Composite Materials FAQ: Weaves and Carbon
When the average person looks at a piece of composite material all they see a light weight piece of material. This section will inform you on what exactly you have just by looking at it. One of the most important factors in determining the properties of composite material is the weave of the reinforcement. The weave is the pattern in which the strands of fiber are positioned in the resin. Stress is transmitted along the length of a fiber. Special consideration should be give to the fiber lay out as part properties vary depending on a number of factors included fiber orientation. Formations include unidirectional where all strands are facing the same direction, multidirectional where strands can be woven together in a predetermined pattern on multiple axis, or randomly directional. The type of weave determines the strength of the part when different types of stresses are applied. For example, a unidirectional weave is strongest in pure tensile stress, (for you non engineering types envision trying to pull a piece of string between your hands, the stress you put on that string is referred to as tensile stress). Rotational stress, similar to the stress found in a driveshaft, is best suited for a multidirectional weave. Multidirectional weaves provide strength in many directions. A random weave is where a part is made up of many small pieces of fiber randomly oriented on the part. The part is equally strong in all directions, relatively less expensive, and relatively weaker that the continuous strand parts.
The weave also refers to how the fibers lay in relation to each other. The most common weaves include Plain, Twill, and Satin. Plain weave styles are the least expensive and least pliable, but they hold together well when cut. The frequent over/under crossings of the threads reduces the strength of the plain weaves, although they are still adequate for all but the highest performance applications. Satin fabrics and Twill weaves are highly pliable and stronger than the plain weaves. In a satin weave, one filling yarn floats over three to seven other warp threads before being stitched under another warp fiber. Threads run straighter much longer in this loosely woven type, maintaining the theoretical strengths of the fiber. Twill weaves offer a compromise between satin and plain types, as well as an often desirable herringbone cosmetic finish.So when you are looking to purchase a part, imagine the stresses that this part will see. If the weave appears to reinforce the areas that will see the most stress then the part has been correctly designed.
"Dry" and "Wet" Carbon
"Dry" and "Wet" carbon fiber refers to different types of layups used with carbon fiber. A wet carbon layup is where resin is prepared separately from the carbon fabric. Resin is applied to the carbon and then the wetted fabric is placed in a mold. A dry layup uses pre-impregnated carbon fiber. This is where carbon fiber is manufactured with the resin already imbedded into the fabric. The resin is not cured and needs heat to begin the cure process. Pre-impregnated fabrics are available in different resin to fabric weight ratios, and with different cure temperature. Some impregnated fabric needs to be kept in freezers or else the resin will begin to cure. Others can be kept at room temperature but require ovens to cure. This is all dependant on the type of resin used. Both types of layups can produce equaly strong parts. The differnce comes in the ease of manufacturability.
Other Composite Material Issues
Buying Carbon Fiber Parts: The majority of carbon fiber parts that you can purchase are not completely carbon fiber. They are in actuality fiberglass parts with a thin layer of carbon fiber for aesthetics. Due to the high cost of the materials required to make carbon fiber parts, full carbon fiber parts are ungodly expensive. Full CF hoods can cost $1500+ depending on who makes them. Another thing to be mindful of when buying parts is that some manufacturers will use low grade materials which lowers their cost and improves their profit margin. To protect yourself from this be sure to ask what type of resin is being used. I know it was mentioned before but it is critical that epoxy resin be used with Carbon Fiber. Other resins will mean reduced strength and in a structural situation can be very dangerous.