifying J and K such that the known heads atnodes 1 and 6 are recovered.

(c) Solve the equations for the heads at nodes 2 to 5. (Result at node 4 should beV4 = 30.8133 m.)

(d) Determine the flow entering and leaving each element.1.2 A plane truss may be described as a standard discrete problem by expressing the char-

acteristics for each member in terms of end displacements and forces. The behaviourof the elastic member shown in Fig. 1.5 with modulus E, cross-section A and length Lis given by

q = Ke u

1

2

4

6

(1)

(2)

(3)

(4)

5

3

Fig. 1.4 Fluid network for Problem 1.1.


where

q ={U 1U 2

}; u =

{u1u2

}and Ke =

EA

L

[1 1

1 1]

To obtain the final assembled matrices for a standard discrete problem it is necessaryto transform the behaviour to a global frame using Eqs 1.23 and 1.25 where

L =[

cos sin 0 00 0 cos sin

]; q =

U1V1U2V2

and u =

u1v1u2v2

(a) Compute relations for q and K in terms of L, q and Ke.(b) If the numbering for the end nodes is reversed what is the final form for K compared

to that given in (a)? Verify your answer when = 30o.1.3 A plane truss has nodes numbered as shown in Fig. 1.6(a).

(a) Use the procedure shown in Fig. 1.3 to define the non-zero structure of the coefficientmatrix K. Compute the maximum bandwidth.

(b) Determine the non-zero structure of K for the numbering of nodes shown in 1.6(b).Compute the maximum bandwidth.

Which order produces the smallest band?1.4 Write a small computer program (e.g., using MATLAB39) to solve the truss problem

shown in Fig. 1.6(b). Let the total span of the truss be 2.5 m and the height 0.8 m and usesteel as the property for each member with E = 200 GPa and A = 0.001 m2. Restrainnode 1 in both the u and v directions and the right bottom node in the v direction only.Apply a vertical load of 100 N at the position of node 6 shown in Fig. 1.6(b). Determinethe maximum vertical displacement at any node. Plot the undeformed and deformedposition of the truss (increase the magnitude of displacements to make the shape visibleon the plot).

You can verify your result using the program FEAPpv available at the publishersweb site (see Chapter 18).

1.5 An axially loaded elastic bar has a variable cross-section and lengths as shown inFig. 1.7(a). The problem is converted into a standard discrete system by consideringeach prismatic section as a separate member. The array for each member segment isgiven as

qe = Keue

x (u,U )

y (v,V ) x 9

y 9

u 92

u19

1

2

u(b) Displacements(a) Truss member description

q

vu9

Fig. 1.5 Truss member for Problem 1.2.

Problems 15

where

Ke = EAeh

[1 1

1 1]

qe ={qeeqee+1

}and ue =

{ueue+1

}

Equilibrium for the standard discrete problem at joint e is obtained by combining resultsfrom segment e 1 and e as

qe1e + qee + Ue = 0

where Ue is any external force applied to a joint. Boundary conditions are applied forany joint at which the value of ue is known a priori.

Solve the problem shown in Fig. 1.7(b) for the joint displacements using the dataE1 = E2 = E3 = 200 GPa,A1 = 25 cm2,A2 = 20 cm2,A3 = 12 cm2,L1 = 37.5 cm,L2 = 25.0 cm, L3 = 12.5 cm, P2 = 10 kN, P3 = 3.5 kN and P4 = 6 kN.

1.6 Solve Problem 1.5 for the boundary conditions and loading shown in Fig. 1.7(c). LetE1 = E2 = E3 = 200 GPa,A1 = 30 cm2,A2 = 20 cm2,A3 = 10 cm2,L1 = 37.5 cm,L2 = 30.0 cm, L3 = 25.0 cm, P2 = 10 kN and P3 = 3.5 kN.

1.7 A tapered bar is loaded by an end loadP and a uniform loading b as shown in Fig. 1.8(a).The area varies as A(x) = Ax/L when the origin of coordinates is located as shownin the figure.

