Fiberglass. Carbon fiber. Kevlar. When you hear these terms you probably think of the items that are made out of them – fiberglass boats, carbon fiber airplane or motorcycle parts, etc. But those parts aren’t made from these materials alone. If they were they’d flop around like a tee shirt. In addition to the “cloth”, or fibers they contain a stiff and hard resin, usually with a top-coating or skin. Together they comprise a “composite”. When people say something is made of fiberglass, what they mean is that it is a composite part containing fiberglass reinforcing fibers.
A composite, in this context, is defined as two or more materials whose combined properties are superior to the sum of those individual components: The whole is greater than the sum of the parts.
In addition to a wide variety of materials, there are a number of common manufacturing processes within the world of composites. Here I will give an overview of the most common materials and their function. My focus will be on fiberglass composites because these are the most commonly used in motorcycle parts like the ones I design and that we sell at Dead Center Cycles, such as fairings and saddlebags. I will cover the various manufacturing methods in a separate article.
The job of the reinforcing fiber is to provide strength by transmitting loads across the structure of the part. These include, in order from lowest to highest performance and cost, fiberglass, aramid (Kevlar) and carbon fiber, with fiberglass being overwhelmingly the most common. All are available as a woven cloth in various thicknesses and weave patterns. Fiberglass is also commonly used in “chopped” form, either in fabric-like chopped mat sheets - the short chopped strands being held together with a glue-like material in order to form a sheet - or chopped up and sprayed into the form of a part during manufacturing.
The job of the resin is to separate the reinforcing fibers and transmit load from one to the next throughout the structure. Resins are a thick syrup-like liquid material that are thremosetting - when mixed with a catalyst they will harden and become stiff, giving off heat. Once catalyzed or “set” they can withstand high temperatures - they cannot be melted and re-used as opposed to the class of materials known as thermoplastics, which can. The most common, again in order from lowest to highest performance and cost are polyester, vinyl-ester, and epoxy. As you might guess, fiberglass is most commonly paired with polyester resin, aramid with vinyl ester, and carbon fiber with epoxy.
The outside or top layer is typically a material called “gel-coat” which is a specially formulated type of polyester resin. It’s role is protective and cosmetic and it is available in a wide variety of colors, although it can also be clear, as is often used with carbon fiber to show off it’s weave. Gel coat provides a smooth, glossy outer surface hiding the reinforcing strand pattern as well as any pits in the structural resin/fiber structure below it, making the part readily paintable. It is harder than standard polyester resin and it can be sanded and polished to a high gloss.
Forces on a part under load
Let’s take a moment to think a bit more about the function of a composite part. The reinforcing fiber does its best work when loaded in tension, like a long string being pulled on each end. Need more strength? Use more strings! But what happens when the load slackens? The strings will flop around and move out of place. What you need is to encase the strings in something such that they stay where you want them, don’t get tangled, and don’t rub against each other. Thus – resin. Note that once you have enough resin to do the job any extra only adds weight without any gain in the performance (strength) of the structure. So during manufacturing various methods are used to “squeeze” out the excess. Another issue – let’s say that you have taken your dry reinforcing fiber and “wet it out” with the thick, syrupy resin. Depending on the process utilized, there may be a tendency for air bubbles to become trapped inside. After the resin hardens large air pockets are known as “voids” and although they are often invisible from the outside of the finished part they can cause major problems. Sometimes finger pressure alone on the outside of a part can cave in these voids. Heat will cause the trapped air to expand and repeated hot/cold cycles over time may weaken the gel coat causing it to crack. This can sometimes be seen when polishing the paint on a painted fiberglass part - bumps show up on the surface of the part . Therefore any air pockets must be removed during manufacturing – another reason to squeeze out the resin, removing air bubbles along with the excess.
What about short reinforcing strands? How can they transfer load across a long structure without spanning it? Imagine a 2x4 made out of a cast resin or plastic of some sort that spans a short distance. If you push down on it mid-span, at some point it will break, likely along a cleavage line. The top of the 2x4 is in compression – the ends are pushing in towards the middle. The bottom is in tension – the ends are trying to pull it apart. The resin is much stronger in compression than it is in tension, so it will crack and a gap will open in the bottom. Now imagine that instead of solid resin your 2x4 was made of densely packed 2”-3” long fibers, soaked with resin which had been catalyzed and solidified into the 2x4 shape. The fibers might be randomly oriented, but some would surely lay across the cleavage line toward the bottom of the 2x4. They would be in tension. Now, in order to break the 2x4 those fibers would either need to break or be pulled out from the grip of the resin surrounding them. A 2x4 made this way would be many times stronger than one made of solid resin.
As you might guess, the carbon fiber strands are the strongest of the 3 types I’ve mentioned – they will take the highest amount of strain or the hardest “pull” before breaking. Aramid (Kevlar) is next, fiberglass last. And epoxy does the best job of gripping those fibers, followed by vinyl ester, and finally polyester. If low weight is critical and cost not a major issue, you would choose carbon fiber with epoxy. If weight is not a major factor, and cost is important, you might choose fiberglass with polyester resin. Note that strength need not be sacrificed. You can likely equal the strength of the carbon fiber/epoxy structure – but you will need to make your part thicker and therefore heavier in order to do so. You might also choose fiberglass woven mat, with its long straight fibers, rather than chopped shorter strands, in order to save some weight (but at increased cost). Often times, as in the case of a part for a cruising motorcycle, cost is more critical than weight, making fiberglass/polyester the best and most common choice. Note that any of these composite structures, including one made of chopped strand fiberglass/polyester is much stronger for the weight than steel. And it will never rust.