Carbon Fiber Characteristics

Carbon Fiber's reputation as a material has taken on mystical proportions! Not only does it have a reputation for being the best and the strongest, but it's also become cool to have something made of carbon fiber!

I don't claim to be an expert. This article combines information that I have found in my research on Carbon Fiber, I am not a primary source. I try to be accurate but I MAKE MISTAKES, I know this might come as a surprise to some of you, but there it is. If you plan to build a mast or other boaty things, do your research. Consult an expert, and be careful. Have Fun.

First, What is Carbon Fiber

Carbon Fibre, not surprisingly, is made of carbon crystals aligned in a long axis. These honeycomb shaped crystals organize themselves in long flattened ribbons. This crystal alignment makes the ribbon strong in the long axis. In turn these ribbons align themselves within fibers. The fiber shape is the original shape of the material (its precursor) used to produce the Carbon Fiber. I don't know of any process where fibers are shaped AFTER carbonizing. These fibers (containing flat ribbons of carbon crystals) in turn are bundled by the manufacturer in thicker fibres and are woven into carbon cloth, made into felt, twisted or bundled without twisting. This is referred to as Roving. Carbon fiber is also offered as chopped strands and powder.

In order to modify the characteristics of the lay up, other materials are sometimes added such as glass fibers, Kevlar or Aluminium. Carbon fiber is rarely used as it. Rather it is embedded in a matrix. In mast-making and boat building we usually think of epoxy or polyester resins, but carbon fiber is also used as reinforcement for thermoplastics, concrete or ceramics.

Manufacturing of Carbon Fibre

There are several methods of making carbon fiber but essentially they all begin with the making of fibers out of a carbon rich precursor material. The original size and shape of the fibre will remain in the finished carbon fiber, but the interior chemical structure will have been greatly modified through the various heating cycles. The first steps are carbonizing and stretching precursor fibers, either PAN: Polyacrylonitrile, Pitch or Rayon. There are several cycles of heating at varying temperatures excluding oxygen. This process drives off most of other elements (hydrogen and nitrogen mainly) of the starting material leaving carbon behind. It also allows the carbon to gradually crystallize in its characteristic honeycomb way. If you haven't seen it yet, go to my Carbon Research page and look at the video on Carbon fibre structure. it's fabulous.

This Youtube video shows carbon fibre being made and is worth a look.

The most important factors determining the physical properties of carbon fiber are degree of carbonization (carbon content, usually more than 92% by weight) and orientation of the layered carbon planes (the ribbons). Fibers are produced commercially with a wide range of crystalline and amorphous contents variations to modify or favour the various properties.

Depending on the starting material and process of carbonization Carbon fiber is modified to suit the end purpose. PAN or polyacrylonitrile is the most common precursor for plastic composites.

The main variations of characteristics is strength vs stiffness. By using different heating cycles either can be emphasized. Research is being done to modify other features such as heat and electrical conductivity.

Not only is internal structure of the fibres important but how they are lined up in the finished products has an enormous impact on the properties of the item being made. Proper alignment of the carbon fibres is essential to maximize their benefits.

Carbon Fiber Properties, What's Not to Love!!

1. High Strength to weight ratio
2. Rigidity
3. Corrosion resistance
4. Electrical Conductivity
5. Fatigue Resistance
6. Good tensile strength but Brittle
7. Fire Resistance/Not flammable
8. High Thermal Conductivity in some forms
9. Low coefficient of thermal expansion
10. Non poisonous
11. Biologically inert
12. X-Ray Permeable
13. Relatively Expensive
14. Requires specialized experience and equipment to use.

I've not written in detail, but carbon fibre is self lubricating, it also has Excellent EMI (Electromagnetic Interference) Shielding Property

1- Carbon Fiber has High Strength to Weight Ratio (also known as specific strength)

Strength of a material is the force per unit area at failure, divided by its density. Any material that is strong AND light has a favourable Strength/Weight ratio. Materials such as Aluminium, titanium, magnesium, Carbon and glass fiber, high strength steel alloys all have good strength to weight ratios. It is not surprising that Balsa wood comes in with a high strength to weight ratio.

The following figures are offered for comparison only and will vary depending on composition, alloy, type of spider, density of wood etc. The units are kN.m/kg.