The problem is converted into a standard discrete system by dividing it into equallength segments of constant area as shown in Fig. 1.8(b). The array for each segmentis determined from

qe = Keue + f e

1 6

10

2 3 4 5

7 8 9

1 10

9

2 4 6

(a)

(b)

8

3 5 7

Fig. 1.6 Truss for Problems 1.3 and 1.4.


u1 u2 u3 u4e=1 e=2 e=3

L1 L2 L3

E1,A1 E2,A2 E3,A3

u1=0 P2 P3 P4

u1=0 P2 P3 u4=0

(a) Bar geometry

(b) Problem 1.5

(c) Problem 1.6

Fig. 1.7 Elastic bars. Problems 1.5 and 1.6.

x

y

L

2AP b

L

A

u1 u2 u3 u4 u5=0

h=L /4

e=1 e=2 e=3 e=4

(a) Tapered bar geometry (b) Approximation by 4 segments

Fig. 1.8 Tapered bar. Problem 1.7.

where Ke and ue are defined in Problem 1.5 and

f e = 12 b h{

11

}

For the properties L = 100 cm, A = 2 cm2, E = 104 kN/cm2, P = 2 kN, b =0.25 kN/cm and u(2L) = 0, the displacement from the solution of the differentialequation is u(L) = 0.03142513 cm.

Write a small computer program (e.g., using MATLAB39) that solves the problemfor the case where e = 1, 2, 4, 8, segments. Continue the solution until the absoluteerror in the tip displacement is less than 105 cm (let error be E = |u(L) u1| whereu1 is the numerical solution at the end).

References 17

References

1. L.F. Richardson. The approximate arithmetical solution by finite differences of physical prob-lems. Trans. Roy. Soc. (London), A210:307357, 1910.

2. R.V. Southwell. Relaxation Methods in Theoretical Physics. Clarendon Press, Oxford, 1stedition, 1946.

3. D.N. de G. Allen. Relaxation Methods. McGraw-Hill, London, 1955.4. S. Crandall. Engineering Analysis. McGraw-Hill, New York, 1956.5. B.A. Finlayson. The Method of Weighted Residuals and Variational Principles. Academic Press,

New York, 1972.6. K. Washizu. Variational Methods in Elasticity and Plasticity. Pergamon Press, New York, 3rd

edition, 1982.7. D. McHenry. A lattice analogy for the solution of plane stress problems. J. Inst. Civ. Eng.,

21:5982, 1943.8. A. Hrenikoff. Solution of problems in elasticity by the framework method. J. Appl. Mech.,

ASME, A8:169175, 1941.9. N.M. Newmark. Numerical methods of analysis in bars, plates and elastic bodies. In L.E.

Grinter, editor, Numerical Methods in Analysis in Engineering. Macmillan, New York, 1949.10. M.J. Turner, R.W. Clough, H.C. Martin, and L.J. Topp. Stiffness and deflection analysis of

complex structures. J. Aero. Sci., 23:805823, 1956.11. R.W. Clough. The finite element method in plane stress analysis. In Proc. 2nd ASCE Conf. on

Electronic Computation, Pittsburgh, Pa., Sept. 1960.12. R.W. Clough. Early history of the finite element method from the view point of a pioneer. Int.

J. Numer. Meth. Eng., 60:283287, 2004.13. R.F. Clebsch. Theorie de lelasticite des corps solides. Dunod, Paris, 1883.14. R.V. Southwell. Stress calculation in frame works by the method of systematic relaxation of

constraints, Part I & II. Proc. Roy. Soc. London (A), 151:5695, 1935.15. Hardy Cross. Continuous Frames of Reinforced Concrete. John Wiley & Sons, NewYork, 1932.16. W.J. Duncan and A.R. Collar. A method for the solution of oscillation problems by matrices.

Phil. Mag., 17:865, 1934. Series 7.17. W.J. Duncan and A.R. Collar. Matrices applied to the motions of damped systems. Phil. Mag.,

19:197, 1935. Series 7.18. R.R. Frazer, W.J. Duncan, and A.R. Collar. Elementary Matrices. Cambridge University Press,

London, 1960.19. J.H. Argyris and S. Kelsey. Energy Theorems and Structural Analysis. Butterworths, London,

1960. Reprinted from a series of articles in Aircraft Eng., 195455.20. G. Kron. Equivalent Circuits of Electrical Machinery. John Wiley & Sons, New York, 1951.21. A. Samuelsson. Personal communication, 2003.22. Lord Rayleigh (J.W. Strutt). On the theory of resonance. Trans. Roy. Soc. (London),

A161:77118, 1870.23. W. Ritz. Uber eine neue Methode zur Losung gewisser variationsproblem der mathematischen

physik. Journal fur die reine und angewandte Mathematik, 135:161, 1908.24. H. Liebman. Die angenaherte Ermittlung: harmonishen, functionen und konformer Abbildung.

