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Back To Basics – Carbon Fiber Characteristics – Part 2 – Compressive Strength, Toughness & Rigidity / Hardness / Stiffness

Jun 1st 2021

In this installment of Back to Basics we continue to look at carbon fiber characteristics. On deck is Compressive Strength, Toughness & Rigidity / Hardness / Stiffness

Compressive Strength is a material’s resistance to withstand loading while a force pushes it together in compression. Ultimate strength is determined by the maximum loading applied as the fibers break or are permanently deformed. Compressive strength is taken generally with an epoxy matrix in laminate form.

Fiber Compressive Strength (MPa)
Carbon Fiber 570
Kevlar 49 190
Kevlar 29 230
E-Glass 425

In compression, Kevlar is much weaker than Carbon Fiber or Fiberglass. The importance being Kevlar is more prone to crack when hit with a sideways blow, causing compression strains in the fiber. This is not to say that Kevlar should not be used, rather designed with a ply architecture that sufficiently covers the needs in which the structure may see.

Toughness is a material’s ability to resist cracking under stress or absorb energy. Although strength and toughness are often related, strength measurement is the highest stress a fiber can handle while toughness is measured on how much the material can give before deforming. It is also the area under a stress strain curve from the testing start measured to the point of failure. It is common to have a weaker strength fiber that will still exhibit “tougher” characteristics. Toughness can characterize a material’s tendency to be resistant to fatigue and for abrasion resistance.

Fiber Toughness (MPa*m^1/2)
Hexcel Carbon Fiber AS4 2.12±0.4
Kevlar KM2 6.63±0.61
Fiber Glass 1.08±0.14

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4901246/

Kevlar, being the lightest fabric widely used in composites, also exceeds Fiberglass and Carbon Fiber in toughness. For this reason, Kevlar fibers are used heavily in vibration dampening applications and to offers better impact resistance than CF or FG. This toughness also helps Kevlar in the fact that it is more resistant to fatigue when loaded repeatedly.

Rigidity / Hardness / Stiffness are all characterized by the material’s ability to not deform under loading. It determines whether certain parts will stretch or move while under a load, which if in design critical areas, tight tolerances may be an issue on load bearing structures.

Fiber Carbon Fiber Kevlar 49 Kevlar 29 S-Glass E-Glass
Strain % 2.00% 2.40% 3.60% 5.40% 4.8%

If a part is needed for keeping strict dimensional tolerancing under load, Carbon Fiber is the answer. While Carbon Fibers have the highest modulus among the three types of fibers, Carbon Fiber composites will keep tighter dimensional tolerances even when it is loaded near their ultimate strengths. Although each of the fibers are categorized as high-modulus materials, each behave differently when loaded near their ultimate strengths and along the entire loading cycle. While CF will only give ~2%, Kevlar 29 and Fiberglass will stretch to loads in the figure of nearly double than Carbon Fiber. The consequence of being stiff is that Carbon Fiber will fail suddenly without showing signs of failure.