Composites

• Composites are a material comprised of two or more constituent materials that have different properties.
• The two materials work together to imrpove the mechanical, electrical, physical or chemical properties of the composite.

• Within the composite you can easily tell the different materials apart as they do not dissolve or blend into each other.

• Composites are an important material in an intensely competitive global market.
• New materials and technologies are being produced frequently for the design and rapid manufacture of high-quality composite products.
• Composites are replacing more traditional materials as they can be created with properties specifically designed for the intended application.

• As designers develop new products, they should always be aware of the materials available.
• In an effort to increase productivity and lose weight, carbon fiber parts are often glued together.
• The use of an epoxy adhesive rather than traditional fastening methods allows manufacturers to create more complex shapes quickly and easily.
• These materials and methods are being transferred to consumer products.

• The idea of creating composite materials is not new.
• One of the earliest examples is straw reinforced bricks, where the mud is the matrix and the straw is the reinforcement.

Form of Composites

• Composite material is made up of a matrix (binder or glue) in which a reinforcement (fibers, sheets or particles) is embedded.
• Composites take advantage of the directional properties of the reinforcement and gluing properties of the matrix.

Reinforcement

• The way in which the reinforcement is applied in a composite can happen in a number of ways, but generally there are three main categories:

• These fibres/sheets/particles can be made from textiles, glass, plastics, wood and carbon.

Laminar (Sheets)

• Consist of two or more layers bonded together, usually with an adhesive.
• Examples are:
  • plywood
  • laminated glass.

Fiber-reinforced

• Fiber-reinforced, which act as the reinforcing material.
• Examples are:
  • glass
  • carbon
  • fiber
  • reinforced concrete.

Particle-reinforced

• Contains particles, which act as the reinforcing material.
• There are many different forms of particulate composites.
• The particulates can be very small particles (< 0.25 microns), chopped fibers (such as glass), platelets, hollow spheres, or new materials such as bucky balls or carbon nano-tubes.
  • Concrete is an example of particle-reinforced composites.

Matrix

• Most composites have two constituent materials: matrix (a binder), and a reinforcement.
• The reinforcement is usually much stronger and stiffer than the matrix, and gives the composite its good properties.
• The matrix holds the reinforcements in an orderly pattern.
• Because the reinforcements are usually discontinuous, the matrix also helps to transfer load among the reinforcements.

• The matrix can be made from:
  • Polymers
    • Polymer Matric Composites (PMC’s): such as thermoplastics and thermosetting plastics.
  • Ceramics
    • Ceramic Matrix Composites (CMC’s): such as concrete.
  • Metals
    • Metal Matrix Composites: such as titanium.

Processes of Making Composites

Weaving

• The act of forming a sheet-like material by interlacing long threads passing in one direction with others at a right angle to them.

Molding

• In open molding, raw materials (resins and fiber-reinforcements) are exposed to air as they cure or harden.
• Open molding utilizes different processes, including hand lay-up, spray-up, casting, and filament winding.

• In closed-molding, raw materials (fibers and resin) cure inside a two-sided mold or within a vacuum bag (shut off from air).
• Closed-molding processes are usually automated and require special equipment, so they’re mainly used in large plants that produce huge volumes of material; up to 500,000 parts a year.

Pultrusion

• A continuous manufacturing process used to create composite materials that have a constant cross-section.
• Reinforcing fibers are saturated with a liquid-polymer resin and then pulled through a heated die to form a part.

Lamination

• Covering the surface of a material with a thin sheet of another material typically for protection, preservation or aesthetic reasons.

Composition and Structure of Composites

Concrete

• Concrete is made with water, Portland cement and aggregates (gravel, etc).
• The aggregate of coarse rock or gravel is embedded in a matrix of cement.
• The hardness of cement is increased significantly by adding gravel as reinforcing filler.
• Concrete is used in skyscrapers, bridges, sidewalks, highways, houses, dams etc.

Reinforced Concrete

• Where the rebar (metal rods) is the fibre and the concrete is the matrix.
• Skyscrapers, bridges, sidewalks, highways, houses and dams.

Engineered Wood

• Also called composite wood, man-made wood or manufactured board.
• Includes plywood, particle board and LVL.

Laminated Veneer Lumber (LVL)

• An engineered wood product that uses multiple layers of thin wood, oriented in the same direction, assembled with resins.
• Structural applications such as in buildings.

Plywood

• A sheet material manufactured from thin layers or “plies” of wood veneer that are glued together with adjacent layers having their wood grain rotated up to 90 degrees to one another.
• Construction, internal (furniture, flooring, etc.) and exterior uses such as marine ply for boats.

Particle Board

• A material made from different sizes of wood chips and joined with glue.
• Furniture (such wardrobes, wall units, TV cabinets), shelving, toys, wall linings etc.

Fiberglass

• Strands of glass, formed into a matt and then covered in resin (polymer/thermoset plastic).
• Used in boats, pools, pipes, bathtubs, motors and car bodies.

Kevlar

• Para-aramid synthetic fiber covered in a resin.
• Used in body armor, high performance canoes or sports equipment, ropes, military applications etc.

Carbon-Reinforced Plastic

• Carbon fibers formed into a mat then covered in resin.
• Carbon fiber has played an important part in weight reduction for vehicles and aircraft.
• Used in high performance car racing, sports equipment (golf clubs, etc) and aviation (the Boeing Dreamliner).

Advantages and Disadvantages of Composite Materials

Advantages

Strength

• Per pound, composites are stronger than other materials such as steel.
• The two primary components of composites (fibers and resins) contribute to their strength.
• Fibers carry the load, while resins (matrix) distribute the weight throughout the composite part as required.

Lightweight

• Composites are light in weight compared to most woods and metals.
• Lower weight contributes to fuel efficiency in cars and airplanes. Lighter objects, ranging from utility poles to bridge decks, are easier to transport and install.

Resistance

• Composites resist damage from weather and harsh chemicals that can eat away at other materials.
• That makes them good choices for applications that face constant exposure to salt water, toxic chemicals, temperature fluctuations and other severe conditions.

Design Flexibility

• A wide range of material combinations can be used in composites, which allows for design flexibility.
• The materials can be custom tailored to fit unique specifications of each application.
• Composites also can be easily molded into complicated shapes.
• This offers designers, engineers and architects a freedom not typically found with other competing materials.

Durability

• Simply put, composites last!
• Structures made with composites have a long lifespan and require little maintenance.
• Many products made with composites, such as boats, have been in service for more than half a century.

Disadvantages

Delamination

• Since composites are often constructed of different ply layers into a laminate structure, they can “delaminate” between layers where they are weaker.

High Cost

• They are a relatively new material, and as such have a high cost.
• However, certain composites such as concrete have been around for a long time and are very cheap.

Complex Fabrication

• The fabrication process of composites is usually labor-intensive and complex, which further increases cost.

Lack of Visual Proof of Damage

• Delamination and cracks in composites are mostly internal and hence require complicated inspection techniques for detection.
  • This is especially a problem in buildings, where concrete is prone to crack around the small granules in the matrix.
  • As these cracks are internal they can be very hard to spot early-on; possibly leading to failure or early renovations which waste resources.

Composite to Metal Joining

• Metals expand and contract more on variations in temperature as compared to composites.
• This may cause an imbalance at joinery and may lead to failure.

Availability

• The dissemination (distribution/availability) of composites globally is limited.