Sitzber. Math. Physik Kl. Bayer Akad. Wiss. Munchen, 3:6575, 1918.25. C.F. Gauss. Werke. Dietrich, Gottingen, 18631929. See: Theoretishe Astronomie, Bd. VII.26. B.G. Galerkin. Series solution of some problems in elastic equilibrium of rods and plates. Vestn.

Inzh. Tech., 19:897908, 1915.27. C.B. Biezeno and J.J. Koch. Over een Nieuwe Methode ter Berekening van Vlokke Platen. Ing.

Grav., 38:2536, 1923.28. D. McHenry. A new aspect of creep in concrete and its application to design. Proc. ASTM,

43:1064, 1943.


29. R. Courant. Variational methods for the solution of problems of equilibrium and vibration. Bull.Am. Math Soc., 49:161, 1943.

30. W. Prager and J.L. Synge. Approximation in elasticity based on the concept of function space.Quart. J. Appl. Math, 5:241269, 1947.

31. O.C. Zienkiewicz and Y.K. Cheung. The finite element method for analysis of elastic isotropicand orthotropic slabs. Proc. Inst. Civ. Eng., 28:471488, 1964.

32. R.S. Varga. Matrix Iterative Analysis. Prentice-Hall, Englewood Cliffs, N.J., 1962.33. M.L. Wilkins. Calculation of elastic-plastic flow. In B. Alder, editor, Methods in Computational

Physics, volume 3, pages 211263. Academic Press, New York, 1964.34. O.C. Zienkiewicz. Origins, milestones and directions of the finite element method. Arch. Comp.

Meth. Eng., 2:148, 1995.35. O.C. Zienkiewicz. Origins, milestones and directions of the finite element method. A personal

view. In P.G. Ciarlet and J.L Lyons, editors, Handbook of Numerical Analysis, volume IV, pages365. North Holland, 1996.

36. O.C. Zienkiewicz. The birth of the finite element method and of computational mechanics. Int.J. Numer. Meth. Eng., 60:310, 2004.

37. J.S. Przemieniecki. Theory of Matrix Structural Analysis. McGraw-Hill, New York, 1968.38. R.K. Livesley. Matrix Methods in Structural Analysis. Pergamon Press, New York, 2nd edition,

1975.39. MATLAB. www.mathworks.com, 2003.

2

A direct physical approach toproblems in elasticity: plane stress

2.1 Introduction

The process of approximating the behaviour of a continuum by finite elements whichbehave in a manner similar to the real, discrete, elements described in the previous chaptercan be introduced through the medium of particular physical applications or as a generalmathematical concept. We have chosen here to follow the first path, narrowing our view toa set of problems associated with structural mechanics which historically were the first towhich the finite element method was applied. In Chapter 3 we shall generalize the conceptsand show that the basic ideas are widely applicable.

In many phases of engineering the solution of stress and strain distributions in elasticcontinua is required. Special cases of such problems may range from two-dimensionalplane stress or strain distributions, axisymmetric solids, plate bending, and shells, to fullythree-dimensional solids. In all cases the number of interconnections between any finiteelement isolated by some imaginary boundaries and the neighbouring elements is con-tinuous and therefore infinite. It is difficult to see at first glance how such problems maybe discretized in the same manner as was described in the preceding chapter for simplersystems. The difficulty can be overcome (and the approximation made) in the followingmanner:

1. The continuum is separated by imaginary lines or surfaces into a number of finiteelements.

2. The elements are assumed to be interconnected at a discrete number of nodal pointssituated on their boundaries and occasionally in their interior. The displacements ofthese nodal points will be the basic unknown parameters of the problem, just as insimple, discrete, structural analysis.

3. A set of functions is chosen to define uniquely the state of displacement within eachfinite element and on its boundaries in terms of its nodal displacements.

