Rank the strength of the following materials from strongest to weakest: carbon fiber, plastic, steel, aluminum

Mechanical Engineering Materials Science
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In general, how do these materials rank regardless of specific alloy or configuration.

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Strength of materials can hold many meanings. As such, it’s important to denote what type of strength (tensile, compressive, etc.) that you’re using as a benchmark to compare materials.

For tensile strength, the strength of a material is linked to the Yield Strength, defined as a material’s ability to resist being deformed elastically (non-permanently). From strongest to weakest, the materials rank in terms of tensile yield strength as:

  1. Carbon Fiber: 3200 MPa
  2. Steel: 350 MPa
  3. Aluminum: 276 MPa
  4. Plastic: 45 MPa (Nylon)

The strongest carbon fiber composites can be 10x stronger than steel while also being 5 times lighter respectively. Glass-reinforced plastics can be stronger, but for the most part plastics are the weakest in terms of tensile yield strength.

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I think it’s important in these situations to define what ‘strong’ means. In materials, ‘Strong’ is a red herring and has no real meaning. There is tensile strength, compressive strength, flexural strength(specifically for plastics), tortional strength, impact resistance.

Clarifying that you’re talking specifically about tensile strength shows that you understand the differences in material properties

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Carbon fiber > steel > aluminum > plastic for tensile and yield strength

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While carbon fiber does have the highest tensile yield strength, it’s important to note that contiguous metals like aluminum and steel have distinct advantages like in fatigue life!

Fatigue life is the number/duration of loading cycles that a material can withstand before it fails under fatigue conditions. Fatigue failure occurs when a material experiences repeated or cyclic loading, leading to progressive damage and ultimately structural failure, even when the applied loads are in the elastic region (i.e. below yield strength).

Metals tend to eventually fail under fatigue at a single crack. Carbon fiber composites do not do this — they tend to degrade under fatigue throughout the entire volume of the structure. Composite materials fail due to fatigue in four basic ways: cracking of the matrix (i.e., the resin that holds the fibers together), delamination (peeling apart of one layer of fabric from another), breakage of fibers, and debonding of individual fibers from the resin. Cracking of the fibers and/or of the matrix can be deep within the layers and may not be visible from the surface. Fatigue cracks in metal structures always propagate from the surface (the trick is finding the cracks when they are small).

It’s hard to model the fatigue life of carbon fiber due to the nature of how this material is processed. When its internal structure is degraded from load in any way, it’s broken.

References: Velo