Note that strength and rigidity are different properties, strength is resistance to breaking, rigidity is resistance to bending or stretching.

Because of the way the crystals of carbon fibre orient in long flat ribbon or narrow sheets of honeycomb crystals, the strength is higher running lengthwise than across the fibre. That is why designers of carbon fibre objects specify the direction the fibre should be laid to maximize strength and rigidity in a specific direction. The fibre being aligned with the direction of greatest stress.

Pan based precursor carbon fibre has higher strength than pitch based carbon fibre which has higher stiffness.

2- Carbon Fiber is very Rigid

Rigidity or stiffness of a material is measured by its Young Modulus and measures how much a material deflects under stress. Carbon fiber reinforced plastic is over 4 times stiffer than Glass reinforced plastic, almost 20 times more than pine, 2.5 times greater than aluminium. For more information on stiffness and how it is measured, plus a comparison table of different materials see my Young Modulus page.

Remember stress is force, strain is deflection such as bending or stretching

3- Carbon fiber is Corrosion Resistant and Chemically Stable.

Although carbon fibers themselves do not deteriorate measurably, Epoxy is sensitive to sunlight and needs to be protected. Other matrices (whatever the carbon fiber is embedded in) might also be reactive.

Carbon fibres can be affected by strong oxydizing agents

Composites made from carbon fibre must either be made with UV resistant epoxy (uncommon), or covered with a UV resistant finish such as varnishes.

4- Carbon fiber is Electrically Conductive

This feature can either be useful or be a nuisance. In Boat building conductivity has to be taken into account just as Aluminium conductivity comes into play. Carbon fiber conductivity can facilitate Galvanic Corrosion in fittings. Careful installation can reduce this problem.

Carbon Fiber dust can accumulate in a shop and cause sparks or short circuits in electrical appliances and equipment.

There is currently quite a lot of R&D on using the electric conductivity of carbon fibre to produce heat either for faster curing of composite materials, or for the heating capabilities themselves. This might have application in winter clothing or clothing destined for harsh environments.

5- Fatigue Resistance is good

Resistance to Fatigue in Carbon Fiber Composites is good. However when carbon fiber fails it usually fails catastrophically without significant exterior signs to announce its imminent failure.

Damage in tensile fatigue is seen as reduction in stiffness with larger numbers of stress cycles, (unless the temperature is high)

Test have shown that failure is unlikely to be a problem when cyclic stresses coincide with the fiber orientation. Carbon fiber is superior to E glass in fatigue and static strength as well as stiffness.

The orientation of the fibers AND the different fiber layer orientation, have a great deal of influence on how a composite will resist fatigue (as it has on stiffness). The type of forces applied also result in different types of failures. Tension, Compression or Shear forces all result in markedly different failure results.

Paper by Oak Ridge National Laboratory, on test of carbon fiber composites intended for automotive use. American Institute of Aeronautics and Astronautics, test for materials to be used in wind turbines blades.

6- Carbon Fiber has good Tensile Strength

Tensile strength or ultimate strength, is the maximum stress that a material can withstand while being stretched or pulled before necking, or failing. Necking is when the sample cross-section starts to significantly contract. If you take a strip of plastic bag, it will stretch and at one point will start getting narrow. This is necking. Tensile Strength is measured in Force per Unit area. Brittle materials such as carbon fiber does not always fail at the same stress level because of internal flaws. They fail at small strains. (in other words there is not a lot of bending or stretching before catastrophic failure) Weibull modulus of brittle materials

Testing involves taking a sample with a fixed cross-section area, and then pulling it gradually increasing the force until the sample changes shape or breaks. Fibers, such as carbon fibers, being only 2/10,000th of an inch in diameter, are made into composites of appropriate shapes in order to test.

Units are MPa This table is offered as a comparison only since there are a great number of variables.

NOTE: When testing carbon fiber, and other fibres and non homogenious materials, samples much be made that are consistant and comparable. This is not a simple procedure. If you read research where strength/stiffness is compared, the researchers will always explain how their samples were manufactured including the type of matrix, alignment of fibres, ratio of fibres to matric among other factors. This difficulty explains why measurements can vary quite a lot between research results.