4. The displacement functions now define uniquely the state of strain within an element interms of the nodal displacements. These strains, together with any initial strains and theconstitutive properties of the material, define the state of stress throughout the elementand, hence, also on its boundaries.

5. A system of equivalent forcesconcentrated at the nodes and equilibrating the boundarystresses and any distributed loads is determined, resulting in a stiffness relationship

20 A direct physical approach to problems in elasticity: plane stress

of the form of Eq. (1.3). The determination of these equivalent forces is done mostconveniently and generally using the principle of virtual work which is a particularmathematical relation known as a weak form of the problem.

Once this stage has been reached the solution procedure can follow the standard discretesystem pattern described in Chapter 1.

Clearly a series of approximations has been introduced. First, it is not always easy toensure that the chosen displacement functions will satisfy the requirement of displacementcontinuity between adjacent elements. Thus, the compatibility condition on such lines maybe violated (though within each element it is obviously satisfied due to the uniqueness ofdisplacements implied in their continuous representation). Second, by concentrating theequivalent forces at the nodes, equilibrium conditions are satisfied in the overall sense only.Local violation of equilibrium conditions within each element and on its boundaries willusually arise.

The choice of element shape and of the form of the displacement function for specificcases leaves many opportunities for the ingenuity and skill of the analyst to be employed,and obviously the degree of approximation which can be achieved will strongly depend onthese factors.

The approach outlined here is known as the displacement formulation.1, 2

The use of the principle of virtual work (weak form) is extremely convenient and pow-erful. Here it has only been justified intuitively though in the next chapter we shall see itsmathematical origins. However, we will also show the determination of these equivalentforces can be done by minimizing the total potential energy. This is applicable to situa-tions where elasticity predominates and the behaviour is reversible. While the virtual workform is always valid, the principle of minimum potential energy is not and care has to betaken. The recognition of the equivalence of the finite element method to a minimizationprocess was late.2, 3 However, Courant4 in 1943 and Prager and Synge5 in 1947 proposedminimizing methods that are in essence identical.

This broader basis of the finite element method allows it to be extended to other con-tinuum problems where a variational formulation is possible. Indeed, general proceduresare now available for a finite element discretization of any problem defined by a properlyconstituted set of differential equations. Such generalizations will be discussed in Chapter 3,and throughout the book application to structural and some non-structural problems willbe made. It will be found that the process described in this chapter is essentially an ap-plication of trial-function and Galerkin-type approximations to the particular case of solidmechanics.

2.2 Direct formulation of finite element characteristics

The prescriptions for deriving the characteristics of a finite element of a continuum,which were outlined in general terms, will now be presented in more detailed mathematicalform.

It appears that Courant had anticipated the essence of the finite element method in general, and of a triangularelement in particular, as early as 1923 in a paper entitled On a convergence principle in the calculus of variations.Kon. Gesellschaft der Wissenschaften zu Gottingen, Nachrichten, Berlin, 1923. He states: We imagine a meshof triangles covering the domain . . . the convergence principles remain valid for each triangular domain.

Direct formulation of finite element characteristics 21

It is desirable to obtain results in a general form applicable to any situation, but toavoid introducing conceptual difficulties the general relations will be illustrated with a verysimple example of plane stress analysis of a thin slice. In this a division of the region intotriangular-shaped elements may be used as shown in Fig. 2.1. Alternatively, regions maybe divided into rectangles or, indeed using a combination of triangles and rectangles. Inlater chapters we will show how many other shapes also may be used to define elements.

2.2.1 Displacement function

A typical finite element, e, with a triangular shape is defined by local nodes 1, 2 and 3, andstraight line boundaries between the nodes as shown in Fig. 2.2(a). Similarly, a rectangularelement could be defined by local nodes 1, 2, 3 and 4 as shown in Fig. 2.2(b). The choiceof displacement functions for each element is of paramount importance and in Chapters 4and 5 we will show how they may be developed for a wide range of types; however, inthe rest of this chapter we will consider only the 3-node triangular and 4-node rectangularelement shapes.

Let the displacements u at any point within the element be approximated as a columnvector, u:

u u =a

Nauea =[N1, N2, . . .

