Intro to Composites
What exactly are composites?
Before we jump into the advanced topics of automated fiber placement, or even additive manufacturing of composites, we need to create a strong foundation of what composites really are.
When people think of composites, they immediately picture the black carbon fiber weave that is prevalent on high-end sports cars. While that is not incorrect, there is much more to composites than that. The very basic definition of a composite material is: a material made up of various recognizable individual components. In the context of modern day advanced composites, these components can be broken down into a reinforcement and a matrix. This is not to be confused with an alloy (usually used when speaking of metals) where the final result is a homogenous mixture.
Composite materials can be naturally formed or man-made, which when combined, creates a product that is stronger than each individual component separately. Advanced composite materials have been developed and heavily invested in the aerospace and defense sectors, and only with the recent advancements of technology are they becoming more accessible and widespread. For example, the cost of carbon fiber decreased by 50% in only 3 years; €30/kg ($15/lb) in 2016 to €15/kg ($7/lb) in 2019. This is why many people are just starting to hear about them, even though modern day "advanced" composites have been around since the 1930s, when strands of glass were used as the primary reinforcement. This was a groundbreaking discovery, as the strength was comparable to steel, while the weight was comparable to aluminum, and could be molded in much more complex shapes than its metallic counterparts. Then in 1960, Richard Millington created a fiber made up of 99% carbon, which is still the most advanced material used in manufacturing today.
However, the use of man-made composites in construction dates as far back as Ancient Mesopotamia, when they invented plywood. That's right! One of the most common building materials of today is technically a composite, as it consists of two or more individual components; wood (reinforcement) and glue (matrix), to make a product stronger than each individual component. Other examples of historical uses of composites includes huts made of straw (reinforcement) and mud (matrix), and paper mâché made of paper (reinforcement) and glue (matrix). Now that the concept of composites is clear, let's dive into the advanced composites of today.
Advanced composites, or more technically called fiber reinforced plastics (FRP), consist mainly of two sub-categories based upon the type of fiber reinforcement: glass fiber reinforced plastics (GFRP) and carbon fiber reinforced plastics (CFRP). With the push to become a more sustainable industry, more and more naturally growing fibers are being evaluated as viable reinforcements, such as flax, hemp, and bamboo to name a few; creating a third category of natural fiber reinforced plastics (NFRP). Nowadays, each reinforcement classification can be broken down further by the type of polymer matrix: thermoset, thermoplastic, or even dry fiber (with only a small amount of binder). Whichever type of composite being used, they all consist of two main parts: a fiber reinforcement and a polymer matrix.
Let's think of the fibers as individual ropes. Ropes are great for pulling heavy loads, but not so good for pushing them. This means that ropes have high tensile strength, and very low compressive strength. When the polymer matrix is introduced, this creates a stiff backbone for the fibers which transfers loads between the fibers. Where one fiber would be in compression, the load gets transferred via the matrix to a different fiber that is in tension. This constant load transfer between fibers, and knowing how to orient the fibers to achieve different strengths and stiffnesses, is what makes composites so unique for many different applications. By volume, a composite part using carbon fiber as the reinforcement is 5 times stronger than steel and 10 times lighter than aluminum.
The biggest advantage of composites is that the engineer is not limited to a single set of material properties, as would be the case with metal alloys. Composites takes advantage of the compressive strength and stiffness of the polymer matrix, and the tensile strength and durability of the embedded fiber reinforcements. These two components are mutually dependent - without the matrix, the fiber has no structure and cannot maintain it's shape; without the reinforcement, the polymer is weak and brittle. The image below is a great visual representation of how the polymer and matrix interact with each other.
Ok, so now that we have a good foundation of why composites are highly sought after, let's dig into some of the design principles of creating a functional part. Continue to Design for Composites to learn about the different variables and requirements for designing a component with composites.