Back To Basics – Carbon Fiber Characteristics – Part 1 – Tensile Strength & Modulus

There are three main types of fibers used in composites today; Carbon Fibers, Fiberglass, and Kevlar (Aramid). When used in composites, fibers define the performance criteria and are held responsible for carrying the load in a designed structure. (Resins are responsible for transferring the loads onto the fibers.) In short, fiber selection is an integral part of the design process. From these 3 fibers, there are thousands of choices to weigh when deciding on which fabric to use for a given project. Factors and characteristics such as modulus, tensile strength, compressive strength, toughness, rigidity, conductivity, and chemical / corrosion resistance are all important when selecting a fiber to use. Although there are thousands of fabric options to choose from, selecting the proper fiber(s) at the beginning the project’s design will alleviate challenges through the project duration and point toward the best fabric(s) to use. Often, composite structures utilize more than one of these fibers to achieve needed design specifications for final use. Although there are endless fiber properties that can further define a components final use, the following high-level characteristics provide a high level for defining purposes for design:

Modulus is known as the change in length a fiber goes through as increased loading is applied upon it. This loading can be compressive or tensile. It is calculated as Stress divided by Strain. The modulus most often characterizes the material’s stiffness or rigidity. On a given stress / strain curve, it is referred to as the slope of the line. Most fibers are categorized by modulus as it helps determine the rigidity or stiffness of a composite material. Generally, composites using Carbon Fiber, Fiberglass, and Kevlar are all categorized as brittle and fail with little stretch in comparison to more plastic or non-brittle materials.

Fiber Fiber
Tensile Strength (MPa)
to Weight Ratio
Toray Carbon Fiber T700 4900 1.80 2722
Toray Carbon Fiber T800S 5880 1.80 3266
Hexcel Carbon Fiber AS4 4447 1.79 2484
Hexcel Carbon Fiber IM8 6170 1.79 3446
Kevlar 49 3000 1.44 2083
Kevlar 29 2920 1.44 2027
S-Glass 4585 2.48 1848
E-Glass 3790 2.40 1579

Tensile Strength is a material’s resistance to withstand loading while being stretched. It is the maximum force or loading that is applied before permanent deformation of the fiber occurs.

As shown, carbon fibers are overall superior in strength and weight. However, a significant difference is shown between S-Glass and E-Glass. Kevlar is the lightest of the materials in terms of density. While exhibiting the lowest ultimate strengths, Kevlar, in terms of strength to weight, exceeds fiberglass and is comparable to that of carbon fiber. Economically speaking, if ultimate strength is the only design criteria, E-Glass is the top choice.