7- Fire Resistance/Non Flammable

Carbon fibre is classified as non conbustible and has no listed flash point. If it is exposed to high heat in the presence of fuel burning, it can eventually oxidize but as soon as the flame and fuel is removed the flame does not continue.

Because carbon fibre is almost always used in a matrix such as epoxy, plastic or concrete, the tolerance of the matrix to high temperature is the more significant factor.

Depending upon the manufacturing process and the precursor material, carbon fiber can be be made to feel quite soft to the hand and can be made into or more often integrated into protective clothing for firefighting. Nickel coated fiber is an example. Because carbon fiber is also chemically very inert, it can be used where there is fire combined with corrosive agents.HIGH TEMP FELT WELDING BLANKET - BLACK, 18" X 24" These felt carbon fibre blankets are also used to protect substrates when doing plumbing soldering.

8- Thermal Conductivity of Carbon Fiber

See my article on Heat Conductivity of Carbon Based materials including carbon fibre, nanotubes and graphene.

Thermal conductivity is the quantity of heat transmitted through a unit thickness, in a direction normal to a surface of unit area, because of a unit temperature gradient, under steady conditions. In other words it's a measure of how easily heat flows through a material.

There are a number of systems of measures depending on metric or imperial units.

1 W/(m.K) = 1 W/(m.^oC) = 0.85984 kcal/(hr.m.^oC) = 0.5779 Btu/(ft.hr.^oF)

This table is only for comparison. The units are W/(m.K)

Because there are many variations on the theme 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. There are also efforts to Enhance this feature.

The Materials Information Society has a page on "graphite" AKA Carbon Fiber

9- Low Coefficient of Thermal Expansion

This is a measure of how much a material expands and contracts when the temperature goes up or down.

Units are in Inch / inch degree F, as in other tables, the units are not so important as the comparison.

Carbon fiber can have a broad range of CTE's, -1 to 8+, depending on the direction measured, the fabric weave, the precursor material, Pan based (high strength, higher CTE) or Pitch based (high modulus/stiffness, lower CTE).

In a high enough mast differences in Coefficients of thermal expansion of various materials can slightly modify the rig tensions.

Low Coefficient of Thermal expansion makes carbon fiber suitable for applications where small movements can be critical. Telescope and other optical machinery is one such application.

10-11-12 Non Poisonous, Biologically Inert, X-Ray Permeable

These quality make Carbon fiber useful in Medical applications. Prosthesis use, implants and tendon repair, x-ray accessories surgical instruments, are all in development.

Although not poisonous, the carbon fibers can be quite irritating and long term unprotected exposure needs to be limited. The matrix either epoxy or polyester, can however be toxic and proper care needs to be exercised.

13- Carbon Fiber is Relatively Expensive

Although it offers exceptional advantages of Strength, Rigidity and Weight reduction, cost is a deterrent. Unless the weight advantage is exceptionally important, such as in aeronautics applications or racing, it often is not worth the extra cost. The low maintenance requirement of carbon fiber is a further advantage.

It is difficult to quantify cool and fashionable. Carbon fiber has an aura and reputation which makes consumers willing to pay more for the cachet of having it.

You might need less of it compared to fiberglass and this might be a saving.

Fibre Glast Real Carbon Fiber Fabric - 3K, 2 X 2 - Twill Weave - 1 yd Roll

Noahs supplies Carbon Fiber and Glass cloth to amateur boat builders, wander in their online catalog and compare the prices. (Canadian Store)

14- Carbon Fibers are brittle

The layers in the fibers are formed by strong covalent bonds. The sheet-like aggregations readily allow the propagation of cracks. When the fibers bend they fails at very low strain. In other words carbon fibre does not bend much before failing.

Effect of varying carbon fibre and matrix ratio

15- 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 fiber construction will be closely linked to the skill and care taken.

Here is a link to a YouTube video by Easy Composite Ltd. on various techniques using Carbon Fibre. They have a number of instructional video.

This article is a work in progress. I will continue to add information and refine the text as I explore the subject. The information I have presented comes from a variety of sources. I have tried to verify it when I could. I try to use "reliable sources" such as manufacturer's data, research papers or university articles. I also rely on Wikipedia for an overview.
Good article from the University of Tennessee on Carbon Fiber Production.
Christine.

email me if you find mistakes, I'll fix them and we'll all benefit: Christine