Carbon Fibre – strong, stiff, & light weight fibres
Properties, process, history, and application of carbon fibres
Carbon fibres are extremely strong, thin fibre, made of aligned carbon crystals with the shape of a honeycomb and consisting of long, chainlike molecules of pure carbon that are made by charring synthetic fibres such as rayon in the absence of oxygen.
Physical/chemical/mechanical properties of carbon fibres
- Tenacity—1.8 —2.4 (kn/mm2)
- Density—-1.95 gms/c.c
- Elongation at break—-0.5%
- Elasticity —not good
- Moisture regain (M. R%)—-0%
- Resilience—–Not good
- Ability to protest friction—-Good
- Ability to protest heat—–Good
- Lustre—–Like silky
- Carbon is a poor conductor of electricity because it is a nonmetal, which in general are poor conductors of electricity.
- Carbon Fiber has High specific Strength to Weight Ratio
- Carbon Fiber is very Rigid
- Carbon fibre is Corrosion Resistant and Chemically Stable.
- Carbon fibre is Electrically Conductive
- Fatigue Resistance is good
- Carbon Fiber has good Tensile Strength
- Carbon fibre is Fire Resistance/Non-Flammable
- Thermal Conductivity of Carbon Fiber–it is not possible to pinpoint exactly the thermal conductivity. Special types of Carbon Fiber have been specifically designed for high or low thermal conductivity.
- Low Coefficient of Thermal Expansion–Carbon fibre can have a broad range and can be designed as per final requirement.
- Non-Poisonous, Biologically Inert, X-Ray Permeable–These qualities make Carbon fibre useful in Medical applications
- Carbon Fibers are brittle—The layers in the fibres are formed by strong covalent bonds.
Carbon Fiber is not yet geared to Amateur techniques
In order to maximize Carbon Fiber Characteristics, a relatively high level of technical excellence must be achieved. Imperfections and air bubbles can significantly affect performance. Typically, autoclaves or vacuum equipment is required. Moulds and mandrels are major expenses as well.
The success of any amateur carbon fibre construction will be closely linked to the skill and care taken.
- Effect of bleaching—-sodium hypochlorite slightly oxidized carbon fibres
- Effect of sunlight—-Do not change carbon fibres
- Protection against flame—–Excellent
- Protection ability against insects——Do not harm carbon fibres.
Carbon fibres are available in different forms
This tow gives a measure of its number of filaments per bundle. Small Tow has 24000 or fewer filaments, Large tow has 48000 to 320000 filaments and sometimes more. Normally is sold in large spools.
Often Carbon tow is used in winding tubes and other structures, selectively reinforcing structures and used in compression moulding. It can be used in chop guns but fibreglass can usually be substituted effectively.
Carbon fibre tape is available in 2″ wide 15feet long. Carbon tapes are used for reinforcement of carbon parts. It is easier to buy in this form than to cut your own stripes.
Carbon fibre is offered as bundled fibres. These are not woven but rather are bunches of unidirectional filaments. The terms Tow, Roving and Strand is more a measure of how large the bundle of fibre is than of actual differences in the form.
Cloth comes in many weights and wide ranges are availableHome builders usually deal with 3.5-12 ounces per square yard. but other weights are possible.
There are many types of weave. Most home builders are familiar with the plain weave. It is the easiest of weaves to wet out.
Twill weaves (right) are also available. Twill weave usually has the fibres spanning 2 fibres. It makes it easier to form to a shape. Twill weave is reputed to be slightly stronger than plain weave. It is certainly very showy.
Because the fibres bend up and down as they follow the weave there is a slight decrease in strength when compared with the uniaxial cloth that is made so that all the fibres run one way (except for the few that keep the cloth together.)
Matting or Felt
It is possible to find felted Carbon fibre but its use is not for reinforcement of matrix. IT has used in heat protection and safety applications. Home builders looking for a matted material normally use fibreglass.
Carbon Fiber Veil
Used in making corrosion resistant barriers and for grounding composite structures and minimizing static electricity build-up. The nonwoven structure allows a resin-rich surface that increases chemical stability and reduces micro-crack formation in the composite surface.
Carbon fibre sleeves or socks
These are available in 2 forms, either biaxially woven or unidirectional, with minimal stitching to keep the tube together until impregnated. These are very useful if making tubes or round pipes or masts. In the woven tubes, the fibres end up running diagonally along the tube. Woven tubes are very adjustable and can be used for several sizes simply by compressing lengthwise or stretching. This allows for tapered construction or for curves.
Unidirectional tubing has the fibres running lengthwise only. This allows the composite to be reinforced in the longitudinal direction. Uniaxial tubes need to be bought in the right size since there is no stretch.
Using these tubes simplifies making pipes, tubes and dinghy masts but they are quite expensive.
Coatings and surface treatment
In order to improve adhesion to the resin matrix, Carbon Fiber is sometimes surface treated. By partially oxidizing the fibre to make it more chemically active. Other coatings or treatments exist to modify the fibre’s handling such as polymer coatings.
Hybrid or Combinations
Carbon Fibre is sometimes combined with other fibres to modify its characteristics or to reduce cost.
One of the most common combinations is with Kevlar. Since Kevlar can be dyed, several companies now offer coloured carbon fibre – Kevlar fabrics.
When combining carbon fibre with metal it is good to remember that it is electrically conductive and can be involved in galvanic corrosion. The so-called Silver Carbon Fiber is actually aluminium and fibreglass.