Representative Volume Elements and Unit Cells

Representative Volume Elements and Unit Cells

Concepts, Theory, Applications and Implementation

1st Edition - November 19, 2019

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  • Authors: Shuguang Li, Elena Sitnikova
  • eBook ISBN: 9780081026397
  • Paperback ISBN: 9780081026380

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Numerical methods to estimate material properties usually involve analysis of a representative volume element (RVE) or unit cell (UC). The representative volume element (RVE) or unit cell (UC) is the smallest volume over which a measurement can be made that will yield a value representative of the whole. RVEs and UCs are widely used in the characterisation of materials with multiscale architectures such as composites. However, finite element (FE) software packages such as Abaqus and Comsol MultiPhysics do not offer the capability for RVE and UC modelling directly on their own. To apply them to analyse RVEs and UCs, the generation of the FE models for them, the imposition of boundary conditions, and the extraction of directly relevant results are essentially the responsibility of the user. These have tended to be incorrectly implemented by users! For the first time, this book will provide a comprehensive account on correct modelling of RVEs and UCs, which will eliminate any uncertainties and ambiguities.The book offers a complete and thorough review on the subject of RVEs and UCs, establishing a framework on a rigorous mathematical and mechanical basis to ensure that basic concepts, such as symmetry and free body diagrams, are applied correctly and consistently. It also demonstrates to readers that rigorous applications of mathematics and mechanics are meant to make things clear, consistent, thorough and, most of all, simple and easy to follow, rather than the opposite as many perceive. As a result, the book shows that the appropriate use of RVEs and UCs can deliver an effective and reliable means of material characterisation. It not only provides a much needed comprehensive account on material characterisation but, more importantly, explains how such characterisation can be conducted in a consistent and systematic manner. It also includes a ready-to-use open source code for UCs that can be downloaded from a companion site for potential users to utilise, adapt and expand as they wish.

Key Features

  • The companion site for the book can be found at
  • The theories presented in this book will give users more confidence when applying RVE and UC models to analyse materials of complex architectures with accuracy and efficiency
  • Systematic explanations of RVE and UC theories have been included, as well as their applications in composites
  • It illustrates in detail how to set up UC models and provides an open source code to implement via Abaqus


Researchers analyzing the multiscale behaviour and characteristics of composite materials. Engineers working in materials selection and characterisation. Materials scientists working on micro-architectures. Software engineers interested in virtual testing platforms for composites characterization

