Many components of our vehicle will be fabricated from carbon fiber; its incredible strength to weight ratio is something any race team drools over. So today we took a trip to NASA’s Jet Propulsion Laboratory to construct a few sample body panels and our centerpiece for the steering wheel. We were kindly met by JPL employee Dominic Aldi and given a quick tour of his lab. We were then given a brief introduction into carbon fiber lay-ups and set to work on producing our own carbon fiber components.
We first needed to cut our prepreg carbon fiber sheets to the specifications we wanted. For our body panels, we utilized a combination of unidirectional and bidirectional sheets, giving us an ideal combination of high bending stiffness and high strength in tension. Unidirectional carbon fiber provides high bending stiffness, but has low shear strength. On the other hand, bidirectional carbon fiber has lower bending stiffness but it has tensile strength in two directions. These properties can be exploited differently depending on the orientation in which sheets are laid up. Once the sheets were spec’d out and an orientation of the sheets was decided upon, we were ready to bind the sheets together. We went with two test designs; a four-layer thick sheet and six-layer thick sheet for our body panels, utilizing a combination of unidirectional and bidirectional fibers. With the steering wheel, we sandwiched an aluminum honeycomb sandwiched in between two bidirectional carbon fiber sheets, to provide us with a lightweight yet rigid steering wheel. Once this was done, we had to infuse the carbon fiber sheets together utilizing a vacuum and pressure in an autoclave.
For most vacuuming processes, a vacuum bag must be measured, cut, and sealed according to the material being infused. However, the JPL lab had a nifty invention that could preprocess de-bulk our prepreg carbon fiber sheets (given they fit into the machine) that reduces the headache of creating a unique bag for every component. The carbon fiber sheets are simply put together prior to curing and the combination of the resin and vacuum initially bind the sheets together to ensure the highest strength to weight ratio is achieved. However, we were unable to put our steering wheel into this machine as it was simply too large, so we had to mockup a vacuum bag to infuse the carbon fiber sheets to the aluminum honeycomb.
Once the de-bulking process was complete, we brought the sheets to the autoclave, a heated and pressurized vessel. The carbon fiber was cured at over 100 psi and 200ºC. Once this process was done, we had completely infused carbon fiber sheets, ready to be machined and placed into our vehicle.