Truss Structures Made of Carbon-fibre-reinforced Plastic
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KeywordsSponge Wind Turbine Fibre Bundle Truss Structure Wind Turbine Tower
Truss structures made of carbon-fibre-reinforced polymer (CFRP) stand out for being lightweight and yet still offer excellent mechanical properties. At the Institute for Textile Technology (ITA) at RWTH Aachen University, a CFRP truss structure derived from biological origins was constructed. Using 3-D rotation weaving and a vacuum infusion method resulted in a glass sponge-like, or Euplectella structure.
Truss structures made of carbon-fibre-reinforced plastic (CFRP) have great potential for use in a wide range of lightweight products. They have also established themselves as a rival to conventional steel structures. The current hurdle to overcome for CFRP truss structures are what tend to be complex and thus expensive manufacturing methods .
The term truss refers to a material system comprising multiple interconnected rod members. The points at which the rods connect to each other are known as nodes. Trusses made of metal or wood may feature nodes in the form of joints, welds or exoskeletons attached to the rod members .
The State-of-the-art: CFRP Truss Structures
Right now, scattered examples of CFRP truss structures are used in a range of applications. So-called isogrid structures are used in rocket construction. Triangular ribs arranged in the form of a grid tube function either directly as a support or alternatively as a means of reinforcing an outer shell. These isogrid structures are more than capable of withstanding tensile, compressive, shear and bending stresses. There is scope to make the construction even more rigid by using unidirectional fibre bundles. There is also the option of local reinforcement to help accommodate any point loads exerted .
A filament winding method is used when manufacturing isogrid structures . Filament winding refers to a method whereby fibres or fibre bundles already impregnated in a matrix material are wound around a certain shape. Using CNC controllers enables fully automated winding and accelerates the overall process. This approach is ideal for manufacturing simple and rotationally symmetrical components .
Another example of a CFRP truss structure is the so-called IsoTruss design from IsoTruss Industries, Pleasant Grove, USA. It combines the advantages of trusses with the material-specific properties of fibre composites. IsoTruss is an open, cage-like grid tube structure formed from a series of intersecting triangles. IsoTruss structures can be based on a range of geometrical designs featuring 6 to 12 intersections. The number of nodes in the base geometry in question is also the parameter that dictates the structural characteristics. For example, a six-node structure excels in terms of resistance to transverse stresses .
The individual beam elements of this structure are in a fibre bundle wound around a metallic mandrel. The fibres of the individual rod elements are interwoven at the nodal points . As well as being highly stable and lightweight, these open structures are also sturdier in response to wind or other buckling loads. The unique construction method means that IsoTruss structures can absorb and retransmit forces from all directions .
Like their isogrid peers, these structures are manufactured using a special filament winding method. The carbon fibres are wound around a metallic mandrel, which facilitates the shaping as well as mechanically supporting the fibres, and then cured. As of now, the technology remains at the prototype stage and is used to make small-scale batches of supporting structures or bicycle frames .
All of which means CFRP truss structures currently claim a niche-like existence. One of the main reasons is the high cost of manufacturing. For industries without a generous budget for lightweight design, manufacturing costs for CFRP truss structures are simply not viable compared to alternative designs made of steel or aluminium. One possible alternative could be 3-D rotation weaving, given its scope for full automation and consequent cost savings and its equal ability to handle complex woven structures. This offers the potential to establish the use of CFRP truss structures in a further area.
Modelled on the Deep-sea Sponge
Its Euplectella structure is based on a natural model: the Euplectella Aspergillum deep-sea sponge. This creature uses its unique skeletal structure to withstand ambient conditions at depths of up to 5000 m. Its skeleton actually comprises a type of biological glass fibre. Despite the very brittle nature of the material making up the deep sea sponge and its lack of resistance, the skeleton of the Euplectella Aspergillum is virtually indestructible. It can withstand both the pressure at sea depths as well as stresses from crab claws.
The skeleton of the Euplectella Aspergillum is made up of seven hierarchically arranged levels. In terms of size, these individual levels range from a few nanometres to several centimetres. The glass fibres of the sponge are what underpin the skeleton structure. These fibres, which are also known as needles, comprise concentrically arranged glass slats. They are composed of silicate nanoparticles, 50 to 200 nm in diameter, which, in turn, comprise individual particles of around 3 nm in diameter. The slats are interconnected via a type of organic matrix. This structure is what prevents the fibres breaking since cracks are interrupted by the interim layers of organic material.
Multiple fibres of varying diameter are what form the so-called structural rods. Here, the individual fibres are interconnected via glass cement, which is also a material made up of silicate nanoparticles.
In general, fibre bundles with fibres of varying thickness exhibit greater strength (defect tolerance) than bundles made up of fibres of equivalent diameter. The bundled structural rods are arranged horizontally and vertically within a cylindrical cage and woven into a loose network. Every second square formed in this way is traversed by two additional diagonal rods, each in a different direction.
The stability of Euplectella Aspergillum is enhanced even further thanks to the so-called ribs. Approximately every other diagonal element forms a slight increase in height, which is a prominent feature on the external structure of the sponge. The deep-sea sponge reaches 20 to 30 cm in length .
Manufacturing Method: From Fibre to End Product
All switches and impellers can be individually controlled. A yarn-breakage sensor monitors the weaving process and stops it if any damage is incurred. This approach to weaving permits braid angles of 20° to 70°.
IsoTruss structures can be based on a range of geometrical designs featuring 6 to 12 intersections.
In future, the Euplectella structure is likely to offer an alternative to conventional support structures. One potential application could be in the wind energy sector. Renewable energy accounts for up to 20 % of domestic German power. The proportion of renewable energy is set to climb far higher in future . To ensure that power is generated efficiently in the process, ever-larger wind turbines will be constructed, well over 100 m high, since the specific energy supplied by the rotor, particularly onshore, rises with increasing tower height .
The tower installed as part of the average wind energy installation constitutes around 25 % of the overall manufacturing costs. The high transport costs and complexity of installation also hamper the current level of tower technology.
Wind turbine towers stand out for their high breaking strength, fatigue resistance and rigidity. Most of the current wind energy installations feature a hollow tower made of steel or reinforced concrete. Alternatively, the tower can be manufactured from a steel-truss structure. As conventional structures increase in height, the specific price per metre for such towers shoots up. For example, the average specific price for an 80 m reinforced concrete tower high is around 6500 Euro/m, rising to 8500 Euro/m for 160 m. This effect makes such constructions less economically viable and opens the door for alternative technologies .
The unique design of the Euplectella structure also offers weight-specific, high compressive strength. Compared to other CFRP truss constructions, the use of special 3-D weaving methods means that the individual fibres and fibre bundles can be woven at various levels together, allowing optimal absorption of all forces exerted on the structure. Having a structure with holes reduces susceptibility to wind stress and also means that the buckling loads exerted are far less than would apply to a solid reinforced concrete tower with a continuous shell surface.
A further potential application for the Euplectella structure is its use as a support structure for drilling rigs. Just like the naturally occurring structural configuration, the open CFRP structure is equally capable of withstanding transverse loads exerted by currents as well as pressure under water. Furthermore, high tensile and compressive strength mean that heavy loads such as oil rigs can also be accommodated. One further plus of CFRP components is their effective water resistance in comparison to their corrosion-prone steel equivalents.
In future, to make such towers or support structures more technically and economically viable, further development fine-tuning of the Euplectella structure will be required: For wind energy installations well in excess of 100 m and drilling rig support structures, which have to cope with seawater several hundred metres deep at times, individual tower segments must be joined together on site since transporting such units presents a huge challenge. This creates the need for a mechanism or method such as using an exoskeleton at joint locations or a special gluing mechanism in order to ensure that the positive mechanical properties of the structure remain unimpaired at all the joints. Other steps must also be taken to optimise the weaving speed as well as the setting-up of the weaving machine to boost the commercial potential.
- Schuetze, R.: Neue Entwicklungen leichter CFK-Fachwerkstrukturen, VDI-Bericht 1080 1994Google Scholar
- Sayir, M.; Dual, J.; Kaufmann, S.; Mazza, E.: Ingenieurmechanik 1 — Grundlagen und Statik. 3rd edition, Wiesbaden: Springer Vieweg, 2015Google Scholar
- Kim, D.: Fabrication and testing of composite isogrid stiffened cylinder. In: Composite Structures, Vol. 2, Iss. 1, 1999Google Scholar
- IsoTruss: http://www.isotruss.com/manufacturing/, accessed 12/08/2016
- Aizenberg, J.; Weaver, J.; Thanawala, M. et al.: Skeleton of Euplectella sp.: Structural Hierachy from the Nanoscale to the Macroscale. In: Science, 2005, Vol. 309, no. 5732Google Scholar
- Jarass, L.; Obermair, G.; Voigt, W.: Windenergie — Zuverlässige Integration in die Energieversorgung. Berlin, Heidelberg: Springer-Verlag, 2009Google Scholar
- Hau, E.: Windkraftanlagen: Grundlagen, Technik, Einsatz, Wirtschaftlichkeit. 5th edition, Berlin, Heidelberg: Springer-Verlag, 2014Google Scholar