]

u1u2...

e

= Nue (2.1)

y

x

3

1

2

e

t =txty

va(Va)

ua(Ua)a

Fig. 2.1 A plane stress region divided into finite elements.


x

y

1

12

3Ni

x

y

Ni

1

1

34

2

(a) 3-node triangle (b) 4-node rectangle

Fig. 2.2 Shape function N3 for one element.

In the case of plane stress, for instance,

u ={u(x, y)

v(x, y)

}

represents horizontal and vertical movements (see Fig. 2.1) of a typical point within theelement and

ua ={uava

}

the corresponding displacements of a node a.The functions Na, a = 1, 2, . . . are called shape functions (or basis functions, and,

occasionally interpolation functions) and must be chosen to give appropriate nodal dis-placements when coordinates of the corresponding nodes are inserted in Eq. (2.1). Clearlyin general we have

Na(xa, ya) = I (identity matrix)while

Na(xb, yb) = 0, a = bIf both components of displacement are specified in an identical manner then we can write

Na = Na I (2.2)and obtain Na from Eq. (2.1) by noting that Na(xa, ya) = 1 but is zero at other vertices.The shape functions N will be seen later to play a paramount role in finite element analysis.

Triangle with 3 nodesThe most obvious linear function in the case of a triangle will yield the shape of Na of theform shown in Fig. 2.2(a). Writing, the two displacements as

u = 1 + 2 x + 3 yv = 4 + 5 x + 6 y

(2.3)

we may evaluate the six constants by solving two sets of three simultaneous equations whicharise if the nodal coordinates are inserted and the displacements equated to the appropriatenodal values. For example, the u displacement gives

u1 = 1 + 2 x1 + 3 y1u2 = 1 + 2 x2 + 3 y2u3 = 1 + 2 x3 + 3 y3

(2.4)


We can easily solve for 1, 2 and 3 in terms of the nodal displacements u1, u2 and u3 andobtain finally

u = 12

[(a1 + b1x + c1y) u1 + (a2 + b2x + c2y) u2 + (a3 + b3x + c3y) u3] (2.5)

in which

a1 = x2y3 x3y2b1 = y2 y3c1 = x3 x2

(2.6)

with other coefficients obtained by cyclic permutation of the subscripts in the order 1, 2, 3,and where

2 = det1 x1 y11 x2 y21 x3 y3

= 2 (area of triangle 123) (2.7)

From (2.5) we see that the shape functions are given by

Na = (aa + ba x + ca y)/(2); a = 1, 2, 3 (2.8)

Since displacements with these shape functions vary linearly along any side of a trianglethe interpolation (2.5) guarantees continuity between adjacent elements and, with identicalnodal displacements imposed, the same displacement will clearly exist along an interfacebetween elements. We note, however, that in general the derivatives will not be continuousbetween elements.

Rectangle with 4 nodesAn alternative subdivision can use rectangles of the form shown in Fig. 2.3. The rectangularelement has side lengths of a and b in the x and y directions, respectively. For the derivation

1

34

x

y

x9

y9

a

b

2

Fig. 2.3 Rectangular element geometry and local node numbers.


of the shape functions it is convenient to use a local cartesian system x , y defined by

x = x x1y = y y1

We now need four functions for each displacement component in order to uniquely definethe shape functions. In addition these functions must have linear behaviour along eachedge of the element to ensure interelement continuity. A suitable choice is given by

u = 1 + x 2 + y 3 + x y 4v = 5 + x 6 + y 7 + x y 8

(2.9)

The coefficients a may be obtained by expressing (2.9) at each node, giving for u

u1 = 1u2 = 1 + a 2u3 = 1 + a 2 + b 3 + ab 4u4 = 1 + b 3

(2.10)

We can again easily solve for a in terms of the nodal displacements to obtain finally

u = 1ab

[(a x )(b y ) u1 + x (b y ) u2 + x y u3 + (a x ) y u4] (2.11)

An identical expression is obtained for v by replacing ua by va .From (2.11) we obtain the shape functions

N1 = (a x )(b y )/(ab)N2 = x (b y )/(ab)N3 = x y /(ab)N4 = (a x ) y /(ab)

(2.12)

2.2.2 Strains

With displacements known at all points within the element the strains at any point canbe determined. These will always result in a relationship that can be written in matrixnotation as

= Su (2.13)where S is a suitable linear differential operator. Using Eq. (2.1), the above equation canbe approximated by

= Bue (2.14)with

B = SN (2.15) It is known that strain is a second rank tensor by its transformation properties; however, in this book wewill normally represent quantities using matrix (Voigt) notation. The interested reader is encouraged to consultAppendix B for the relations between tensor forms and the matrix quantities.


