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Composite Materials

Outcomes 

This section addresses aspects of the following syllabus outcomes:   

H1.2 differentiates between properties of materials and justifies the selection of materials, components and processes in engineering

H2.1 determines suitable properties, uses and applications of materials in engineering

H4.1 investigates the extent of technological change in engineering

H4.2 applies knowledge of history and technological change to engineering- based problems

H4.3 appreciates social, environmental and cultural implications of technological change in engineering and applies them to the analysis of specific problems

Source: Stage 6 Engineering Studies syllabus. Board of Studies NSW, 1999.

What is a composite?

A Composite material is formed from a combination of two or more materials that differ in composition or form when bonded together, but retain their identities and properties. The outcome of this “composition” is that the newly formed material has superior properties to the individual components.

You may be familiar with the exceptionally tough material known as ‘fibreglass’ (glass-fibre reinforced plastic) that is commonly used to build boat hulls, fishing rods and surfboards, but many are unaware that the parent materials for fibreglass are both brittle and fragile. In this material the polyester resin that forms the matrix is prevented from cracking by the fibres of glass that criss-cross the structure. The threads of glass in fibreglass are very strong under tension but they are also brittle and will snap if bent sharply. The matrix not only holds the fibres together, it also protects them from damage by sharing applied stresses among them.

Naturally Occurring Composites

There are composites that exist in nature, such as timber and bone. A piece of wood is a composite, with long fibres of cellulose (a very complex form of starch) that provide the strength, held together by a much weaker substance called lignin which acts as the “glue” that bonds and stabilises the fibres. Cellulose is also found in cotton and linen, but it is the binding power of the lignin that makes a piece of timber much stronger than a bundle of cotton fibres.

Bamboo, which has been used extensively in civil engineering in the past, is a very efficient wood composite structure. The same components of cellulose and lignin are found in this material but bamboo is hollow. This results in a very light yet stiff structure. Composite fishing rods and golf club shafts copy this natural design.

The Early Beginnings of Man-Made Composites

The most primitive man made composite materials comprised of straw and mud in the form of bricks for building construction. When a cake of dried mud has a bending force applied, it is likely to fail due to the tensile forces on the underside. The material is, however, strong in compression. Straw, on the other hand, has considerable strength when you try to stretch it but almost none when you crumple it up. When the straw is embedded into mud and let dry hard, the resulting mud brick resists both squeezing and tearing and makes an excellent building material. Put more technically, it has both good compressive strength and good tensile strength.

Mud brick construction may take one of two forms. Individual bricks may be made and then laid using a mud mortar in a similar way to laying bricks, or the entire wall can be formed and a continual moulding process employed so that a wall is effectively one big brick. (For more information see web links)

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Activity 1.

Aim:
To investigate the strength characteristics of composite materials by comparing the bend strength of plain ice with straw-reinforced ice.

Preparation:
To carry out this experiment you will need:

iceblock wrapper

Fig 1. Typical iceblock wrapper suitable for this experiment

Method:
Step 1. Fill some of the water iceblock wrappers with rice straw. (see figure 2.)

filler wrapper straw

Fig 2. Fill wrapper with straw

Step 2. Stand wrappers in a suitable container, both straw-filled and plain, fill with water and place in freezer (see figures 3 & 4)

trim straw and stand wrappers in tumbler

Fig 3. Trim straw and stand wrappers in tumbler

fill with water

Fig 4. Then fill with water

Step 3. Place two tables such that the iceblock can span between them with the bucket resting as shown below.

bucket hangs below beam ready fo testing

Fig 5. Bucket hangs below the beam ready for testing

Step 4. Add water to the bucket until the “beam” (iceblock) breaks. Measure and record the quantity of water used.

Answer

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Composites in Civil Engineering

The Civil Engineering profession is said to be the oldest Engineering profession in the world. Civil engineers have been involved over many centuries to build and maintain the basic infrastructure, from pyramids in Egypt to modern transportation systems.

Today Civil Engineering involves analysis, planning, design, construction, and maintenance of many types of facilities for government, commerce, industry, and the public. Civil Engineers design and supervise the construction of roads, airports, tunnels, bridges, water supply, sewage systems, and buildings. It is therefore a sector that provides an essential service to society. Civil engineers are concerned with the impact of their projects on the public and the environment, and they co-ordinate the needs of society with technical and economic feasibility.