Table of Contents

  • Preface xi

    Part One: Basics

    1. Introduction d background, objectives and basic

    concepts 3

    1.1 The concept of length scales and typical length scales in physics and

    engineering 3

    1.2 Multiscale modelling 4

    1.3 Representative volume element and unit cell 5

    1.4 Background of this monograph 6

    1.5 Objectives of this monograph 6

    1.6 The structure of this monograph 8

    References 10

    2. Symmetry, symmetry transformations and symmetry conditions 11

    2.1 Introduction 11

    2.2 Geometric transformations and the concept of symmetry 12

    2.3 Symmetry of physical fields 16

    2.4 Continuity and free body diagrams 24

    2.5 Symmetry conditions 28

    2.6 Concluding remarks 41

    References 42

    3. Material categorisation and material characterisation 43

    3.1 Background 43

    3.2 Material categorisation 45

    3.3 Material characterisation 60

    3.4 Concluding remarks 64

    References 64

    4. Representative volume elements and unit cells 67

    4.1 Introduction 67

    4.2 RVEs 68

    4.3 UCs 71

    4.4 Concluding remarks 76

    References 77

    5. Common erroneous treatments and their conceptual sources

    of errors 79

    5.1 Realistic or hypothetic background 79

    5.2 The construction of RVEs and their boundary 82

    5.3 The construction of UCs 84

    5.4 Post-processing 96

    5.5 Implementation issues 98

    5.6 Verification and the lack of ‘sanity checks’ 101

    5.7 Concluding remarks 102

    References 103

    Part Two: Consistent formulation of unit cells and

    representative volume elements

    6. Formulation of unit cells 107

    6.1 Introduction 107

    6.2 Relative displacement field and rigid body rotations 108

    6.3 Relative displacement boundary conditions for unit cells 114

    6.4 Typical unit cells and their boundary conditions in terms of relative

    displacements 115

    6.5 Requirements on meshing 176

    6.6 Key degrees of freedom and average strains 177

    6.7 Average stresses and effective material properties 179

    6.8 Thermal expansion coefficients 182

    6.9 “Sanity checks” as basic verifications 183

    6.10 Concluding remarks 185

    References 187

    7. Periodic traction boundary conditions and the key degrees of

    freedom for unit cells 189

    7.1 Introduction 189

    7.2 Boundaries and boundary conditions for unit cells resulting from

    translational symmetries 193

    7.3 Total potential energy and variational principle for unit cells under

    prescribed average strains 197

    7.4 Periodic traction boundary conditions as the natural boundary

    conditions for unit cells 198

    7.5 The nature of the reactions at the prescribed key degrees of freedom 202

    7.6 Prescribed concentrated ‘forces’ at the key degrees of freedom 209

    7.7 Examples 211

    7.8 Conclusions 219

    References 220

    8. Further symmetries within a UC 223

    8.1 Introduction 223

    8.2 Further reflectional symmetries to existing translational symmetries 225

    8.3 Further rotational symmetries to existing translational symmetries 251

    8.4 Examples of mixed reflectional and rotational symmetries 291

    8.5 Centrally reflectional symmetry 303

    8.6 Guidance to the sequence of exploiting existing symmetries 314

    8.7 Concluding statement 315

    References 317

    9. RVE for media with randomly distributed inclusions 319

    9.1 Introduction 319

    9.2 Displacement boundary conditions and traction boundary conditions

    for an RVE 320

    9.3 Decay length for boundary effects 323

    9.4 Generation of random patterns 327

    9.5 Strain and stress fields in the RVE and the sub-domain 330

    9.6 Post-processing for average stresses, strains and effective properties 336

    9.7 Conclusions 345

    References 345

    10. The diffusion problem 347

    10.1 Introduction 347

    10.2 Governing equation 347

    10.3 Relative concentration field 351

    10.4 An example of a cuboidal unit cell 353

    10.5 RVEs 355

    10.6 Post-processing for average concentration gradients and diffusion fluxes 356

    10.7 Conclusions 360

    References 360

    11. Boundaries of applicability of representative volume elements

    and unit cells 361

    11.1 Introduction 361

    11.2 Predictions of elastic properties and strengths 361

    11.3 Representative volume elements 363

    11.4 Unit cells 366

    11.5 Conclusions 366

    References 367

    Part Three: Further developments

    12. Applications to textile composites 371

    12.1 Introduction 371

    12.2 Use of symmetries when defining an effective UC 381

    12.3 Unit cells for two-dimensional textile composites 383

    12.4 Unit cells for three-dimensional textile composites 398

    12.5 Conclusions 414

    References 415

    13. Application of unit cells to problems of finite deformation 417

    13.1 Introduction 417

    13.2 Unit cell modelling at finite deformations 419

    13.3 The uncertainties associated with material definition 433

    13.4 Concluding remarks 436

    References 437

    14. Automated implementation: UnitCells© composites

    characterisation code 439

    14.1 Introduction 439

    14.2 Abaqus/CAE modelling practicality 441

    14.3 Verification and validation 450

    14.4 Concluding remarks 456

    References 456

    Index 459


Product details

  • No. of pages: 482
  • Language: English
  • Copyright: © Woodhead Publishing 2019
  • Published: November 19, 2019
  • Imprint: Woodhead Publishing
  • eBook ISBN: 9780081026397
  • Paperback ISBN: 9780081026380

About the Authors

Shuguang Li

Shuguang Li is a Professor of Aerospace Composites at the Institute for Aerospace Technology, Faculty of Engineering, University of Nottingham, UK. He obtained his PhD from the University of Manchester in 1993 and returned to his academic track as a lecturer at the University of Manchester in 1995 and was appointed to his present position in 2012. Professor Li has published well over 100 academic papers, most of them in highly reputable international journals. His main research interest is in the area of analysis of composite materials and structures, in particular, on modelling damage and failure, micromechanics and material characterisation. As an outcome of his research on micromechanical modelling of composites, a piece of software UnitCells© has emerged which offers material scientists and structural designers a useful tool for characterisation of composites in terms of effective elastic properties as well as strength properties.

Affiliations and Expertise

Professor of Aerospace Composites, the Institute for Aerospace Technology, Faculty of Engineering, University of Nottingham, UK

Elena Sitnikova

Elena Sitnikova is currently a Research Fellow at the Faculty of Engineering, the University of Nottingham. She received her MSc degree in Applied Mathematics, Mechanics from Saint Petersburg State University in 2004 and PhD in Engineering from the University of Aberdeen in 2010. Prior to joining the University of Nottingham in 2013, she took a position of Research Associate at the University of Liverpool. Dr Sitnikova authored a number of research papers in several fields of engineering. Her current interests are in the field of textile composites, with particular emphasis on problems of damage, material characterisation and high strain rate response of these materials.

Affiliations and Expertise

The Faculty of Engineering, The University of Nottingham, University Park, Nottingham, UK

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