For the plane stress case the relevant strains of interest are those occurring in the planeand are defined in terms of the displacements by well-known relations6 which define theoperator S

=xyxy

=

u

x

v

y

u

y+ vx

=

x, 0

0,

y

y,

x

{u

v

}

With the shape functions N1, N2 and N3 already determined for a triangular element, thematrix B can easily be obtained using (2.15). If the linear form of the shape functions isadopted then, in fact, the strains are constant throughout the element (i.e., the B matrix isconstant).

A similar result may be obtained for the rectangular element by adding the results forN4; however, in this case the strains are not constant but have linear terms in x and y.

2.2.3 Stresses

In general, the material within the element boundaries may be subjected to initial strainssuch as those due to temperature changes, shrinkage, crystal growth, and so on. If suchstrains are denoted by 0 then the stresses will be caused by the difference between theactual and initial strains.

In addition it is convenient to assume that at the outset of the analysis the body isstressed by some known system of initial residual stresses 0 which, for instance, couldbe measured, but the prediction of which is impossible without the full knowledge of thematerials history. These stresses can simply be added on to the general definition. Thus,assuming linear elastic behaviour, the relationship between stresses and strains will belinear and of the form

= D( 0)+ 0 (2.16)where D is an elasticity matrix containing the appropriate material properties.

Again for the particular case of plane stress three components of stress corresponding tothe strains already defined have to be considered. These are, in familiar notation,

=xyxy

and for an isotropic material the D matrix may be simply obtained from the usual stressstrain relationship6

x x0 = 1E(x x0)

E(y y0)

y y0 = E(x x0)+ 1

E(y y0)

xy xy0 = 2(1 + )E

(xy xy0)


i.e., on solving,

D = E1 2

1 0 1 0

0 0 (1 )/2

2.2.4 Equivalent nodal forces

Let

qe =

qe1qe2...

define the nodal forces which are statically equivalent to the boundary stresses and dis-tributed body forces acting on the element. Each of the forces qea must contain the samenumber of components as the corresponding nodal displacement ua and be ordered in theappropriate, corresponding directions.

The distributed body forces b are defined as those acting on a unit volume of materialwithin the element with directions corresponding to those of the displacements u at thatpoint.

In the particular case of plane stress the nodal forces are, for instance,

qea ={UeaV ea

}

with components U and V corresponding to the directions of u and v, respectively (viz.Fig. 2.1), and the distributed body forces are

b ={bxby

}

in which bx and by are the body force components per unit of volume.In the absence of body forces equivalent nodal forces for the 3-node triangular element can

be computed directly from equilibrium considerations. In Fig. 2.4(a) we show a triangularelement together with the geometric properties which are obtained by the linear interpolationof the displacements using (2.1) to (2.8). In particular we note from the figure [and (2.6)]that

b1 + b2 + b3 = 0 and c1 + c2 + c3 = 0The stresses in the element are given by (2.16) in which we assume that 0 and 0 are

constant in each element and strains are computed from (2.14) and, for the 3-node triangularelement, are also constant in each element. To determine the nodal forces resulting fromthe stresses, the boundary tractions are first computed from

t ={txty

}= t

[nx 0 ny0 ny nx

] xyxy

(2.17)

where t is a constant thickness of the plane strain slice and nx , ny are the direction cosinesof the outward normal to the element boundary. For the triangular element the tractions


1

2

3

b1

b2

b3

c1c2

c3

1

2

3

sx

txy

txy

sy

sx

sy(a) Triangle and geometry (b) Uniform stress state

Fig. 2.4 3-node triangle, geometry and constant stress state.

are constant. The resultant for each side of the triangle is the product of the triangle sidelength (la) times the traction. Here la is the length of the side opposite the triangle node aand we note from Fig. 2.4(a) that

la nx = ba and la ny = ca (2.18)