Composites are regularly a part of the materials forming the basis for Civil Engineering projects. Over the last thirty years composite materials, plastics, and ceramics have been the dominant emerging materials. The volume and number of applications of composite materials has grown steadily, penetrating and conquering new markets relentlessly. Modern composite materials constitute a significant proportion of the engineered materials market.

Plywood

Combining natural and synthetic materials, plywood is a man-made composite, more specifically known as a laminated product. Thin layers of timber veneer are bonded together with adhesive to form flat sheets of laminated wood that are stronger than natural wood. Alternate veneers are laid at right angles producing a material with more uniform strength properties than natural timber, which possesses a strong and a weak grain direction. Veneers are found in odd numbers to ensure the external veneers are both aligned.

Asphalt

Asphalt is a commonly used composite in civil engineering used for the construction of pavement, highways and parking lots. Mineral aggregate (such as crushed basalt)and crushing fines are mixed into a bitumen binder and then laid down in layers and compacted. Mixing and laying may be carried out hot, warm or cold depending on the nature and properties required. Hot mix asphalt provides the toughest surface and so is the method of choice on many highly trafficked pavements such as major highways and airfields. (For more information see web links)

Geotextiles

Geotextiles are permeable fabrics that have become irreplaceable in civil engineering. As their use has expanded there has been the introduction of geotextile composites and the development of products such as geogrids and meshes. Referred to as geosynthetics, these products have a wide range of applications and are currently used to advantage in many civil engineering applications including roads, airfields, railroads, embankments, retaining structures, reservoirs, canals, dams, bank protection and coastal engineering.
Geotextile fabrics perform four basic functions: filtration, drainage, separation  and reinforcement. (For more information see web links)

Cermets

A material consisting of processed ceramic particles bonded with metal and used in high-strength and high-temperature applications.

Cermets are bonded materials containing ceramics and metal, widely used in jet engines and nuclear reactors. They behave much like metals but have the great heat resistance of ceramics. Tungsten carbide, titanium carbide, zirconium bromide, and aluminium oxide are among the ceramics used; iron, cobalt, nickel, and chromium are among the metals.

Advanced composites are revolutionising virtually every aspect of Engineering, including thermal management.  Industrial applications are now the largest user of composites, outstripping aerospace and sports equipment.  There are now an extremely large and increasing number of applications, including disk drives, semiconductor manufacturing equipment, drive shafts, automobile and truck bodies, brakes, clutches, high-speed and precision machinery, flywheels, natural gas and hydrogen vehicle storage tanks, wind turbine blades, gas turbine engines, process industries equipment, x-ray tables and cassettes, prosthetics, orthotics as well as the numerous applications in Civil Engineering.

In addition to outstanding strength and stiffness combined with light-weight, composites offer unique and predictable physical properties, including thermal conductivities many times that of copper and thermal expansions which can be varied from high to near zero.  Advanced materials, some with ultrahigh thermal conductivities are being used in industrial motor cover/heat sinks, servers, notebook computers, power modules, plasma displays, printed circuit board heat sinks, and radiators.

Composites include a wide range of polymeric, metallic and ceramic materials having both high-temperature and low-temperature capabilities, making them useful for applications such as gas turbine engines, automobile and aircraft brakes, process industries equipment and cryogenic systems.

Over recent decades many new composites have been developed, some with very valuable properties. By carefully choosing the reinforcement, the matrix, and the manufacturing process that brings them together, engineers can tailor the properties to meet specific requirements. They can, for example, make the composite sheet very strong in one direction by aligning the fibres that way, but weaker in another direction where strength is not so important. They can also select properties such as resistance to heat, chemicals, and weathering by choosing an appropriate matrix material.

In terms of stress, the fibres serve to resist tension, the matrix serves to resist shear, and all materials present serve to resist compression.

Concrete

Concrete is a useful civil engineering material where aggregate (small stones or gravel) and sand are bound together with a water curing powder called Portland cement. The water in the concrete mix reacts with the cement in a chemical process known as hydration. This water is absorbed by the cement, which hardens, bonding the other components together and eventually creating a stone-like material. This is a classic composite material where there is a synergy between materials in that the composite (or combination) of materials is stronger and performs better than the individual materials themselves. Concrete is rigid and has good compression strength, while steel has high tensile strength. The result is a structure that is strong in both tension and compression.

Concrete is used more than any other man made material on the planet. It is used to make pavements, building structures, foundations, motorways/roads, over-passes, bridges, parking structures, brick/block walls and footings for gates, fences and poles.

