Aims and scope

Multifunctional composites refer to composite materials that are specifically made, or modified, to provide more than one enhanced property, or functionality. Composites’ main traditional functionalities have been high structural stiffness and strength with the lowest possible weight. Although polymer matrix composites have other additional advantages such as corrosion resistance, they also bear shortcomings such as relatively poor acoustic damping, damage under impact, and vulnerability to erosion. Multifunctional composites transcend these weaknesses by providing added functionalities without adversely affecting the signature high stiffness and strength densities of composites. For example, carbon nanotubes incorporated into composites enhance electrical conductivity. Added functionalities can be active, such as active vibration control or passive ones such as constrained layer damping. By the very nature of their processing and fabrication, multifunctional composites can not only enhance properties, but include functionalities which do not exist at all in classical composites, such as self-healing and microvascular transport.

From ameliorating relative weaknesses to providing functionalities that do not exist in classical materials, the study of multifunctional composites has become a vibrant area of research and development. The aim of this book is to provide researchers and practitioners with a comprehensive introduction to the subject, from which they can advance to performing state-of-the art research or translate current advances into commercial products. After studying this book, the reader will have a broad understanding of the challenges and opportunities afforded by various functionalities that affect the application of composite materials to aircraft and other weight sensitive structures. Prepared by this solid understanding of the field, the reader will then be ready to tackle further technical papers specifically related to individual properties of interest.

Each chapter deals with a different functionality. To achieve the aim of this book, the scope entails a full and instructive description of each. Each chapter therefore includes: (a) a brief description of the physics of the function or property that is being added or improved; (b) a brief description of testing and analysis methods used to quantify that functionality; (c) a review of contemporary approaches that implement or improve the functionality, which may include new process and materials development. The relative emphasis within these sections varies from chapter to chapter, as the current state of understanding, characterization, and materials development varies greatly among the functionalities addressed in this book. Furthermore, each chapter draws on the unique expertise of its author. As subject expert, each author is best suited to determine the most fitting presentation of the chapter’s subject matter.

Composites are commonly utilized in aircraft, and are routinely subjected to a variety of electromagnetic effects, including static precipitation, high intensity radiated fields, electrostatic discharge, nuclear electromagnetic pulses, atmospheric radiation, and others, as described in Chapter 1. These effects are especially threatening to composites, which have lower conductivity and higher contact resistance than aluminum. This complicates efforts to provide shielding, grounding, and other features protecting the aircraft from electromagnetic phenomena. In addition, lighting strikes on aircraft cause both direct effects (Chapter 2) and indirect effects (Chapter 1). Alternative materials which mitigate both classes of effects are described in detail in Chapters 1 and 2.

Despite their outstanding in-plane strength, laminated composites are susceptible to damage when subject to low velocity/low energy impact normal to their surfaces, as discussed in Chapter 3. The main property affected is compression after impact (CAI), which is the controlling design property in many applications. Although there are many ways to toughen a laminated composite (described in Chapter 3), these solutions tend to degrade some of the in-plane strengths. Therefore, careful consideration should be given during material selection and structural design.

Erosion of polymer matrix composites (PMC) is a very complex problem involving removal of surface material by repetitive impact of raindrops or sand, as described in Chapter 4. The result is a roughened surface that may compromise aerodynamic performance, optical and radiation transmission, and even compromise the structural integrity of the part. Aircraft, rotorcraft, windmills, and missiles are among the applications negatively affected by erosion. Multifunctional composites can meet the crucial challenge of protecting against erosion or increasing the erosion resistance of PMCs.

Composites have conquered aerospace markets due to their high strength to weight ratio, but their subsequent high stiffness to weight ratio facilitates the spread of acoustic and mechanical vibration. This disadvantage causes passenger discomfort and even mechanical damage. Despite the inherent damping capacity of the polymer matrix, energy dissipation in composites may be insufficient for high performance applications. Therefore, new materials, manufacturing processes, and design methods continue to be developed to improve the damping performance of composite structures. Chapter 5 introduces the problem, emphasizing materials, solutions, and design. Chapter 6 further discusses viscoelastic damping treatments and their design, numerical simulation, and optimization.

Healing is no longer a power exclusive to live organisms, but an ability that has been thoroughly implemented in various composites which are now able to self-repair many types of damage, including distributed intralaminar cracking, large delaminations, erosion damage, and damage to anti-corrosion coatings. These and other fascinating aspects of this innovative technology can be seen in Chapter 7.

Early in their development, self-healing ingredients were incorporated into the composite during fabrication, in a process similar to interleaving tough layers between laminas to improve impact resistance (Chapter 3). The problem with that approach is that the self-healing agent is eventually depleted. To circumvent this obstacle, microvascular transport technology was created to deliver healing agents, and eventually many other fluids, throughout the composite. This technology mirrors the microvascular systems present in living organisms (Chapter 8).

Since polymers are more permeable to gases than metals, PMCs have faced challenges as materials for gas storage tanks, requiring metal liners that introduce their own problems, or polymer liners that themselves must be highly impervious. Other products such as food packaging, anti-corrosion coatings, fire resistance (Chapter 10), and thermal protection systems (Chapter 11) also benefit from lower gas permeability. Nanotechnologies applied to composites can solve these challenges, as discussed in Chapter 9.

Vulnerabilities to fire such as ignitability, flame spread, heat release, smoke opacity, and toxicity, and consequent fire resistant properties including thermal insulation, structural integrity, and residual bearing capacity, are crucial factors determining whether a material can be used in any application where human life is at risk. These applications encompass the wide fields of transportation and architecture among others. Chapter 10 thoroughly investigates the problem of measuring composites’ fire safety, as well as new approaches for improvement.

Chapters 9 and 10 provide a natural introduction for Chapter 11: Thermal protection systems (TPS), concerning design of materials that protect an underlying structure such as a spacecraft from the tremendously harsh environment of atmospheric re-entry. TPS are also widely applicable in other situations that pose simultaneous challenges of thermal gradient and load that no classical material can meet.

Finally, Chapter 12 introduces a new type of composite able to generate a physical response that is completely absent from any of its constituents. The specific example illustrated involves composites that produce electricity when excited by a magnetic field; a peculiar response that neither of the two constituents (a piezoelectric and a magnetostrictive phase) could produce alone. This is in stark contrast to classical additive composites, in which added components amplify existing attributes. For instance, a fiber might increase the stiffness already present in the matrix. This new class of composites, termed product composites, opens up new possibilities for previously unimagined material responses.