Therefore,

lat ={latxlaty

}= t

[ba 0 ca0 ca ba

] xyxy

The resultant acts at the middle of each side of the triangle and, thus, by sum of forces andmoments is equivalent to placing half at each end node. Thus, by static equivalence thenodal forces at node 1 are given by

q1 = t2

([b2 0 c20 c2 b2

]+[b3 0 c3

0 c3 b3])

= t2

[b1 0 c10 c1 b1

] = BT1 t

(2.19a)

Similarly, the forces at nodes 2 and 3 are given by

q2 = B2 tq3 = B3 t

(2.19b)

Combining with the expression for stress and strain for each element we obtain

q = BT [D (Bue 0)+ 0] t= Keue + f e (2.20a)


whereKe = BTD B t and f e = BT(0 D 0) t (2.20b)

The above gives a result which is now in the form of the standard discrete problemdefined in Sec. 1.2. However, when body forces are present or we consider other elementforms the above procedure fails and we need a more general approach. To make the nodalforces statically equivalent to the actual boundary stresses and distributed body forces, thesimplest general procedure is to impose an arbitrary (virtual) nodal displacement and toequate the external and internal work done by the various forces and stresses during thatdisplacement.

Let such a virtual displacement be ue at the nodes. This results, by Eqs (2.1) and (2.14),in virtual displacements and strains within the element equal to

u = N ue and = B ue (2.21)respectively.

The external work done by the nodal forces is equal to the sum of the products of theindividual force components and corresponding displacements, i.e., in matrix form

ueT1 qe1 + ueT2 qe2 . . . = ueTqe (2.22)

Similarly, the internal work per unit volume done by the stresses and distributed body forcessubjected to a set of virtual strains and displacements is

T uTb (2.23)or, after using (2.21),

ueT(BT NTb) (2.24)

Equating the external work with the total internal work obtained by integrating (2.24)over the volume of the element, e, we have

ueTqe = ueT(

e

BT d

e

NTb d

)

(2.25)

As this relation is valid for any value of the virtual displacement, the multipliers must beequal. Thus

qe =

e

BT d

e

NTb d (2.26)

This statement is valid quite generally for any stressstrain relation. With (2.14) and thelinear law of Eq. (2.16) we can write Eq. (2.26) as

qe = Keue + f e (2.27)where

Ke =

e

BTD B d (2.28a)

andf e =

e

NTb d

e

BTD 0 d+

e

BT0 d (2.28b)

Note that by the rules of matrix algebra for the transpose of products (A B)T = BTAT.


For the plane stress problem

e

() d =Ae

() t dA

where Ae is the area of the element. Here t now can be allowed to vary over the element.In the last equation the three terms represent forces due to body forces, initial strain, andinitial stress respectively. The relations have the characteristics of the discrete structuralelements described in Chapter 1.

If the initial stress system is self-equilibrating, as must be the case with normal residualstresses, then the forces given by the initial stress term of Eq. (2.28b) are identically zeroafter assembly. Thus frequent evaluation of this force component is omitted. However, iffor instance a machine part is manufactured out of a block in which residual stresses arepresent or if an excavation is made in rock where known tectonic stresses exist a removalof material will cause a force imbalance which results from the above term.

For the particular example of the plane stress triangular element these characteristicswill be obtained by appropriate substitution. It has already been noted that the B matrixin that example was not dependent on the coordinates; hence the integration will becomeparticularly simple and, in the absence of body forces, Ke and f e are identical to those givenin (2.20b).

The interconnection and solution of the whole assembly of elements follows the simplestructural procedures outlined in Chapter 1. This gives

r =e

qe = 0 (2.29)

A note should be added here concerning elements near the boundary. If, at the boundary,displacements are specified, no special problem arises as these can be satisfied by specifyingsome of the nodal parameters u. Consider, however, the boundary as subject to a distributedexternal loading, say t per unit area (traction). A loading term on the nodes of the elementwhich has a boundary facee will now have to be added. By the virtual work consideration,this will simply result in

f e f e e

NTt d (2.30)

with integration taken over the boundary area of the element. It will be noted that t musthave the same number of components as u for the above expression to be valid. Such aboundary element is shown again for the special case of plane stress in Fig. 2.1.

