Part 1
The Ultimate Guide to Carbon Fiber
Carbon Fiber Design, Analysis, Prototyping & Manufacturing
Element 6 Composites specializes in carbon fiber design, analysis, prototyping, and manufacturing.
We are experts in carbon fiber composites and other high-performance materials. This guide will walk you through everything you need about carbon fiber design and application.
What is Carbon Fiber?
Carbon fiber is composed of strands of fibers 5 to 10 microns in diameter that consist of long, tightly interlocked chains of carbon atoms in a microscopic crystalline structure.
These fibers are extremely stiff, strong, and light, and are used in many processes to create high-performance building materials. Carbon fiber reinforcements come in a variety of weaves, braids, and other formats such as tow, and uni-directional.
These are combined with various resins to produce carbon fiber-reinforced composites in a wide range of shapes and fiber patterns.
How is Carbon Fiber Made?
Precursor
To produce carbon fiber, an organic polymer precursor is needed. This raw material is processed with heat and chemical agents to convert it to carbon fiber. The first high-performance carbon fiber materials were made from a rayon precursor. Currently, approx 90% of carbon fiber is made from polyacrylonitrile, while the other 10% or so is made from rayon or petroleum pitch.
Manufacturing
The carbon fiber manufacturing process begins with carbonization. To achieve high-quality carbon fiber, the precursor polymer needs to contain a high percentage of carbon atoms. The majority of the non-carbon atoms within the structure will be removed in the process. First, the precursor is pulled into long fibers. These fibers are then heated to very high temperatures in an anaerobic gas mixture (without the presence of oxygen) to ensure the material doesn’t burn. The heat energizes the atomic structure of the fibers and drives off most of the non-carbon atoms from the material.
Treatment
Following carbonization, the surface of the carbon fibers must be treated to improve bondability with epoxies or other resins. Careful oxidation of the surface of the carbon fibers improves chemical bonding properties, while simultaneous roughening of the surface provides improved mechanical bonding. This oxidation can be accomplished in a number of different ways. The carbon fiber can be exposed to various gases such as carbon dioxide or ozone, or liquids such as nitric acid, or even processed electrolytically.
Sizing
Prior to weaving, the carbon fibers must be sized, or coated, with a polymer to protect them during the weaving process. The sizing is selected for compatibility with the laminating resin to be used. The fibers are then wound onto bobbins, spun, and processed into various weaves and other formats
The Ultimate Guide to Carbon Fiber Design and Application
Why Would You Use Carbon Fiber as Opposed to Another Material?
Reason 1: Strength
The primary reason why one would consider the use of carbon fiber is its high stiffness to weight ratio. Carbon fiber is very strong, very stiff, and relatively light.
The stiffness of a material is measured by its modulus of elasticity. The modulus of carbon fiber is typically 34 MSI (234 Gpa). The ultimate tensile strength of Carbon Fiber is typically 600-700 KSI (4-4.8 Gpa). Compare this with 2024-T3 Aluminum, which has a modulus of only 10 MSI and ultimate tensile strength of 65 KSI, or with 4130 Steel, which has a modulus of 30 MSI and ultimate tensile strength of 125 KSI.
Ultra-High Modulus Carbon Fiber
High and Ultra-High Modulus carbon fiber or High Strength carbon fiber are also available due to refinements in the materials and the processing of carbon fiber.
A composite carbon fiber part is a combination of carbon fiber and resin, which is typically epoxy. The strength and stiffness of a carbon fiber composite part will be the result of the combined strengths and stiffnesses of both the fiber and the resin.
The magnitude and direction of local strength and stiffness of a composite part are controlled by the local fiber density and orientation in the laminate.
Balanced & Symmetric Carbon Fiber Weave
It is typical in engineering to quantify the benefit of structural material in terms of its strength to weight ratio (Specific Strength) and its stiffness to weight ratio (Specific Stiffness), particularly where reduced weight relates to improved performance or reduced life cycle cost.
A carbon fiber plate fabricated from standard modulus plain weave carbon fiber in a balanced and symmetric 0/90 layup has an elastic bending modulus of approx. 10 MSI. It has a volumetric density of about .050 lb/in3.
Thus the stiffness to weight ratio or Specific Stiffness for this material is 200 MSI The Strength of this plate is approx. 90 KSI, so the Specific Strength for this material is 1800 KSI
Greater Than Aluminum or Steel
By comparison, the bending modulus of 6061 aluminum is 10 MSI, the Strength is 35 KSI, and the volumetric of density is 0.10 lb.
This yields a Specific Stiffness of 100 MSI and a Specific Strength of 350 KSI. 4130 steel has a stiffness of 30 MSI, a strength of 125 KSI and a density of .3 lb/in3 which yields a Specific Stiffness of 100 MSI and a Specific Strength of 417 KSI.
Hence, even a basic plain-weave carbon fiber panel has a specific stiffness 2x greater than aluminum or steel. It has a specific strenght 5x that of aluminum and over 4x that of steel.
Sandwich Structures Utilizing Lightweight Cores
When one considers the option of customizing carbon fiber panel stiffness through strategic fiber placement and includes the significant increase in stiffness possible with sandwich structures utilizing lightweight core materials, is it obvious the advantage that carbon fiber composites can make in a wide variety of applications.
The specifics numbers depend on the details of construction and the application. For instance, a foam-core sandwich has an extremely high strength to weight ratio in bending, but not necessarily in compression or crush. In addition, the loading and boundary conditions for any components are unique to the specific structure.
Thus it is impossible to provide the thickness of a carbon fiber plate that would directly replace a steel plate in a given application without careful consideration of all design factors. This is accomplished through careful engineering analysis and experimental validation.
Patent Pending Methods
One example of design flexibility in carbon fiber is the custom design of beams with tailored stiffness along specific axes.
Element 6 Composites has developed patent-pending methods for the fabrication of carbon-fiber tubes for optimum stiffness along each bending axis. Such tubes are similar to I-Beams in their resistance to bending, yet retain the high torsional stiffness found in a tube.
Dimensional Stability
Reason 2: Low Thermal Expansion
One important benefit of choosing carbon fiber is its dimensional stability with changes in temperature. Carbon fiber has a coefficient of thermal expansion of less than one-millionth of an inch per degree F, vs 7 millionths of an inch/inch per degree F for steel, or 13 millionths in/in for aluminum.
Homogeneous / Isotropic Properties
Reason 3: Anisotropic Properties
When designing composite parts, one cannot simply compare the properties of carbon fiber versus steel, aluminum, or plastic. These materials have homogeneous (properties are the same at all points), and isotropic (properties are the same along all axes).
By comparison, carbon fiber parts are neither homogeneous nor isotropic. In a carbon fiber part, the strength resides along the axis of the fibers, and thus fiber density and orientation greatly impact mechanical properties. This provides the ability to tailor the mechanical properties of a part along any axis.
Carbon Fiber Specialists
