Preface

Composite materials are pervasive throughout our world and include both natural and man-made composites. For example, in nature, wood is a composite consisting of wood fibers (cellulose) bound together by a matrix of lignin. Composite materials have been used by mankind for thousands of years; many of the sun-dried mud brick buildings of the earliest known civilization in Mesopotamia at Sumer were reinforced with straw as early as 4900 B.C. However, with the advent of high-strength man-made fibers and the tremendous advances in polymer chemistry during the twentieth century, in many instances composite materials now can be made that offer advantages comparable to those of competing materials. The advantages of these advanced composites are many, including lighter weight, the ability to tailor composites for optimum strength and stiffness, improved fatigue life, corrosion resistance, and, with good design practice, reduced assembly costs due to fewer detail parts and fasteners. The specific strength (strength/density) and specific modulus (modulus/density) of high-strength fiber-reinforced composites, especially those with carbon fibers, are higher than those of comparable metal alloys. This translates into greater weight savings, resulting in improved performance, greater payloads, longer ranges (for vehicles), and fuel savings.

This book is intended primarily for technical personnel who want to learn more about modern composite materials. It would be useful to designers, structural engineers, materials and process engineers, manufacturing engineers, and production personnel involved with composites.

The book deals with all aspects of advanced composite materials: what they are, where they are used, how they are made, their properties, how they are designed and analyzed, and how they perform in service. It covers continuous- and discontinuous-fiber composites fabricated from polymer, metal, and ceramic matrices, with an emphasis on continuous-fiber polymer matrix composites. The book covers composite materials at the introductory to intermediate level. Throughout the book, practical aspects are emphasized more than theory. Because I spent 38 years in the industry, the information covers the current state-of-the-art in composite materials.

The book starts with an overview of composite materials (Chapter 1) and how highly anisotropic composites differ from isotropic materials, such as metals. Some of the important advantages and disadvantages of composites are discussed. Chapter 1 wraps up with some of the applications for advanced composites. Chapter 2 examines the reinforcements and their product forms, with an emphasis on glass, aramid, and carbon fibers. Chapter 3 covers the main thermosetting and thermoplastic resin systems. Thermoset resin systems include polyesters, vinyl esters, epoxies, bismaleimides, cynate esters, polyimides, and phenolics. Thermoplastic composite matrices include polyetheretherketone, polyetherketoneketone, polyetherimide, and polypropylene. The principles of thermoset resin toughening are also presented, along with an introduction to the physiochemical tests that are used to characterize resins and cured laminates.

Chapters 4 through 11 describe the progression of composite fabrication steps. Chapter 4 covers the basics of cure tools. This is followed by a discussion of thermoset composite fabrication processes (Chapter 5). Important thermoset lay-up methods include wet lay-up, prepreg lay-up, automated tape laying, fiber placement, filament winding, and pultrusion. Vacuum bagging in preparation for cure is also discussed, along with the cure processes for both addition and condensation curing thermosets. Thermoset liquid molding covers preforming technology (weaving, knitting, stitching, and braiding) followed by the major liquid molding processes, namely, resin transfer molding, resin film infusion, and vacuum-assisted resin transfer molding.

In Chapter 6, thermoplastic composite consolidation is covered, along with the different methods of thermoforming thermoplastics. Finally, the joining processes that are unique to thermoplastic composites are discussed. After these processing fundamentals are fully described, Chapter 7 deals with some of the detailed processing issues unique to thermoset and thermoplastic composites. The concept of cure modeling is introduced along with the importance of both lay-up and cure variables, hydrostatic resin pressure, chemical composition, resin and prepreg, debulking, and caul plates. Residual cure stresses and exothermic reactions are also covered, followed by a brief description of in-process cure monitoring.

Adhesive bonding, sandwich, and integrally cocured structures are introduced in Chapters 8 and 9. The basics of adhesive bonding are covered, along with its advantages and disadvantages. The importance of joint design, surface preparation, and bonding procedures is discussed, along with honeycomb bonded assemblies, foam bonded assemblies, and integrally cocured assemblies. Large, one-piece composite airframe structures have demonstrated the potential for impressive reductions in part counts and assembly costs.

The properties and fabrication technology for discontinuous-fiber polymer matrix composites are addressed in Chapter 10, with an emphasis on spray-up, compression molding, structural reaction injection molding, and injection molding.

Assembly (Chapter 11) can represent a significant portion of the total manufacturing cost, as much as 50 percent of the total delivered cost. In this chapter, the emphasis is on mechanical joining, including the hole preparation procedures and fasteners used for structural assembly. Sealing and painting are also briefly discussed.

Chapters 12 through 15 cover the test methods and properties for composite materials. Important nondestructive test methods (Chapter 12) include visual, ultrasonics, radiographic, and thermographic inspection methods. Mechanical property test methods (Chapter 13) include tests for both composite materials and adhesive systems. In Chapter 14, the strength and stiffness for both discontinuous and continuous reinforced composites are compared. Chapter 15 covers the important topic of environmental degradation, including moisture absorption, fluids exposure, ultraviolet radiation and erosion, lightning strikes, thermo-oxidative behavior, heat damage, and flammability.

Chapters 16 through 19 cover the analysis, design, and repair of composites. Structural analysis (Chapter 16) starts with analysis at the lamina, or ply, level and then uses classical lamination theory to illustrate the analysis methods for more complex laminates. The concept of interlaminar free edge stresses is introduced. Four failure theories are discussed: the maximum stress criterion, the maximum strain criterion, the Azzi-Tsai-Hill maximum work theory, and the Tsai-Wu failure criterion. The important topic of analysis of composite joints, both bolted and bonded, is covered in Chapter 17. Chapter 18 deals with composite design and certification considerations, including materials and process selection, design trade studies, the building block approach to certification, design allowables, and design guidelines. Considerations for handling damage tolerance and environmental issues are also discussed. Repair of composites (Chapter 19) includes fill repairs, injection repairs, bolted repairs, and bonded repairs.

Metal matrix composites (Chapter 20) offer a number of advantages compared to their base metals, such as higher specific strengths and moduli, higher elevated-temperature resistance, lower coefficients of thermal expansion, and, in some cases, better wear re sistance. On the downside, they are more expensive than their base metals and have lower toughness. Because of their high costs, commercial applications for metal matrix composites are limited. As with metal matrix composites, there are few commercial applications for ceramic matrix composites (Chapter 21), also because of their high costs, as well as concerns for reliability. Carbon-carbon composites have been used in aerospace applications for thermal protection systems. However, metal and ceramic matrix composites remain an important material class, because they are considered enablers for future hypersonic flight vehicles.

The reader is cautioned that the data presented in this book are not design allowables. The reader should consult approved design manuals for statistically derived design allowables.

I would like to acknowledge the help and guidance of Ann Britton, Eileen De Guire, Steve Lampman, and Madrid Tramble, ASM International, and the staff at ASM for their valuable contributions. I would also like to thank my wife, Betty, for her continuing support.