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Composite materials are nothing new. Early builders put the principle to good use by reinforcing mud with twigs, with some of these structures still standing today. Wood is a naturally occurring composite, consisting of strong, reinforcing cellulose fibers in a weak lignin resin matrix. In recent years, an increasing amount of research has been made towards understanding the properties of composites to ultimately allow for their widespread acceptance among engineering designers.
Materials hardly ever approach their theoretical strength because of the presence of microflaws. Fine fibers can experience dramatic improvements in strength compared to the bulk material because they tend to contain fewer microflaws. In principle, composites benefit from the superior properties of these fibers, with the matrix able to transfer load to the fibers while also protecting them from environmental and physical damage.
‘Advanced composite’ has become the accepted term for materials manufactured from long or continuous reinforcing fibers, usually in excess of 50% by volume, embedded in a compatible matrix. One of the first systems developed was glass reinforced polyester (GRP).
Today, a large range of fibers and matrix materials are available and are now referred to using the more general term of fiber reinforced plastics (FRP). Advanced composites offer low density, corrosion resistance and good insulation properties, in addition to high strength and stiffness. They have become well-established materials for application in a diverse range of industries ranging from consumer products to space research.
Common Thermoset Compositions
Thermoset composites, which are commonly based on glass, carbon or aramid fibers, are usually incorporated with resins such as polyesters, vinyl esters, epoxies, bismaleimides, cyanate esters, polyimides or phenolics. The relative performance of fiber-reinforced epoxy composites is provided in Table 1.
- Composite materials are highly anisotropic, unlike most ‘traditional' engineering materials, which allows for the reinforcement to be placed only where needed in efficient designs
- The cured composite material is simultaneously produced with the structure, thereby limiting the ability to modify the material following the completion of its manufacturing
- Relevant property data can be difficult to find
- Material cost can form a high proportion of the product cost, although the lifetime cost analysis can be favorable
- Adhesive or bolted joints need careful consideration
- The recycling options for thermoset composites are limited
Between 1991 and 1994, the average annual growth rate for thermosetting composites in the U.S. was 8.9%. However, the growth rate during 1994 was around 11.6%, thereby indicating an increasing acceptance for structural thermosetting composite materials. As of 2017, the thermoplastic composites market had an estimated worth of $17 billion USD, and this value is expected to increase to as much as $42 billion USD by the year 2022.
More recently, various international companies within the car and aircraft manufacturing industries, such as BMW and Boeing, have shown an increased interest in utilizing thermoplastic composites for their products. Some of the most significant driving forces behind this accelerated use of thermoplastic composites is attributed to their lightweight and corrosion-resistant properties, as well as the lower costs associated with its manufacturing requirements.
Additional prospective industries that are expected to benefit from the growth of the thermoset composite market include wind blade, construction, infrastructure, tooling, robotics and biomedical industries.
Some designers are still not entirely comfortable with composite materials as a result of certain limitations including impact, creep and fatigue properties. Those not familiar with composites are justifiably uneasy about specifying materials when this information is not readily available. The situation is improving, particularly with respect to impact data.
The composites industry in general is now moving towards developing manufacturing and product standards. Several UK government projects, such as the Composites Innovation Cluster, which involved 31 different organizations that participated in 17 projects between 2012 and 2016.
With support from the Waste and Resources Action Programme’s Resource Efficiency Action Plan (REAP) for the Composites Industry, this largescale research initiative was primarily focused on reducing composite material-related waste and improving resource efficiency within applicable industrial sectors.
Still others view polymer composites as ‘plastics’, and any problem that they may have encountered with plastics can sometimes color their view of composites. For example, their fire performance is generally thought to be inferior, yet there are many grades of polymers that can be used for the composite matrix that are fire resistant; it is a matter of selecting the correct material for the application required.
Indeed, composite materials are finding application on offshore installations, where their balance of light weight, corrosion resistance and good mechanical performance, as well as excellent fire performance are brought to bear. Following the Piper Alpha disaster, several rigs have opted for composite fire and blast protection systems.
As composites are increasingly used, particularly in high profile structures such as the footbridge over the Aberfeldy River (UK), and understood, their place in the engineer's armory of materials will be assured.
Sources and Further Reading
This article was updated on 25th June, 2019.