Analysis of Carbon Fiber Characterization Techniques

Accurate and efficient fiber characterization is critical to developing a successful process for producing low cost, high quality carbon fibers from pitch. This study analyzes current methods for characterizing diameter, tensile strength, and tensile modulus for carbon fibers using a Dia-stron® automated system consisting of a FDAS 765 laser scan micrometer and a LEX 810 linear extensometer. Fiber diameters measured by the Dia-stron® system are compared with diameters measured by Scanning electron microscope (SEM) and by optical microscope at 20X and 40X magnifications. Tensile measurements were also analyzed. This study found that fiber alignment during tensile testing is critical to determining a fiber’s strength accurately. Most of the uncertainty in determining tensile strength, which is reported in terms of stress, comes from error in diameter measurements.

The Dia-stron® had an estimated uncertainty of about 3.7 % for single measurements of a 9.8-micron diameter fiber. The optical method had an estimated uncertainty of about 9.6% for an 8.58-micron diameter. The SEM micrometer had a resolution of ± 1 micron which for a 9.8-micron fiber is an uncertainty of about 10.2%. The range of uncertainty for the SEM method for fibers with diameters between 7 – 12 microns is about 14.3-8.3%. The SEM and Dia-stron® agreed within 7% for a sample group. Individual fiber comparisons between the Dia-stron® system and the optical microscope had agreement between 2% and 15%. The SEM and optical methods both produced images of the fiber, but could only provide one view of the fiber due to its fixed mounting whereas the Dia-stron® system measured 180 degrees around the fiber circumference. The Dia-stron® was found to be the most efficient of these methods for taking a large number of measurements in a relatively short amount of time.