The word “composite” is thrown around often in technology, materials and scientific publications, with little thought on what makes a composite, well, composite. Composites are new materials made from one or more different materials. Sounds simple? Well, there is more to it.

Most materials scientists and materials engineers do not consider new materials made from a phase change a composite. Therefore, adding alloying components to a melt does not make the new alloy a “composite.” Likewise, the addition of copolymers that are melted together do not make a composite, either. Finally, “natural” composites, such as wood and stone, which may contain multiple materials but are used without any chemical changes are not typically called composites.

Source: A sandwich structure of a multi-directional, fiber-reinforced composite. Source: Public domain

There are still many types of human-made composites left to characterize, and many more to invent.

Component name confusion

There are and continue to be new ways to combine materials into composites. First, one must consider the two basic components, the matrix and the reinforcement. The matrix is the bulk material, the component that typically takes up more than 50% of the volume. The reinforcement is an additive that is placed in the matrix.

There is no hard and fast rule as to which component is the matrix and which is the reinforcement. Measurement by volume can be misleading, as well. For example, consider a lightweight foam composite with steel rebar; the foam may make up 80% of the volume, but only 10% of the mass. For certain calculations, the mass percentage is useful.

The reinforcement can refer to the component with the smallest volume. It can also refer to the component that is added to a liquid phase, such as fibers that are immersed in epoxy in a fiber-reinforced composite. Perhaps the most common terminology is to use reinforcement to describe the material that improves the composite’s most desirable property.

Driving force for creating new composites

Why create new composites at all? The biggest driving force for composite design is the ability to favorably combine materials properties from two different materials. In theory, the advantages of each material will be added into the composite material, yielding some sort of average between the two.

Composites, being combinations of different material classes, usually take the advantages of one material class and combine it with those of another. For example, a ceramic matrix composite (CMC) is made of metal rods or fibers in a ceramic matrix. Reinforced concrete is an example of a CMC. The ceramic has high compressional strength but is weak in tension. The steel rebar is equally good in compression and tension. By combining the two materials together, the resultant composite has the potential to be good in compression and tension, but much stronger in tension than the steel rebar alone.

While much of the popular focus on composites is about the strength, it is important to note that other materials properties can be favorably changed. Consider a grounding mat for working with semiconductors that must dissipate static charge. In this case, rubber, soft and pleasant to stand on, is mixed with a more conductive material, such as carbon. The carbon provides the desirable electrical properties without significantly hardening the rubber mat.

This sandwich structure composite features a thin aluminum honeycomb for strength set between two woven carbon-fiber and epoxy sheets that provide stiffness. It is used for large satellite dishes, which require the structure to withstand strong winds, yet maintain its shape for optimal reception. Source: Seth Price

Rule of mixtures

Predicting the properties of a composite is as much an art as it is a science. A starting point in approximating the materials properties of a new composite is to use the “Rule of Mixtures” calculation. Rule of Mixtures simply states that a composite’s material property will be the weighted average of the materials properties of the components. By weighted average, it literally means the mass fraction of each component. The concept is demonstrated below:

Fig7

For a two-component composite. Where C_p is the materials property of the composite, M_a and M_b are the mass fractions of components a and b, respectively, and P_a and P_b are the materials property of interest for components a and b, respectively.

Importance of testing

The Rule of Mixtures calculation is a starting point. Combining two materials might bring the best of both materials, but it also can bring the worst of both materials. Revisiting the grounding mat example from above, a single copper wire that goes from the top of the mat to the bottom will drastically increase the conductivity measurement of the whole mat, even if its mass fraction is very low. The Rule of Mixtures is not an actual rule, but a back-of-the-envelope calculation.

The key to successful composite development is in the testing phase. Materials must be rigorously tested with ASTM standards to determine their true materials properties. Special attention and focus on the standard deviation, not just the mean is vital. New composites with a large standard deviation in yield strength, for example, will fail unpredictably and will have few applications.

Composites should be tested to failure many times, and the failure surface analyzed for trends. Failure modes should be consistent. For example, a fiber-reinforced composite may fail along the interface between the reinforcement and the matrix, or the crack may go through both components. Testing should show one or the other mode is significantly more common.

Sensitivity analysis is another great tool for determining the viability of a composite during testing. The effects of variation in composition, heat treatments and so on must be well-understood. A composite that can tolerate a wider range of manufacturing conditions will be more affordable and less susceptible to quality excursions and recalls.

Final thoughts

One of the most interesting things about composites research is the variability and the new discoveries in materials and manufacturing processes that make them possible.

As an undergraduate in materials science, preparing for graduate school, an advisor told this author to take another composite course in graduate school. He guaranteed that one could take 50 composite classes, and every one of them would be fundamentally different. Perhaps that is both the challenge and the fun of composites work.