The last 100 years has seen the development of many exciting engineering materials, one type of which, fiber-reinforced plastic composites, has found outer space, military, and recently, solar car applications alike. Composed of fibers embedded in a plastic material (thematrix), these composites can be formed from an array of constituent materials.
For instance, more than twenty types of synthetic fibers have been produced. Of these, three are commonly used for engineering applications: glass, carbon (graphite), and Kevlar. Glass is very strong due to the strength of its Si-O bonds, but as a result of its susceptibility to imperfections, it is also very brittle. Because of its low fracture toughness, once a crack forms at an imperfection it propagates to failure with little additional loading. However, by drawing molten glass through a small-diameter orifice such that only a small number of these imperfections occur, this surface during the drawing process protect against crack propagation by closing any small imperfections that appear, further strengthening the fiber.
Unlike glass, graphite is not a strong material because it consists of hexagonally-arranged carbon atoms in weakly-bonded, stacked planes. But graphite can be made into a strong fiber by exploiting the extraordinary strength of the individual planes: by rolling them into a long cylindrical structure, a fiber whose axial direction is in the plane of the carbon atoms is formed. This structure accounts for graphite fiber’s high tensile but low transverse strength.
Kevlar can also be made into a strong fiber. Derived from poly (paraphenylene terephthalamide), Kevlar’s structure consists of copolymer chains that are hydrogen bonded in a plane that is pleated and aligned radially. When the fiber is pulled along its axis the sheet stretches like an accordion before the load is applied directly to the bonds in the sheet plane.
These fibers are ubiquitous in the engineering world, but nevertheless, a fiber does not always stand alone. In a composite material it can be embedded in a matrix material, commonly an epoxy resin. The resin is either combined with the fibers at the time of application or is preimpregnated by the manufacturer and maintained at temperatures near 0 degrees Celsius until it is to be applied.
The main reason that composites are used so extensively in solar cars is that they have a high stiffness to weight ratio and are versatile enough to be formed into either non-structural panels or highly stressed components of the solar car, such as the chassis. In a competition where weight is such a key issue, composite materials are a necessity for any team. Luckily, the ease of manufacturing allows both novice and experienced teams to incorporate composites into their solar car. And once these parts are made, integrating the composite materials with other materials in the car, such as aluminum or steel, can be done effectively with the correct knowledge and materials.
One wonderful aspect of composites is how they can be incorporated with a core material to produce extremely strong and light panels. Basically, the composite laminates are placed on either side of a core material, such as foam or nomex-honeycomb, like a sandwich. By spacing out the two laminates of composite on either side of the core, the strength of the panel increases tremendously. Even highly structural parts of the solar car, such as the chassis, can be made with composite panels.
A useful manufacturing technique associated with panels is cut-and-fold. Unaltered, composite panels remain flat and stiff. But by removing a small strip of one of the laminates, but not from the laminate on the other side of the honeycomb core, the panel can be folded along the removed strip. This technique is analogous to folding a flat piece of paper into a box or other shape. After folding, composite brackets can be bonded to the folded seam to cause the panel to hold its shape.
Choice of type of composite can be critical in some cases. When high stiffness is necessary, carbon fiber is often the best choice. In solar cars this can often mean most of the body/shell, sometimes the chassis, bulkheads and ribs, all of which are done with carbon fiber panels. Kevlar, while not as stiff, is extremely tough. Most importantly, it is electrically non-conductive, unlike carbon fiber. Thus, kevlar can be used for making battery boxes, where conductive material would be quite hazardous. One key note is that kevlar is most likely the best choice for a top shell, upon which the solar cells sit. While teams can hope for electrically insulated solar cell encapsulation and top shells, the risk is still high for some solar cells shorting and destroying themselves against the conductive carbon fiber shell. The loss in stiffness of a top shell due to not using carbon fiber can easily be made up with extra support ribs. Lastly, fiberglass is also non-conductive, and thus can be used in similar applications as kevlar and where high stiffness is not essential.
The raw materials, though, are not the only requirement for working with composites. One also needs a mold in order to give the composite part its final shape. Making a composite part basically consists of placing the layers of composite and core on top of a mold. Peel-ply is placed on bottom of the composites to prevent the part from sticking to the mold and breather is placed on top to allow air to be sucked out. Lastly a vacuum bag surrounds the entire layup. Curing composites is done under vacuum to allow the weight of the atmosphere to compress the piece and increase bonding between laminates and core and also to remove air bubbles which compromise strength. Composites are often also heated in an oven to increase the epoxy’s cured strength as well as decrease curing time.
These composites combine the properties of each constituent material and, thanks to their high stiffness to weight ratios and ease of formability to complex geometries, have become indispensable to producing lightweight, aerodynamic solar cars. Exploiting each composite for its different properties has allowed the team to build a lightweight, strong solar car. Composites expertise is therefore a continuing requirement for the team. Thankfully, learning to use composites is fun, and should be part of everyone’s solar car experience.