Qin / Ye Toughening Mechanisms in Composite Materials

1. Auflage 2015
ISBN: 978-1-78242-291-4
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 416 Seiten

Reihe: Woodhead Publishing Series in Composites Science and Engineering

Toughening Mechanisms in Composite Materials aims to provide a comprehensive and technically detailed coverage of composites and their toughening mechanisms. Unique in its direct and comprehensive approach, the book presents fundamental knowledge on composites' toughening mechanisms as well as a comprehensive treatment of numerical methods. This volume summarizes the current state-of-the-art and presents the most recent research outcomes in the field. It details the development of each of the techniques, beginning with basic principles, and new concepts are illustrated with examples wherever possible. - Covers particle-reinforced composites, fibre-reinforced composites and other toughening mechanisms - Analyses toughening mechanisms in a broad range of composite materials - Developments in nanotube toughened composites and toughened graphene ceramic composites are examined

Autoren/Hrsg.

Qin, Qing-Hua

Ye, Jianqiao

Leseproben

1

Introduction to the composite and its toughening mechanisms

Q.H. Qin    Australian National University, Acton, ACT, Australia

Abstract

This chapter presents a brief discussion on the basic concepts of composite materials and the corresponding toughening mechanisms. The chapter begins with a basic definition of composites and their matrix and fiber components and then introduces the classifications and engineering applications of composites. Section 1.4 presents several approaches for evaluating the effective material properties of composites. It includes the mechanics of materials, finite element method, homogenization, and combined micromechanics-boundary element method. Several toughening mechanisms of composites are described at the end of this chapter.

Keywords

Composite

Toughening mechanism

Mechanics of material

Effective material property.

1.1 Basic concepts

The word “composite” usually signifies that two or more separate materials are combined on a macroscopic scale to form a structural unit for various engineering applications. Each of the material components may have distinct thermal, mechanical, electrical, magnetic, optical, and chemical properties. It is noted that a composite composed of an assemblage of these different materials gives us a useful new material whose performance characteristics are superior to those of the constituent materials acting independently (Ye, 2003; Qin and Yang, 2008). One or more of the material components is usually discontinuous, stiffer, and stronger and known as the reinforcement; the less stiff and weaker material is continuous and called the matrix. Sometimes, because of chemical interactions or other processing effects, an additional distinct phase, called an interphase, exists between the reinforcement and the matrix (Damiel and Ishai, 2006). Composite materials have some advantages when compared to their components or metal parts. Some material properties that can be improved by forming a composite material are (Jones, 1999):

• Strength

• Stiffness

• Wear resistance

• Weight

• Fatigue life

• Extreme temperature response

• Thermal insulation or conduction

• Electrical insulation or conduction

• Acoustical insulation or conduction

• Response to nuclear, X-ray, or magnetic radiation

• Chemical response or inertness to an environment (corrosion resistance)

• Electromagnetic and radar insulation or conduction

• Crack (fracture) resistance and arrest

• Cost

• Fabrication

• Temperature-dependent behavior

• Attractiveness.

Further, composite materials have the following advantages: (1) composites can have unique properties (e.g., specific strength and modulus) that are significantly better than their metal, polymer, and ceramic counterparts; (2) composites offer a greater flexiblity in designing and manufacturing a specific engineering structure; (3) composites can be fabricated to a final product from raw materials; and (4) composites can be tailored to have given properties required by the end users.

1.1.1 Matrix materials

Polymers, metals, and ceramics are all used as matrix materials in composites. They are the constituents that are continuously distributed in a composite. Examples of matrix materials are (1) polymers: epoxies, polyesters, phenolics, silicone, polyimide, nylon, polyethelene, polystyrene, and polycarbonate. The first five belong to the category of thermoset plastic, which is the material that can be melted and shaped only once (if it is heated a second time, it tends to crack or disintegrate); whereas the last four are categorized as thermoplastic, which is, in contrast, a material that can be melted and shaped over and over again; (2) metals: steel, iron, aluminum, zinc, carbon, copper, nickel, silver, titanium, and magnesium; and (3) ceramics: alumina, silicon carbide, aluminum nitride, silicon nitride, zirconia, and mullite. The functions of the matrix are to transmit forces between fibers, hold fibers in proper orientations, protect fibers from the environment, and stop cracks from spreading between fibers. To effectively realize those functions, a desired matrix material should have good ductility, high toughness and interlaminar shear strength, stable temperature properties, and high moisture/environmental resistance. In addition, a strong interface bond between the fiber and matrix materials is desirable, so the matrix must be capable of developing a mechanical or chemical bond with the fiber (Gibson, 2012).

