Composite Materials

Manufacturing Methods

• Hand lay-up Method• Spray Lay up Method• Injection Moulding• Pultrusion Method

Hand lay-up Method

Spray Lay up Method

Injection Moulding

Pultrusion Method

Metal Matrix Composites (MMC)• Metal matrix composites (MMCs), as the name

implies, have a metal matrix. Examples of matrices in such composites include aluminum, magnesium, and titanium.

• Typical fibers include carbon and silicon carbide.• Metals are mainly reinforced to increase or decrease

their properties to suit the needs of design. For example, the elastic stiffness and strength of metals can be increased, and large coefficients of thermal expansion and thermal and electric conductivities of metals can be reduced, by the addition of fibers such as silicon carbide.

Fabrication Method

Fabrication methods for MMCs are varied. One method of manufacturing them is diffusion bonding, which is used in manufacturing boron/aluminum composite parts. A fiber mat of boron is placed between two thin aluminum foils about 0.05 mm thick. A polymer binder or an acrylic adhesive holds the fibers together in the mat. Layers of these metal foils are stacked at angles as required by the design. The laminate is first heated in a vacuum bag to remove the binder. The laminate is then hot pressed with a temperature of about 500°C and pressure of about 35 MPa in a die to form the required machine element.

Squeeze casting

Stir casting

Ceramic Matrix Composites (CMC)

Ceramic matrix composites (CMCs) have a ceramic matrix such as alumina, glass, carbon reinforced by fibers such as carbon, silicon carbide, boron carbide.

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Processing Ceramic Matrix Composites (CMCs)

Slurries: for continuous fibre reinforced composite 1 ） Fibers (glass fibers),

impregnated with slurry (powder glass (1-50m) in water and water soluble resin binder), are wound on to a mandrel to form a tape. 2) The tape is cut into pies.3) The types are stacked (lay-up).4) Burnout of the binder5) Heat pressure e.g. glass fiber reinforced glass-ceramic matrix)

Characteristics of Fiber-Reinforced CompositesMany factors must be considered when designing a fiber-reinforced composite including the length, diameter, orientation, amount and properties of the fibers and matrix, and the bonding between the fibers and matrix.

Fiber length and diameter: Fiber dimensions are characterized by their aspect ratio l/d where l is the fiber length and d is its diameter.

The strength improves when the aspect ratio is large.

Typical fiber diameters are from 10 m to 150 m.

Fibers often fracture because of surface imperfections. Making the diameter small reduces its surface area, which has fewer flaws.

Long fibers are preferred because the load carrying capacity is less at the ends than the remainder. Thus the longer the rod, the fewer the ends, the higher the load carrying capacity.

Different Forms

Effect of fiber Orientation

Maximum strength is obtained when long fibers are oriented parallel to the applied load.

Effect of fiber orientation on tensile strength of E-glass fiber-reinforced epoxy composite.

Effect of fiber Orientation

Figure (a) shows a unidirectional arrangement. Figure (b) shows a quasi-isotropic arrangement

The properties of fiber composites can be tailored to meet different loading requirements. By using combinations of different fiber orientation quasi-isotropic materials may be produced

Advantages:

• High strength to weight ratio (low density high tensile strength) or high specific strength ratio!

• High creep resistance• High tensile strength at elevated temperatures• High toughness• Generally perform better than steel or aluminum in applications where cyclic loads

are encountered leading to potential fatigue failure (i.e. helicopter blades).• Some composites can have much higher wear resistance than metals.• Corrosion resistance

• Disadvantages :• Material costs

• Fabrication/ manufacturing difficulties

• Repair can be difficult

• Operating temperature can be an issue for polymeric matrix (i.e. 500 F). Less an issue for metal matrix (2,700 F).

• Inspection and testing typically more complex.

Nature of Composites: