Attributes and Production: There are a number of names applied to the same material: CRP, carbon fibre, carbon fibre reinforced plastic (hence the abbreviation) to name a few. Basically, the material is a composite of fibres that contain carbon, which in their “thread form” run parallel to one another and are embedded in a matrix: in the case of non-crimp fabrics they are lain in a criss-crossing pattern but in the case of textiles they are woven into a well-ordered weave. The matrix is a pourable, duroplastic plastic, most often epoxy resin or polyester.
CRPs have markedly good physical characteristics, especially their extraordinary firmness (strength) and rigidity – reactions actually run the gamut from surprise to disconcertedness when a three millimetre thin rod is placed in someone’s hand and he tries to bend it. Considering the amount of effort expended, it just will not bend the way you would expect. At the same time, this material is particularly lightweight, has great vibration dampening attributes and little tendency to fatigue even under conditions of continuous dynamic strain. To put it simply and graphically: CRP is much lighter than aluminium while being at least as strong as steel. Further advantages include a low amount of thermal expansion and a high resistance to corrosion.
During production the fine carbon fibres are bundled into individual strands (rovings) of a certain thickness. These are then further processed depending on the intended use: for example, they could be cut into small pieces and used for lamination purposes. In such cases, single or multidirectional non-crimp materials or, alternatively, fibre mats are formed by arranging the fibres in a criss-cross pattern or in one fixed direction, respectively. The sturdiest form of this material is achieved when it is further processed into textile weaves whereby the construction of the mesh can be exactly matched to the type of strain or load which the future structural element will have to endure. The highest degree of strength always corresponds with the direction of the fibres – a fact that obviously leads to the conclusion that a fabric structure in which the fibres run in many different directions will be stronger than one in which they run in only one direction.
Semi-finished products are mainly produced by means of three different processes:
- Pultrusion: Rovings are impregnated with a resin (polyester or epoxy – epoxy is more sturdy but also more expensive) and pulled into strands through an extrusion orifice, which shapes them before the subsequent hardening process. The rovings, resp. carbon fibres are in this case aligned in one direction, the semi-finished product being especially strong in the corresponding direction.
- Filament winding: Rovings are impregnated in resin and wound onto rotating cores in changeable angles. Different grades and strengths are produced depending on the thickness of the rovings and the thickness of the coating that has been applied.
- Prepreg: the easily seen textile structure, which is typically associated with carbon (and gladly imitated by adhesive film), comes about as a result of a reinforcement material that is preimpregnated with a resin matrix being wound onto a metal mandrel. In this process as well, it is possible to match the layer stackup to the requirements the material must meet and to adjust the angle of the fibres as needed. Prepregs have the highest degree of strength but are also the most expensive version.
In general, a higher amount of fibre is what is wanted in a material – usually between 50 and 80 percent: 60 percent fibre content should be considered as a standard to go by.
Applications: The future belongs to carbon! This statement may sound a bit oversimplified and platitudinous but the fact is that carbon is being used for more and more purposes. CRP is mainly used for rigid constructions, including:
- machine construction parts capable of withstanding a high degree of stress and which are highly vibration resistant, sturdy housings,
- sail battens, tennis racquets (because of its great vibration resistance, tennis arm has become much less of a problem than it was in the 70s), rigid kite frames and tent poles, frame pieces and, in the meantime, also seat supports for bicycles,
- flag and telescope poles, pointers, etc.
It is primarily used, however, for making very sturdy tubes, profiles or rods that are incredibly strong for their thickness and weight!
Treatment: CRP articles can be cut with a hacksaw or a circular saw; the ends, however, will indeed tend to fray and even further treatment will not improve the situation a great deal. A band saw with a diamond band saw blade is the ideal tool to work with; i.e. a very hard band saw blade with a relatively tight pattern of small teeth (gulleted edge configuration). A PRECISION DRILL GRINDER with a (diamond coated) CUTTING DISC can also be used with good results.
CRP can be drilled using a conventional carbide drill (with rather low rpms) but, here too, a bit that is diamond coated is recommended. The holes will not fray as much if a soft material (wood, plastic) is placed underneath when drilling.
Best gluing results are achieved when using a two-component epoxy adhesive. When elongating tubes or rods, it is best to employ an (additional) outer or inner mechanical connector like a sleeve or coupler.
Semi-finished products made from CRP take well to being painted but it should be noted that they often have a film of releasing agent on their exterior surface. So, as is generally the case with painting work, you should sand and clean the surface beforehand.