Chapter 16 – Composites: Teamwork and Synergy in Materials

Chapter Outline 16.1 Dispersion-Strengthened Composites 16.2 Particulate Composites 16.3 Fiber-Reinforced Composites 16.4 Characteristics of Fiber-Reinforced Composites 16.5 Manufacturing Fibers and Composites 16.6 Fiber-Reinforced Systems and Applications 16.7 Laminar Composite Materials 16.8 Examples and Applications of Laminar Composites 16.9 Sandwich Structures

Figure 16.1 Some examples of composite materials: (a) plywood is a laminar composite of layers of wood veneer, (b) fiberglass is a fiber-reinforced composite containing stiff, strong glass fibers in a softer polymer matrix ( 175), and (c) concrete is a particulate composite containing coarse sand or gravel in a cement matrix (reduced 50%).

Section 16.1 Dispersion-Strengthened Composites A special group of dispersion-strengthened nanocomposite materials containing particles 10 to 250 nm in diameter is classified as particulate composites. Dispersoids - Tiny oxide particles formed in a metal matrix that interfere with dislocation movement and provide strengthening, even at elevated temperatures.

Section 16.2 Particulate Composites Rule of mixtures - The statement that the properties of a composite material are a function of the volume fraction of each material in the composite. Cemented carbides - Particulate composites containing hard ceramic particles bonded with a soft metallic matrix. Electrical Contacts - Materials used for electrical contacts in switches and relays must have a good combination of wear resistance and electrical conductivity. Polymers - Many engineering polymers that contain fillers and extenders are particulate composites.

Figure 16.4 Microstructure of tungsten carbide—20% cobalt-cemented carbide (1300). (From Metals Handbook, Vol. 7, 8th Ed., American Society for Metals, 1972.)

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.5 The steps in producing a silver-tungsten electrical composite: (a) Tungsten powders are pressed, (b) a low-density compact is produced, (c) sintering joins the tungsten powders, and (d) liquid silver is infiltrated into the pores between the particles.

Figure 16.6 The effect of clay on the properties of polyethylene. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.6 The effect of clay on the properties of polyethylene.

Section 16.4 Characteristics of Fiber-Reinforced Composites Many factors must be considered when designing a fiber-reinforced composite, including the length, diameter, orientation, amount, and properties of the fibers; the properties of the matrix; and the bonding between the fibers and the matrix. Aspect ratio - The length of a fiber divided by its diameter. Delamination - Separation of individual plies of a fiber-reinforced composite.

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.11 Effect of fiber orientation on the tensile strength of E-glass fiber-reinforced epoxy composites.

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.12 (a) Tapes containing aligned fibers can be joined to produce a multi-layered different orientations to produce a quasi-isotropic composite. In this case, a 0°/+45°/90° composite is formed.

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.10 Increasing the length of chopped E-glass fibers in an epoxy matrix increases the strength of the composite. In this example, the volume fraction of glass fibers is about 0.5.

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.13 A three-dimensional weave for fiber-reinforced composites.

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.14 Comparison of the specific strength and specific modulus of fibers versus metals and polymers.

Section 16.5 Manufacturing Fibers and Composites Chemical vapor deposition - Method for manufacturing materials by condensing the material from a vapor onto a solid substrate. Carbonizing - Driving off the non-carbon atoms from a polymer fiber, leaving behind a carbon fiber of high strength. Also known as pyrolizing. Filament winding - Process for producing fiber-reinforced composites in which continuous fibers are wrapped around a form or mandrel. Pultrusion - A method for producing composites containing mats or continuous fibers.

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.20 A scanning electron micrograph of a carbon tow containing many individual carbon filaments (x200).

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.22 Producing composite shapes in dies by (a) hand lay-up, (b) pressure bag molding, and (c) matched die molding.

Figure 16.24 Producing composite shapes by pultrusion. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.24 Producing composite shapes by pultrusion.

Figure 16.23 Producing composite shapes by filament winding. ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.23 Producing composite shapes by filament winding.

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.21 Production of fiber tapes by encasing fibers between metal cover sheets by diffusion bonding.

Section 16.7 Laminar Composite Materials Rule of Mixtures - Some properties of the laminar composite materials parallel to the lamellae are estimated from the rule of mixtures. Producing Laminar Composites - (a) roll bonding, (b) explosive bonding, (c) coextrusion, and (d) brazing.

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.30 Techniques for producing laminar composites: (a) roll bonding, (b) explosive bonding, and (c) coextrusion, and (d) brazing.

Section 16.8 Examples and Applications of Laminar Composites Laminates - Laminates are layers of materials joined by an organic adhesive. Cladding - A laminar composite produced when a corrosion-resistant or high-hardness layer of a laminar composite formed onto a less expensive or higher-strength backing. Bimetallic - A laminar composite material produced by joining two strips of metal with different thermal expansion coefficients, making the material sensitive to temperature changes.

Section 16.9 Sandwich Structures Sandwich - A composite material constructed of a lightweight, low-density material surrounded by dense, solid layers. The sandwich combines overall light weight with excellent stiffness. Honeycomb - A lightweight but stiff assembly of aluminum strip joined and expanded to form the core of a sandwich structure.

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.32 (a) A hexagonal cell honeycomb core, (b) can be joined to two face sheets by means of adhesive sheets, (c) producing an exceptionally lightweight yet stiff, strong honeycomb sandwich structure.

©2003 Brooks/Cole, a division of Thomson Learning, Inc ©2003 Brooks/Cole, a division of Thomson Learning, Inc. Thomson Learning™ is a trademark used herein under license. Figure 16.33 In the corrugation method for producing a honeycomb core, the material (such as aluminum) is corrugated between two rolls. The corrugated sheets are joined together with adhesive and then cut to the desired thickness.