Composite components are made up of 2 or more individually recognizable components, but advanced composites only utilize 2 main ingredients: fiber reinforcement and polymer matrix. Fibers are the main component in composite structures as they provide the strength, durability, and flexibility of the final component.
A fiber is defined as a material that has a long axis, usually many times greater than its diameter. The way fibers, but especially short fibers, are defined is by their aspect ratio. The aspect ratio refers to the fiber length divided by its diameter (l/d), and in order to be considered a fiber, the material usually will have an aspect ratio greater than 100.
As we discussed in Intro to Composites, an individual strand of fiber can be thought of as a rope, where it has really high tensile strength but really low compressive strength. When combined with the polymer matrix, the resulting part is extremely strong and lightweight. As technology develops and the need for more environmentally friendly practices forges ahead, the availability of fiber reinforcements is ever-expanding. However, there are three main types of fiber reinforcements: glass, carbon, and natural.
Incorporating glass fiber with a polymer matrix in the 1930s was really the beginning of modern-day advanced composites, resulting in a common product known as fiberglass. Fiberglass is used in everyday items such as metro seats, fair rides, and sailboats to name a few.
It is not as strong or as stiff as carbon fibers, but it has other characteristics that make it desirable in many applications. It is considered an insulator and is invisible to most types of transmissions, making it a good choice for electrical or broadcasting applications.
The main benefits of using fiberglass over metals are primarily the cost, but also the significant weight savings, which is important in transportation and recreation applications. Fiberglass is cheap, lightweight, and has good structural integrity. This is why fiberglass is so common in household, recreational, and transportation equipment.
Carbon fiber is even stronger and lighter than fiberglass, but these benefits are reflected in the increase in cost, hence why it is most common in aerospace or high-end competition equipment. Ever since its creation in the 1960s, it has been revolutionizing industries such as aerospace, motorsports, and professional equipment, creating very strong and very lightweight components. Some examples of carbon fiber components are in Formula 1 cars, planes, and hockey sticks.
In high competition sports, winning is the main priority and reducing weight and increasing performance is the best way to achieve this. In aerospace, the main goal is to reduce fuel consumption, and reducing weight is the best way to achieve this. The increased performance is a huge benefit, allowing the engineers to further optimize the weight savings.
Natural fibers have been used as a reinforcement in advanced composites for many years, but research has ramped up recently fueled by the regulations pushing towards sustainability. There are many different natural fibers that are being evaluated for use in advanced composites, but the most viable ones at the time of writing this blog are flax, hemp, bamboo, and basalt. Biocomposites will most likely be used as a replacement for fiberglass, and flax is leading the way in this aspect due to its very similar properties. Some examples of biocomposite components are automotive body panels, snowboards, and wind turbine blades.
As a result of the push for renewability, research is constantly evolving, resulting in more and more information being released regularly and applications being developed.
Earlier we spoke about the different applications for the different classifications of composites, and how required strength is a determining factor of which structural material is chosen. Charts 1 and 2 below provide a great snapshot of how each different type of fiber compares to the most common structural metals: steel and aluminum.
Chart 1 above shows the relation of the most common structural metals and fibers, without carbon fiber in order for a clearer snapshot of how the other materials compare to each other. As is evident, steel and aluminum have lower specific strength than all the fibers, but have relatively high specific modulus.
Let's take a look at how carbon fiber compares to all other structural materials in Chart 2 below.
Carbon fiber was added into the chart in order to show how much stronger and stiffer it is than all other structural materials, hence its popularity in aerospace, defense, and space applications.
While applications, such as in construction for example, would probably prefer to use stronger and stiffer material for certain projects, the company wouldn't win many bids if they used primarily carbon fiber as opposed to steel or aluminum. Chart 3 below shows the cost relations of each of the structural materials, in USD per kilogram.
Based on this chart, the construction industry isn't going to be incorporating carbon fiber into their bids anytime soon. It is up to the product designer to determine which material to use for their product by weighing the benefits of strength, weight, and price of each different material. For example, the average price of fiberglass is $1.80/kg, while aluminum costs $2.26/kg, and steel costs $0.62/kg. While steel is about 1/3 the price of fiberglass, a steel component would weigh about 4.5 times more than the fiberglass component, while exhibiting the same structural properties. These are all things that product designers have to take into consideration in order to create a successful product.