The most valuable characteristics of carbon fiber are its strength, stiffness, and lightweight. Carbon fiber is made of very fine, crystalline filaments of carbon in "tows", which contain thousands of individual filaments. These yarn-like strands are woven together into cloth of various weaves. When layers of carbon fiber cloth are bonded together, into a carbon fiber plate or sheet, the carbon fiber laminate is exceedingly strong. So how strong is it?
The Strength of Carbon Fiber
Carbon fiber is twice as stiff and five times stronger than steel. While stronger and stiffer than stainless steel, carbon fiber is lighter, which makes it an ideal manufacturing material.
Classifying Carbon Fiber Stiffness and Strength
Tensile modulus is defined as "the ratio of stress (force per unit area) along an axis to strain (ratio of deformation over initial length) along that axis." Also known as stiffness, tensile modulus can predict the elongation or compression of material as long as the stress is lower than the tensile strength of the material.
Carbon fibers are classified based on the tensile modulus of the fibers. The English unit of measure is pounds of force per square inch of cross-sectional area, abbreviated as psi (or ksi for thousand psi and MSI for million psi) . There are five carbon fiber classifications: low modulus, standard modulus, intermediate modulus, high modulus, and ultrahigh modulus.
| Carbon Fiber Class | Low Modulus | Standard Modulus | Intermediate Modulus | High Modulus | Ultrahigh Modulus |
| Tensile Modulus GPa | <227 | 227 | 289 | 393 | 758 |
| Tensile Modulus MSI | <33 | 33 | 42 | 57 | 110 |
The stiffness and strength of a laminated carbon fiber epoxy sheet or plate are determined by:
The carbon fiber material properties
The layup schedule of the fibers (fiber orientation, weave type and thickness of laminate plies)
The fiber/resin ratio
A good estimate for tensile modulus for a balanced, symmetrical, 0/90deg layup schedule would be 10 MSi (70 GPA). A good estimate for tensile strength for the same layup would be 87 KSI (600 MPA).
By comparison, steel has a tensile modulus of around 29 MSI (200 GPa) and a tensile strength of 61 KSI (420 MPA ). The tensile modulus of aluminum is about 10 MSI (69 GPa) and tensile strength is 40 KSI (276 MPA)
As can be seen from the table below carbon fiber laminate has the highest specific Tensile Strength and the highest Specific Stiffness
| Material | Tensile Modulus GPa | Tensile Strength MPa | Density g/cm3 | Specific Tensile Strength Pa m3/kg |
Specific Tensile Modulus MPa m3/kg |
| Aluminum | 69 | 276 | 2.7 | 102 | 25.5 |
| Steel | 200 | 420 | 7.9 | 53 | 25.3 |
| Carbon Fiber Laminate | 70 | 600 | 1.5 | 400 | 46.7 |
Ultra high modulus carbon fibers are approximately 3 times stiffer than standard modulus but are not as strong. In addition, carbon fiber has superior fatigue resistance properties when compared to both steel and aluminum. Also, when combined with appropriate resins, carbon fibers are among the most corrosion-resistant materials on the market.