1.1.2 Reinforcement materials

Reinforcement materials usually add rigidity and greatly impede crack propagation. In particular, they enforce the mechanical properties of the matrix and, in most cases, are harder, stronger, and stiffer than the matrix. The reinforcement can be divided into four basic categories: fibers, particulates, fillers, and flakes.

Flakes are in flat platelet form and have a primarily two-dimensional geometry with strength and stiffness in two directions. They can form an effective composite material when suspended in a glass or plastic. Ordinarily, flakes are packed parallel to one another with a resulting higher density than fiber-packing concepts. Typical flake materials are mica, aluminum, and silver. Mica flakes embedded in a glassy matrix provide composites that can be machined easily and are used in electrical applications. Aluminum flakes are commonly used in paints and other coatings in which they orient themselves parallel to the surface of the coating. Silver flakes are used where good conductivity is required.

Fillers are particles or powders added to material to change and improve the physical and mechanical properties of composites. They are also used to lower the consumption of a more expensive binder material. In particular, fillers are used to modify or enhance properties such as thermal conductivity, electrical resistivity, friction, wear resistance, and flame resistance. Typical fillers are calcium carbonate, aluminum oxide, lime (also known as calcium oxide), fumed silica, treated clays, and hollow glass beads.

Particulates used in composites can be small particles (< 0.25 µm), hollow spheres, cubes, platelets, or carbon nanotubes. In each case, the particulates provide desirable material properties, and the matrix acts as a binding medium necessary for structural applications. The arrangement of particulate materials may be random or with a preferred orientation. In general, particles are not very effective in improving strength and fracture resistance. Typical particle materials are lead, copper, tungsten, molybdenum, and chromium.

Finally, a fiber is a rope or string used as a component of composite materials whose aspect ratio (length/diameter) is usually very large (> 100). The cross-section can be circular, square, or hexagonal. Commonly used fibers in the composite include the following: (1) glass fiber, which consists primarily of silicon dioxide and metallic oxide modifying elements and are generally produced by mechanical drawing of molten glass through a small orifice. They are widely used due to low cost and high corrosion resistance. Glass fibers can be used in fishing rods, storage tanks, and aircraft parts; (2) aramid fiber, which has higher specific strength and is lighter than glass, is more ductile than carbon. Examples of industrial application are armor, protective clothing, and sporting goods; (3) carbon fiber, which is often produced from an oxidized polyacrylonitrile or via pyrolysis carbonized polymers. The carbon fiber can have a modulus as high as 950 GPa with low density. Its diameter is usually between 5 and 8 µm, smaller than a human hair (50 µm); (4) boron fiber, which usually has high stiffness, good compressive strength, and large diameters (0.05–0.2 mm) compared to other types of fibers. Composites with boron fibers are widely used in aerospace structures where high stiffness is needed; and (5) silicon carbide fiber, which is usually used in high-temperature metal and ceramic matrix composites (CMC) because of its excellent oxidation resistance, high modulus, and strength in high-temperature atmosphere.

1.2 Historical developments

Although it is difficult to say when or where people first learned about composites, nature and literature provide us with numerous examples. About 3000 BC, people used brick made of straw and mud for construction. Mud reinforced with bamboo shoots and glued laminated wood were used in houses built by the Egyptians in 1500 BC. Mongols invented the first composite bow in 1200 AD. Glass–polyester radomes were introduced in 1938 for application in aerospace structures: here, the fiberglass was combined for the first time with good unsaturated polyester resins. The first molded fiberglass...