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The purpose of this design guide is to provide general information on graphite
(carbon fiber) composite materials and some guidelines for designing lightweight
high performance products with graphite composites. By LightSpeed Performances
engineering consultant, Francis Hu
Graphite (Carbon Fiber) Composite Materials (Definition and History)
Composite materials are made by combining reinforcement (fiber) with matrix
(resin), and this combination of the fiber and matrix provide characteristics
superior to either of the materials alone. In a composite material, the fibers
carry majority of the loads, and are the major factor in the material properties.
The resin helps to transfer load between fibers, prevents the fibers from buckling,
and binds the materials together.
Graphite composites have exceptional mechanical properties which are unequaled
by other materials. The material is strong, stiff, and lightweight. Graphite
composite is the material of choice for applications where lightweight &
superior performance is paramount, such as components for spacecrafts, fighter
aircrafts, and racecars.
Graphite fibers (sometimes called carbon fibers) are made from organic polymer
such as polyacrylonitrile. The material is drawn into fibers and kept under
tension while it is heated under high temperature (> 1000C). 2 dimensional
carbon-carbon crystals (graphite) are formed when the hydrogen is driven out.
The carbon-carbon chain has extremely strong molecular bonds (diamond is a 3
dimensional carbon-carbon crystal), and that is what gives the fibers its superior
mechanical properties.
Historically, graphite composites have been very expensive, which limited
its use to only special applications. However, over the past fifteen years,
as the volume of graphite fiber consumption has increased and the manufacturing
processes have improved, the price of graphite composites has steadily declined.
Today graphite composites are economically viable in many applications such
as sporting goods, performance boats, performance vehicles, and high performance
industrial machinery.
Applications of Graphite Composite Materials
Composite materials are extremely versatile. The engineer can choose from
a wide variety of fibers and resins to obtain the desired material properties.
Also the material thickness and fiber orientations can be optimized for each
application.
The three greatest advantages of carbon fiber (graphite) composites are:
- High specific stiffness (stiffness divided by density)
- High specific strength (strength divided by density)
- Extremely low coefficient of thermal expansion (CTE)
Please see table 1 for a comparison of costs and mechanical properties of
carbon fiber composite, fiberglass composite, aluminum, and steel. Due to the
wide variety of graphite fibers and resins available, and the numerous combinations
of the materials, the properties are listed in ranges.
TABLE 1
| |
Carbon fiber Composite (aerospace grade) |
Carbon fiber Composite (commercial grade) |
Fiberglass Composite |
Aluminum, 6061 T-6 |
Steel, Mild |
| Cost $/LB |
$20-$250+ |
$10-$40 |
$1.50-$3.00 |
$6 |
$0.60 |
| Strength (psi) |
90,000-200,000 |
50,000-90,000 |
20,000-35,000 |
35,000 |
60,000 |
| Stiffness (psi) |
10 x 106 - 50 x 106 |
8 x 106 - 10 x 106 |
1 x 106 - 1.5 x 106 |
10 x 106 |
30 x 106 |
| Density (lb/in3) |
0.050 |
0.050 |
0.055 |
0.10 |
0.30 |
| Specific Strength |
1.8 x 106 - 4 x 106 |
1 x 106 - 1.8 x 106 |
363,640 - 636,360 |
350,000 |
200,000 |
| Specific Stiffness |
200 x 106 - 1,000 x 106 |
160 x 106 - 200 x 106 |
18 x 106 - 27 x 106 |
100 x 106 |
100 x 106 |
| CTE (in/in-F) |
-1 x 10-6 - 1 x 10-6 |
1 x 10-6 - 2 x 10-6 |
6 x 10-6 - 8 x 10-6 |
13 x 10-6 |
7 x 10-6 |
Applications for High Specific Stiffness
Carbon fiber composites are ideally suited for applications where high stiffness
and low weight is required. Most metals used for structural applications have
very similar specific stiffness, which is around 100 x 106. If an application
demands high stiffness and lightweight, carbon fiber composites is the only
material of choice.
Examples are:
- Motorcycle components (fork guards, canisters, end caps)
- Spacecraft structure
- Aircraft structure
- Drive shaft for trucks and high performance vehicles
- Machinery rollers
- Sail boat mast and boom
- Bicycle frame
- Machinery components that experience high acceleration & require stiffness & precision
Applications for High Specific Strengths
Carbon fiber composites are widely used for lightweight structures that need to carry extremely high loads.
Examples are:
- Motorcycle components (skid plates, rock guards)
- Fishing pole
- Golf club shaft
- Aircraft structure
- Satellite antenna structures
- Racecar chassis
Applications for Low CTE
Graphite fiber has a negative coefficient of thermal expansion, which means
when it is heated it will shrink. When the graphite fibers are put into a resin
matrix (positive CTE), the composite can be tailored to have almost zero CTE.
Graphite composites are used for high precision and thermally stable applications.
Examples are:
- High precision antennas
- Scanning & imaging machines
- Precision optical devices
- Metrology equipment
Manufacturing Process
Carbon fiber composite components are manufactured utilizing a molding process.
The graphite fibers can be woven into cloth, braided into tubes, or made into
unidirectional tapes. The fibers are next coated with resin. This fiber &
resin mix can be partially cured then frozen to create a pre-preg, or the fiber
& resin mix can be used wet. The graphite fiber & resin mix is then
placed into a mold in layers. The number of layers and the orientation of the
layers will depend on the mechanical properties desired. The layers of graphite
is then compacted and consolidated in the mold by pressure from a press, autoclave
and or from a vacuum bag. Depending on the resin system, the part can be cured
at room temperature or elevated temperature. Once the part is cured, the part
is removed from the mold, and it is ready for finishing operations such as trimming
and drilling.
Design Information
Carbon fiber composites are considered designer’s material, because the parts
can be tailored to have strength and or stiffness in the directions and locations
that are necessary by strategically placing materials and orienting fiber direction.
Also the design and manufacturing flexibility that graphite composites offers
provide opportunities to consolidate parts and to integrate many features into
the part to further enhance the design and reduce the total part price. Some
general design guidelines are listed below:
| Material Thickness |
Typically range from .040" to 1/2". Can use sandwich construction to achieve lighter and stiffer parts. |
| Corner Radius |
Recommend 1/8" or larger. |
| Shape |
Will duplicate the shape of the mold. Can be heavily contoured. |
| Dimensional Tolerance |
+ .010" |
| Surface Finish |
Tool side can be class A
Can be gel coated painted, or use any other surface coating |
| Shrinkage |
.0005 in/in |
| Electrical Properties |
Electrical conductive |
| Fire Retarding |
Resins available in fire retardant applications meeting various ASTM classes & smoke generation requirements |
| Corrosion |
Resins available for corrosion applications, especially for hot brine, most acids, caustics, & chlorine |
Tooling
Molds are used to define the shape of the fiberglass parts. The graphite
composite part will pick up all shapes and features of the molds; therefore
the quality of the part is heavily influenced by the quality of the mold. The
molds can be either male or female. The female molds are the most common and
they will produce a part with a smooth exterior surface while a male mold will
produce a smooth interior surface. A matched mold (male and female) is required
if the part is consolidated using a press. The molds can be made with composite
materials, metal filled epoxy, or machined from aluminum or steel. The type
of mold and materials used depends on the type of part and the production quantity.
LightSpeed Performance design team
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