Carbon fiber has a reputation problem. Not a bad one, exactly. The problem is that it sounds like one thing. Light, strong, used in bike frames and fighter jets, done. Nobody who works with it talks that way, because there is no single carbon fiber. Asking for carbon fiber is about as specific as asking for metal.
A Family, Not a Product
The terminology is best treated as a surname. There are many, many members in the family, and they don’t all behave alike. Standard modulus, intermediate, high modulus. Each step up is a tradeoff in toughness for stiffness. Some fibers stretch quite far before breaking. Others hardly move at all, and then break without warning. Then there is the question of the architecture. Woven cloth, uni directional tape, 3k tow, 12k. All with different properties.
A race car manufacturer and a company that builds hospital equipment use it, but they are not using the same material. The racing end wants maximum strength per gram. The scanner table has to remain dimensionally stable and invisible to the imaging process, which eliminates a wide number of materials from consideration.
The Resin Is Half the Part
Fibers alone are simply threads. Resin is needed to hold them in place and transfer loads. This part is usually ignored but leads to most notable issues. Both parts have identical fiber. However, one endures 350F whereas the other deforms significantly before reaching that temperature. One withstands hydraulic fluids, but the other discolors and warps upon contact. The fiber carries the load. Nevertheless, the resin defines the conditions under which it can be carried.
Layup Does the Rest
Third and final component. This is the tricky one. Fibers are arranged in a desired pattern in multiple layers at selected angles. This is the plywood concept of alternating grain direction, except far more precise. Lay all fibers in the same direction for optimal strength in that direction and low strength everywhere else. Parts identical in mold design, material selection, and weight may carry different loads before failing. Same materials. Different formula.
How a Supplier Can Add Value
Material selection is where the quality of the supplier is clearly demonstrated. The best composite material suppliers don’t simply sell a product. Axiom Materials is one of those that takes things one step further and does some engineering. They will ask you what temperature your part is going to see, what it is going to be surrounded by, and what it is going to have to endure. The fiber and resin selections are made according to that information.
Prepreg needs to come into that discussion as well. It is delivered pre-impregnated and with a specified resin content, giving you consistency and repeatability instead of running the risks inherent to wet layup. Batch twelve will behave the same way batch one did. In aerostructures and medical hardware, where the part must be proven safe before anyone signs off, the value of consistency is greater than the value of the materials themselves.
The Cost of Guessing
Bad decision and the invoice comes later. Parts that warp after a season outdoors. Parts that begin to crack the first time there is a cold day. Parts that are actually heavier than the aluminum parts they were meant to replace, in a way that stings because that was the whole point of making them out of composite materials. On anything that flies or holds a person up, a poor selection of materials isn’t simply an engineering frustration anymore.
Conclusion
Strong material, wide range, no patience for assumptions. Determine the requirements of the part and ask the hard questions up front, and let the material work.
