# Carbon fiber reinforced plastic

properties

Fiber type: Carbon fiber HT
Matrix type: Epoxy resin Fiber
volume fraction 60%
All values ​​are typical
average values

Basic elasticity quantities
${\ displaystyle E _ {\ |}}$ 140,000 N / mm²
${\ displaystyle E _ {\ perp}}$ 12,000 N / mm²
${\ displaystyle G _ {\ perp \ |}}$ 5,800 N / mm²
${\ displaystyle G _ {\ perp \ perp}}$ 5,400 N / mm²
${\ displaystyle \ nu _ {\ | \ perp}}$ 0.26
density
${\ displaystyle \ rho}$ 1.5 g / cm³
Basic strengths
${\ displaystyle R _ {\ |} ^ {+}}$ 2,000 N / mm²
${\ displaystyle R _ {\ |} ^ {-}}$ 1,500 N / mm²
${\ displaystyle R _ {\ perp} ^ {+}}$ 70 N / mm²
${\ displaystyle R _ {\ perp} ^ {-}}$ 230 N / mm²
${\ displaystyle R _ {\ perp \ |}}$ 90 N / mm²
Coefficient of thermal expansion
${\ displaystyle \ alpha _ {\ |}}$ 0.2 · 10 −6 K −1
${\ displaystyle \ alpha _ {\ perp}}$ 30 · 10 −6 K −1

Carbon fiber reinforced plastic , also carbon fiber reinforced plastic ( CFRP ), also shortened carbon fiber , carbon or carbon , is a composite material in which carbon fibers are embedded in a plastic matrix . The matrix is ​​used to connect the fibers and to fill the gaps. The material epoxy resin is usually chosen as the matrix . However, other thermosets or thermoplastics are also possible as matrix material.

CFRP is used in particular where the increased costs are accepted for a low mass and high rigidity at the same time. Examples include bicycle frames , fishing rods , row boats and accessories, walking sticks, and pressurized gas containers. In 2015, the global production of CFRP was 91,000 tons with an annual growth of approx. 12%.

## description

CFRP consists of carbon fibers that are embedded in a matrix made of synthetic resin. The mechanical properties of the cured composite benefit above all from the tensile strength and rigidity of the carbon fibers. The matrix prevents the fibers from shifting against each other under load. For this, the matrix must adhere to the fiber, otherwise the components will fail due to fiber pull-out . Since the composite material benefits primarily from the properties of the fiber material, the highest possible fiber volume fraction is usually sought - as much fiber as possible, as little matrix as possible, no air pockets or cavities, but all fiber surfaces should be wetted with matrix.

The strength and rigidity of a material made from CFRP are significantly higher in the direction of the fibers than across the direction of the fibers. The strength across the fiber is lower than that of an unreinforced matrix. Therefore z. T. individual fiber layers laid in different directions. The fiber directions are determined by the designer in order to achieve a desired strength and stiffness. The entire design of a component is usually supported by calculations based on the classic laminate theory .

In comparison to materials like steel, carbon fibers have a significantly lower density (~ factor 4.3). Their weight-specific stiffness in the direction of the fibers is, depending on the type of fiber, somewhat (approx. 10–15%) or even significantly (approx. Factor 2) higher than steel. In this way, a very stiff material is created, which is particularly suitable for applications with one main load direction, where a low mass with high rigidity is important. Often, fiber composite components have to be designed to be more voluminous in order to withstand the same forces as a corresponding metal component, which reduces the weight advantage.

## Applications

The mountain bike Biria unplugged from 1998
Ice hockey stick, broken.

Components made from fiber-reinforced materials are expensive to manufacture compared to metal components with the same load capacity. Therefore, they are mainly used in areas in which their advantages (mostly weight savings) result in at least a correspondingly high cost-saving potential:

Sometimes the weight saving is also supported by other interests, such as B. more comfortable handling or slower user fatigue:

Some automobile manufacturers offer elements of the vehicle interior made of carbon or with a carbon look , mainly for aesthetic reasons . Often only the top layer of a component is coated with carbon , since the mechanical properties of the material are not required and costs can be reduced.

As a filament for the Fused Deposition Modeling 3D printing process , carbon fiber reinforced plastic (as a filament: polyamide CF) is used due to its high strength and tear length, especially for the production of stable but light tools and parts.

## Manufacturing process

Carbon fiber fabric for the production of CFRP
The sculpture Mae West in Munich is made of CFRP pipes with a diameter of about 25 cm (base up to 15.5 m made of CFRP-wrapped steel).

The manufacturing processes correspond to those of glass fiber reinforced plastic (GRP). Above all, processes are used with which high-quality fiber composites can be produced ( prepreg in a press or autoclave process , fiber winding, infusion or RTM processes). CFRP hand laminates, on the other hand, are used almost exclusively in small series production and in single-item production, as only lower fiber volume percentages can be achieved with them.

A high fiber volume fraction is desired for the greatest possible rigidity . Air inclusions reduce the bonding of the matrix to the fiber and introduce notch stresses into the laminate. Under stress, individual air bubbles can combine and lead to delamination, i.e. H. local detachment of the individual layers from one another. Furthermore, the humidity in the bubbles can freeze on the bubble wall at low temperatures. Due to the higher specific volume of the ice, this can lead to the formation of a crack in the fine pores of the bladder wall. Therefore, CFRP manufacturing processes aim to achieve a product free of air bubbles.

### Carbon fiber reinforced carbon

If a plastic matrix phenolic resin used and the matrix is then pyrolyzed at temperatures of 800-900 ° C under protective gas (nitrogen), a new class of materials, which can carbon fiber reinforced carbon (engl. Carbon-fiber-reinforced carbon , CFC , CRC or CFRC ) to be developed. Phenolic resin shows a carbon yield> 50  % by weight , which results in a porous carbon matrix. This is reinforced by the carbon fibers. Repeated impregnation and pyrolysis with phenolic resin or other materials with a high carbon yield, for example liquid pitch , can fill the porous portion and the carbon matrix can be made more dense with each impregnation and pyrolysis stage.

The porous matrix structure can also be filled up via gas phase pyrolysis of carbon-containing gases. However, this process is more tedious than the liquid-phase infiltration process with subsequent pyrolysis .

## Health risks

The mechanical (synonym: cutting) processing of CFRP (sawing, milling, drilling, grinding, etc.) generates potentially carcinogenic carbon fiber particles.

“With the increasing use of CFRP, the increase in adhesive bonds to ensure fiber-compatible joining is closely linked. However, chip-removing processes are required in places in order to produce bondable surfaces. The resulting carbon fiber particles are considered to be potentially carcinogenic, so that the need for appropriate occupational safety precautions arises. "

- Dr. Hubert Pelc : Machining adhesive surface preparation for CFRP components

According to Bundeswehr experts, CFRP fibers should be released in a fire, which could have an effect comparable to that of asbestos . Above all, there would be a danger for helpers at accident sites, such as fire fighters or police officers. An expert cites a radius of approx. 300 meters around an accident with burning CFRP as a guide value.