Once the nodal displacements have been determined by solution of the overall structural-type equations, the stresses at any point of the element can be found from the relations inEqs (2.14) and (2.16), giving

= D (Bue 0)+ 0 (2.31)

Example 2.1: Stiffness matrix for 3-node triangle. The stiffness matrix for an individualelement is computed by evaluating Eq. (2.28a). For a 3-node triangle in which the moduliand thickness are constant over the element the solution for the stiffness becomes

Ke = BT D B t (2.32)


where is the area of the triangle computed from (2.7). Evaluating (2.15) using the shapefunctions in (2.8) gives

Ba = 12

ba 00 caca ba

(2.33)

Thus, the expression for the stiffness of the triangular element is given by

Kab = t4

[ba 0 ca0 ca ba

]D11 D12 D13D21 D22 D23D31 D32 D33

bb 00 cbcb bb

(2.34)

where Dij = Dji are the elastic moduli.

Example 2.2: Nodal forces for boundary traction. Let us consider a problem in whicha traction boundary condition is to be imposed along a vertical surface located at x = xb.A triangular element has one of its edges located along the boundary as shown in Fig. 2.5and is loaded by a specified traction given by

t ={txty

}= t

{xxy

}

The normal stress x is given by a linearly varying stress in the y direction and the shearingstress xy is assumed zero, thus, to compute nodal forces we use the expressions

x = kxy and xy = 0in which kx is a specified constant.

Along the boundary the shape functions for either a triangular element or a rectangularelement are linear functions in y and are given by

N1 = (y2 y)/(y2 y1) and N2 = (y y1)/(y2 y1)

(xb,y1)

(xb,y2)

(tx,ty)

Fig. 2.5 Traction on vertical face.

Generalization to the whole region internal nodal force concept abandoned 31

thus, the nodal forces for the element shown are computed from Eq. (2.30) and given by

f1 = y2y1

t N1

{xxy

}dy =

{kx t (2y1 + y2)(y2 y1)/6

0

}

and

f2 = y2y1

tN2

{xxy

}dy =

{kx t (y1 + 2y2)(y2 y1)/6

0

}

2.3 Generalization to the whole region internal nodalforce concept abandoned

In the preceding section the virtual work principle was applied to a single element and theconcept of equivalent nodal force was retained. The assembly principle thus followed theconventional, direct equilibrium, approach.

The idea of nodal forces contributed by elements replacing the continuous interaction ofstresses between elements presents a conceptual difficulty. However, it has a considerableappeal to practical engineers and does at times allow an interpretation which otherwisewould not be obvious to the more rigorous mathematician. There is, however, no needto consider each element individually and the reasoning of the previous section may beapplied directly to the whole continuum.

Equation (2.1) can be interpreted as applying to the whole structure, that is,

u = N u and u = N u (2.35)in which u and u list all the nodal points and

Na =e

Nea (2.36)

when the point concerned is within a particular element e and a is a node point associatedwith the element. If a point does not occur within the element (see Fig. 2.6)

Na = 0 (2.37)A matrix B can be similarly defined and we shall drop the bar, considering simply that

the shape functions, etc., are always defined over the whole domain, .For any virtual displacement u we can now write the sum of internal and external work

for the whole region as

uTr =

T d

uT b d

uT t d = 0 (2.38)

In the above equation, u, u and can be completely arbitrary, providing they stemfrom a continuous displacement assumption. If for convenience we assume they are simplyvariations linked by relations (2.35) and (2.14) we obtain, on substitution of the constitutiverelation (2.16), a system of algebraic equations

K u + f = 0 (2.39)


y

x

a

Na

Fig. 2.6 Shape function Na for whole domain.

where

K =

BTD B d (2.40a)

and

f =

NTb d

NT t d +

BT(0 D 0) d (2.40b)

The integrals are taken over the whole domain and over the whole surface area onwhich tractions are given.

It is immediately obvious from the above that

Kab =e

Keab and fa =e

f ea (2.41)

by virtue of the property of definite integrals requiring that the total be the sum of the parts:

()d =e

e

()d and

()d =e

e

()d (2.42)

The same is obviously true for the surface integrals in Eq. (2.40b). We thus see that thesecret of the approximation possessing the required behaviour of a standard discretesystem of Chapter 1 lies simply in the requirement of writing the relationships in integralform.

The assembly rule as well as the whole derivation has been achieved without involvingthe

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