Concrete ( a composite itself) and steel (a metal alloy) combine to create composite structures that are rigid and strong. In its simplest form, steel rods placed in the lower portion of a concrete beam, provide an enormous increase in the materials tensile strength. This reinforced concrete is suitable for fully supported structures in buildings, bridges and slabs that are laid on the ground. This strength can, however, be increased considerably by applying a tensile load to the steel, either before or after the concrete has set. This pre-tensioned steel provides a compressive load to the concrete that, upon loading, must be overcome before the concrete begins to suffer tensile failure.

Pre-tensioning may take one of two forms. Prestressed concrete is where the reinforcement is stretched in the mould prior to the pouring of the concrete. When the concrete has set and cured, the load is released from the reinforcement. In attempting to return to its original length, the reinforcement applies a compressive load to the concrete.

Post-tensioned concrete involves the concrete being cast in short sections with holes left for the reinforcing rods to be added later. These sections are assembled in position on falsework and threaded together with the reinforcing rods. These rods apply a compressive load to the concrete when they are tensioned.

Concrete is not, however, the perfect material. Its poor durability of is a major cause of problems in modern building and civil engineering structures in all countries: the annual cost of investigating and repairing deteriorating reinforced concrete structures runs into many millions. For more information visit corrosion.

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Activity 2:

Aim:
Nowadays it is common for breakfast juices to be packed in small boxes with a straw attached for convenient consumption.

fruit juice container

Fig 6. A typical fruit juice container

Often called poppas, these containers are manufactured from a composite material. The task here is to closely examine a fruit juice container by cutting open the box and determining the composition of the construction material.

Preparation:
To carry out this experiment you will need:

Method:
Step 1. After emptying the contents (enjoy) cut the box in half to observe the nature of the materials.
Step 2. Justify your observations

Answer

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So What Of The Future?

The modern material known as Carbon-fibre reinforced plastic is a similarly structured composite material to glass-fibre reinforced plastic (GFRP) with extremely low density and high strength making it suitable for aeronautical uses. It is likely that this material will find increasing uses in the Civil Engineering field.

Reinforced Carbon-Carbon (RCC) is a composite material consisting of carbon fibre reinforcement in a matrix of graphite, often with a silicon carbide coating to prevent oxidation. It was developed for the nose cones of intercontinental ballistic missiles, and is most widely known as the material for the nose cone and leading edges of the space shuttle. Although an expensive material, engineers are investigating the possibilities within civil engineering.

Civil Engineers are known to test the limits of building structures, by going higher, longer or lighter. On the other hand Civil Engineers are by definition very conservative. These two professional characteristics come together when Civil Engineers are exploring the exciting opportunities offered by the high-tech Engineering materials available to them today. The challenges to reduce weight, increase spans, build higher or slender constructions; automatically mean they must look at new engineering materials in their daring designs.

The use of composite structural solutions in civil engineering applications is increasing however. This continued growth requires the further development of these materials and an understanding of their behaviour. Particulate reinforced polymer composite systems, in particular vinyl ester/cenosphere composites appear to have significant potential to meet the requirements of civil engineering structures.

While bone is a naturally occurring composite material made of calcium phosphate (mineral) in a collagen (protein) matrix, recent attempts to grow artificial hydroxylapatite-bone composite grafts have proved successful. Although not relevant to civil engineering, this will be a milestone in the bio-engineering field.

Nanotechnology is, however, the field where we are likely to see the greatest research and development as the possibilities here are extraordinary. Nanotechnology involves the manipulation of structures at the atomic level. The matrix substance of many composites can contain micro-cracks that may seriously decrease the strength of the composite. Carbon nanotubes have the potential be used to detect defects at onset by embedding them uniformly throughout the composite material as a network capable of monitoring the health of the composite structures.

Nanotubes are so small they can penetrate the areas in between the bundles of fibre or between the layers of the composite, in the matrix rich areas. They are minimally invasive and just 0.15 percent of the total composite volume. Because the carbon nanotubes conduct electricity, they create a nanoscale network of sensors that work like the nerves in a human body. Researchers can pass an electrical current through the network and if there is a microcrack, it breaks the pathway of the sensors and can be measured.

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Web Links to Useful Resources

Food Packaging (external website)
Artificial Bone (external website)
Reinforced Carbon-Carbon (external website)
Thermal Protection System (external website)
Corrosion in Civil Structures
Geotextiles (external website)
Geofabrics (external website)
Sydney Harbour Bridge (external website)

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