1 chapter 16 – composites: teamwork and synergy in materials

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Chapter 16 – Composites: Teamwork and Synergy in Materials

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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

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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%).

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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 16.1 Dispersion-Strengthened Composites

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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.

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

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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.

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Figure 16.6 The effect of clay on the properties of polyethylene.

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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.

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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.

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Figure 16.11 Effect of fiber orientation on the tensile strength of E-glass fiber-reinforced epoxy composites.

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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.

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Figure 16.13 A three-dimensional weave for fiber-reinforced composites.

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Figure 16.14 Comparison of the specific strength and specific modulus of fibers versus metals and polymers.

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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.

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Figure 16.20 A scanning electron micrograph of a carbon tow containing many individual carbon filaments (x200).

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Figure 16.24 Producing composite shapes by pultrusion.

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Figure 16.23 Producing composite shapes by filament winding.

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Figure 16.21 Production of fiber tapes by encasing fibers between metal cover sheets by diffusion bonding.

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Figure 16.22 Producing composite shapes in dies by (a) hand lay-up, (b) pressure bag molding, and (c) matched die molding.

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Section 16.6 Fiber-Reinforced Systems and

Applications Advanced Composites - The advanced composites

normally are polymer–matrix composites reinforced with high-strength polymer, metal, or ceramic fibers.

Metal-Matrix Composites - These materials, strengthened by metal or ceramic fibers, provide high-temperature resistance.

Ceramic-Matrix Composites - Composites containing ceramic fibers in a ceramic matrix are also finding applications.

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Figure 16.25 A comparison of the specific modulus and specific strength of several composite materials with those of metals and polymers.

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Figure 16.26 The specific strength versus temperature for several composites and metals.

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Figure 16.27 The manufacturer of composite super-conductor wires: (a) Niobium wire is surrounded with copper during forming. (b) Tim is plated onto Nb-Cu composite wired. (c) Tin diffuses to niobium to produce the Nb3Sn-Cu composite.

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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.

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Figure 16.30 Techniques for producing laminar composites: (a) roll bonding, (b) explosive bonding, and (c) coextrusion, and (d) brazing.

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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.

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Figure 16.31 Schematic diagram of an aramid-aluminum laminate, Arall, which has potential for aerospace applications.

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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.

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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.

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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.

1 chapter 16 – composites: teamwork and synergy in materials

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