Thermal Expansion Coefficient

One of the advantages of carbon fiber is its low coefficient of thermal expansion, or CTE. Thermal expansion and contraction occurs for nearly all materials at a rate proportion to temperature change. Plastics typically have some of the highest values for thermal expansion coefficient, and thus change shape the greatest when temperatures increase or decrease. Some examples of high CTE metals are aluminum, magnesium, and zinc. By contrast, metals like invar steel and chromium have much lower values. Invar in particular is often used specifically for its very low CTE in equipment that requires tight tolerances on alignment (e.g. scientific test equipment with mirrors, lasers, optics, and telescopes).

Carbon fiber offers a CTE, in the direction of the fibers, similar to Invar steel, and thus is an excellent alternative when properly designed for and integrated into the surrounding system. For example, a standard modulus all-woven carbon fiber panel will have a CTE value of about 1.5 x 10^-6 /K. By incorporating pitch fibers and aligning the fibers properly, it is possible to create a part with CTE values of zero (or very close to zero). This can be important for high temperature applications (for example, if the part needs to heat up to 400 or 500F and retain alignment), as well as low temperature cryogenic applications where temperatures may be cooled to below -200F.

One challenge when designing carbon fiber parts and integrating them into surrounding assemblies is taking into account the low thermal expansion coefficient of the carbon relative to most other materials. For example, if aluminum is bonding to carbon fiber it will attempt to expand if the temperature is raised. The bond line, however, will resist this expansion, thus creating large shear stresses within the adhesive. The same is true for large decreases in temperature (for example, if a part is cooled 50, 100, or even 200 degrees). If not designed properly, the bond may not hold at the extremes. Assembly technique is important to avoid these types of failures. Some example include overlapping joints, sufficient bond surface area, flexible adhesives, designed-in local compliance, and proper CTE matching between the carbon fiber and non-carbon fiber parts (e.g. metal or plastic). Properly engineered and manufactured composite assemblies can make excellent use of the low CTE properties of carbon fiber.