Understanding the fundamentals of carbon fiber used for FRP reinforcement is very beneficial to engineering structural reinforcement and strengthening applications. These are very high strength fibers used in fiber reinforced plastics, otherwise known as fiber reinforced polymers (FRPs). Carbon FRP, or CFRP, has a significant role in structure strengthening applications and will no doubt become more popular in its use as original, internal reinforcement.
This page describes the source of carbon based fiber, an explanation of the different qualities of fiber manufactured, the mechanical properties of each fiber type, and the difference between "carbon" and "graphite" fibers.
Carbon fiber is produced from a petroleum pitch, rayon, or polyacrylonitrile (PAN) precursor filaments. Of these, PAN-based fiber is the predominate precursor used in CFRP for structural reinforcement and strengthening due to its resulting high quality and strength characteristics.
The process of making PAN-based fibers starts with acrylonitrile, a monomer, which is a yellowish liquid with the chemical formula C3H3N. It’s basically a vinyl with a nitrile attached to it. Acrylonitrile is copolymerized with another monomer to form the polymer polyacrylonitrile (PAN) resin.
The PAN precursor is spun and stretched into acrylic fibers, orienting the polymer molecules and then oxidized at 200 °C to 300 °C. The fibers are then carbonized by heating in an inert gas atmosphere under tension to temperatures between 1,500 °C and 2,000 °C for high strength fibers and up to 2,500 °C for high modulus fibers. After carbonization, the filaments are sized and given a surface treatment to allow them to bond well to matrix resin in a future FRP. Manufacture of PAN-based fibers results in a fiber that is about 94% carbon atoms.
Carbon fiber made from PAN precursor have a diameter of 5 to 7 microns, about 10 times thinner than a human hair.
The filaments are produced and wound onto bobbins in groups called tows. “Tow” is a synonym in the textile industry to “roving” which is simply a parallel group of untwisted filaments. Carbon fiber tows generally contain between 3,000 and 60,000 filaments each, and some up to 100,000 filaments. The number of filaments in a tow is reported in the industry by its thousand, or K value. For example, a 12K carbon tow has 12,000 carbon filaments in a tow.
The desired characteristics of the end product determines which size carbon tow to use. A product, say a carbon textile, weaved with 24K tows will have a lighter weight and less tensile strength than a product using 48K tows. CFRPs used in structural reinforcement generally use large tows (greater than 40K) because they are generally applied to consistent and uniform surfaces (as externally applied) or formed in straight sections (as pultruded reinforcing bar or plate).
Carbon fiber has the widest range of strength and stiffness than all
other fibers used in FRPs. Carbon filament is grouped into
categories based on its modulus of elasticity. The classification names
are: low, standard, intermediate, high, and ultra-high modulus. There is no testing standard related to carbon filament classification therefor the elastic
modulus values for each class vary from manufacturer to manufacturer.
Image credit: By Cjp24 (Own work) [Public domain], via Wikimedia Commons
Even though the class names are generalized, they divide up how and where these fibers are generally used. Described below are the values obtained from the Japan Carbon Fiber Manufacturers Association (JCMA).
A table of varying carbon-based fibers is shown below.
Image credit: Japan Carbon Fiber Manufacturers Association
Low Modulus fiber is generally not used for carbon fiber used for FRP reinforcement. Their low elastic modulus and low tensile strength is not worth the higher price of carbon versus other fibers such as glass.
Note that the tensile strengths of High and Ultra-High Modulus fibers
are equal to or lower than the strengths of Standard and Intermediate
Modulus fibers. Generally speaking, a combination of high tensile strength and high modulus is desired of carbon fiber used for FRP reinforcement and strengthening of civil
Standard and Intermediate Modulus fiber is generally known as commercial or industrial grade fiber. They have high strength and do not have the very high price tag of High and Ultra-High Modulus fibers. The popular carbon fiber used for reinforcement and strengthening of structures generally have a modulus of elasticity in the range of 32,000 to 35,000 ksi (220 to 240 GPa) and tensile strengths between 550 and 725 ksi (3,800 to 5,000 MPa). These fibers give the highest value in terms of tensile strength and stiffness versus cost.
High and ultra-high stiffness fibers are made for the aerospace industry. They are expensive and used in specialized applications such as airfoils. At $900 USD per pound ($1,980 USD per kg) for Ultra-High Mod fiber compared to $25 USD per pound ($55 USD per kg) for Standard Modulus fiber, they are not economical for the civil infrastructure industry. (2010 prices).
Note: The cost of continuous carbon fiber tows varies inversely with the number of filaments per tows, that is, the cost of 3K standard modulus carbon fiber town (3,000 filaments per tow) can be over three times the cost of 100K tows.
Prince-Lund Engineering has provided FRP reinforcement design
assistance to engineers and projects in the U.S. and Central America.
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 Nunna, Srinivas & Blanchard, Patrick & Buckmaster, Derek & Davis, Sam & Naebe, Minoo. (2019). Development of a cost model for the production of carbon fibres. Heliyon. 5. e02698. 10.1016/j.heliyon.2019.e02698.