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.
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.
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
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 This yields a Specific Stiffness of 100 MSI and a Specific Strength of 417 KSI.
| Material | Specific Stiffness | Specific Strength |
| Carbon Fiber | 200 MSI | 1800 KSI |
| 6061 Aluminum | 100 MSI | 350 KSI |
| 4130 Steel | 100 MSI | 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.
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.
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.
