Properties for Design of Composite Structures. Neil McCartney

James Clerk Maxwell in his book on electromagnetism, where an ingenious analytical method is used to estimate the effective electrical conductivity of a uniformly distributed assembly of isotropic spherical particles. His result is thought to be the first expression ever developed for an effective property of a composite. His methodology is also extremely powerful and worthy of extensive use in this book.

      During service, composite components are often exposed to loadings that are sufficient to cause the formation of microstructural damage such as fibre failures or matrix cracking, which are usually associated with the occurrence of fibre/matrix interface debonding. For laminates, delamination at ply interfaces is an additional form of load-induced damage. The initiation and growth of microstructural damage affects the effective properties of composite materials, which degrade. It is remarkable that the degradation of very many thermoelastic constants is characterised by a single damage dependent function, as will become clear in the book. Sometimes damage mechanisms lead to local stress-transfer within complex structures, which can delay the onset of structural failure during progressive loading. Chapters 8–14 of this book will focus on analytical and semi-analytical methods of predicting both the effect of damage on effective properties and, more importantly, the dependence of effective properties on local loading conditions when using an energy balance approach. Chapters 13 and 14 concern, respectively, a model of damage development during the fatigue loading of laminates and a model of composite degradation due to environmental damage (in fibres).

      Chapters 15–19 comprise more advanced analyses, providing mathematical details of many complex derivations that arise when considering: i) the effective properties of aligned systems of spheroids, ii) interface debonding associated with a fibre fracture or matrix crack in a unidirectional composite, iii) the behaviour of bridged cracks of finite length in a unidirectional composite, iv) the behaviour of ply cracks of finite length in a cross-ply laminate, v) stress-transfer mechanics for general symmetric laminates subject to general in-plane loading and vi) stress-transfer mechanics associated with out-of-plane orthogonal bending of cross-ply laminates that do not need to be symmetric. The models considered can lead to analytical formulations that require the use of numerical methods to deal with the complexity arising from the use of analytical techniques. Software implementing the analysis described in this book is provided at the Wiley website link (www.wiley.com/go/mccartney/properties).

      The author would like to take this opportunity to thank all colleagues at NPL, and in other institutions around the world, for all the technical interactions that have helped greatly to formulate and investigate the various aspects of composite materials that are described in this book. My thanks also extend to the staff at John Wiley, who have been very patient while the book has been completed, and for their considerable help in the final stages of book preparation.

      This book is dedicated to my late wife, Irene, and all our family, who have encouraged the completion of this book despite the many years in preparation. The author cannot thank them enough for allowing him the time in their lives, first to undertake the research that is described, and then the writing of the book itself.

      L N McCartney

      April 2022

About the Companion Website

      Properties for Design of Composite Structures: Theory and Implementation Using Software is accompanied by a companion website:

       www.wiley.com/go/mccartney/properties

      The website includes:

       Software Notes, where relevant by chapter

1 Introduction

      Improving the properties and performance of materials by reinforcing them with different types of stiffer and/or stronger phases, such as particles or fibres, leads to the class of material known as composites. This approach was first exploited during natural developments (both living plants and organisms) and later by mankind, e.g. the use by Egyptians when making clay building bricks reinforced with straw to improve handling and performance, and when using gravel to reinforce cement forming a much stronger concrete material. Over the centuries, increasingly sophisticated composites have been developed, responding especially to the advent of higher-performance fibres (for high stiffness, strength and/or high temperature resistance). Even greater benefits can arise by the manufacture of composites reinforced with hollow and/or multicoated inclusions, and nanotubes.

      The simplest inclusion geometry for matrix reinforcement is a set of spherical particles which might exhibit a range of radii. When the matrix and reinforcement is homogeneously mixed, the resulting material has improved properties which are usually considered to be isotropic. Another simple geometry uses aligned continuous fibres to reinforce an isotropic matrix forming a material that is anisotropic such that the properties in the fibre (or axial) direction differ from transverse properties in the plane normal to the fibre direction. This type of material is known as unidirectionally fibre-reinforced composite where the transverse stiffnesses and strengths are usually much lower than the stiffness and strength in the fibre direction. To overcome this significant practical problem, composite laminates are considered where a stack of unidirectional composites known as plies are bonded together where the fibres in each ply in the laminate are aligned in a direction that varies from ply to ply. Laminates are often weak under compression because of a damage mode known as delamination where debonding occurs at or near the interfaces between the various plies. To overcome this practical problem, woven or stitched fibre architectures are used. Such composites can be analysed effectively only if numerical methods are used. This topic will not, therefore, be considered in this book, as the intention is to focus here on the development and use of analytical methods. Composites, often made where the reinforcement is a set of short fibres which are either aligned in a given direction or are randomly oriented, are also not considered in this book.

The engineering application of composite materials for equilibrium calculations requires a full understanding the key materials properties which can be thought of as being classified as elastic, thermal, electrical and magnetic. In addition, when nonequilibrium transport phenomena occur, such as heat flow and electrical conduction, an additional type of property known as conductivities must also be considered. All these properties are well known, and for isotropic materials often encountered in engineering, a limited number of material properties are sufficient to characterise, fully, the physical behaviour of such materials. However, when composite materials are considered, the properties are usually no longer isotropic, and many more properties describing the directional behaviour of the material, and even of the fibres themselves, need to be considered in the engineering design process. This complexity also leads to needs for suitable measurement methods capable of determining the values of the multitude of properties required to fully characterise an anisotropic composite material, an important topic that will not be considered in this book. It is noted that the numerical analysis of three-dimensional composite components requires a very large number of materials properties, associated with the anisotropic nature of composite materials, some of which are extremely difficult to measure (e.g. through-thickness shear moduli). Analytical methods provide a pragmatic way of providing such data, but this leads on to needs to have access to reliable data for fibre properties, which are frequently anisotropic, and some properties are seldom known reliably (e.g. Poisson’s ratios, shear moduli and transverse thermal expansion).

      The effective properties of composite materials, which arise when samples are considered as homogeneous materials having anisotropic properties, depend in a complex way on the properties of the