One of the most common materials structures is a fiber-reinforced composite, consisting of a network of small rods, cylinders or fibers that are immersed in a matrix. The fibers and the matrix are of different materials, and often of different classes, such as metal fibers in a ceramic matrix, though not always.

Their composition, design, applications, failure modes and manufacturing techniques must be considered before choosing them for a specific application. Typically, fiber-reinforced composites are used in applications where neither material by itself will meet the design requirements. Ultimately, the advantages of each material are present in the final design. Unfortunately, the disadvantages of each material are also present in the final design.

This is a ceramic-matric, ceramic fiber composite. It is currently a slurry, made from chopped alumna fibers and ball clay. When fired, the fibers added toughness (crack resistance) to the object. Source: Seth Price

Considerations for fibers

The key to making fiber-reinforced composites suitable for a specific design task is to have a good understanding of how the fibers behave and how they are placed in the composite. Three of the important considerations are the fiber and matrix compositions, the fiber length (or the length to radius ratio) and the fiber orientation.

Composition

Virtually any combination of materials can be made into a fiber-reinforced composite. A few common models are the ceramic with metal “fibers,” such as rebar reinforced concrete, metal matrix composites (MMCs) that have either a polymer or ceramic fibers. Fiberglass is ceramic (glass) fibers embedded in an epoxy (polymer). Metal-reinforced polymers are less common, but they do exist.

Length

Fibers are typically specified as the ratio between the average length of the fiber over the radius of a nominally circular fiber. In some composites, the fibers run the length of flat panels. Sometimes, they are woven to increase strength. These composites have a high ratio, as they are much longer than the fiber radius. These composites show strongly anisotropic properties, meaning their strength and other materials properties are more favorable in some orientations versus others. Other composites used “chopped” fibers, which are much shorter. They still have a high ratio, but not nearly as high as woven or panel-length fibers. The lower ratio means they are more likely to be dispersed in the composite, making them more isotropic.

Corvette body panels are fiberglass: glass fibers embedded in polymer epoxy. This 1954 Corvette uses this process, as do the 2024 Corvettes being produced today. Source: Public domain

Orientation

Long, panel-length fibers and weaves have anisotropic materials properties. They are fixed in their orientation, though sheets of this material can be stacked such that the fibers run in different directions to limit the anisotropy.

Short, chopped fibers can be more randomly distributed, which helps limit the anisotropy. However, sometimes a particular design might require aligned, short fibers. Ferrous fibers can be aligned magnetically, and others can be aligned using static, surface tension of the slurry and other means.

Layout and fabrication techniques

Fiber-reinforced composites can be fabricated in numerous ways. The final structure and composition of the composite will help the engineers choose which manufacturing technique should be used.

For aligned fibers, melt dipping can be used. In this technique, fibers are dipped or placed in a melt, epoxy or slurry. Depending on the specific situation, fibers may be spooled continuously through the liquid phase of another material. This technique is particularly common for embedding ceramic or metal fibers in a polymer matrix. Reinforced concrete is made using a similar method — the rebar is placed first and then immersed in concrete.

Aligned fiber-reinforced composite sheets can be stacked in different orientations to build a three-dimensional structure. While not isotropic, the stacked structure can be highly customizable, perhaps placing different fiber spacings, orientations or materials in different layers.

Chopped fibers or randomly aligned fibers may be dumped into a liquid slurry or melt. Theoretically, the fibers will randomly disperse themselves in the liquid, generating a composite with nearly isotropic properties in two dimensions. Thanks to surface tension, effects of static charge and other problems, it is a little more challenging to force the chopped fibers to disperse in three dimensions.

Stirring the melt or slurry can help distribute the fibers in three dimensions, but the use of a single stirrer can lead to fiber clustering in pockets around the stirrer. To optimize the stirring parameters (speed, stirrer shape, etc.), multiple tensile tests can be conducted from dogbone tensile test samples cut in all directions. Through statistical analysis, engineers can conduct an ANOVA test to ensure that no direction shows stronger or weaker tensile tests.

Failure modes

The two common types of failure in fiber-reinforced composites are fiber failure and fiber pullout failure. Neither is necessarily better or worse than the other, they are simply two failure modes that must be studied.

Fiber failure is where a crack propagates through the matrix and through a fiber as well. In this case, the fiber did little to strengthen the material. With enough energy, or a rapidly expanding crack, a crack can propagate directly through the fiber with little crack tip deflection at the fiber/matrix interface.

Fiber pullout failure is where the crack propagation is deflected by the fiber. Cracks must go much farther, meaning more energy dissipated and more time from visible damage to total failure. However, fiber pullout failure may also indicate that adhesion between the fiber and matrix is poor. In this case, little energy is dissipated, even though the crack is “longer.”

Fiber pullout failures have a fibrous appearance, with hairlike pieces extending from the broken piece. It can also look like a sponge, where fibers have broken free and been pulled out of the matrix, though porosity can look like this as well. Regular fiber failure shows very few fibers sticking out. These failures can look brittle and jagged, and the crack surface will cut across the fibers at almost the same height as the matrix.

Combination failures are possible as well. A crack may start as a fiber-pullout failure, but then as the crack tip gains speed or as it approaches stress concentrators in individual fibers, it may cut directly across them. Furthermore, fibers that are spaced too closely may quickly fracture the matrix, leading to the same effect.

Composite engineers should not view a single failure and treat it as a characteristic failure mode. As with all materials testing, multiple tests should be conducted to fully understand how the composite will be expected to perform and fail in service.

Final thoughts

Overall, the science of fiber-reinforced composites is mature, yet still developing. On one hand, fiber-reinforced composites have been used for generations. On the other, new composites and testing techniques are being developed, and the consistency of the components is improving. If a fiber-reinforced composite was dismissed for a design 10 years ago, it needs to be reevaluated given new technologies.