Single Fiber Tensile Testing (SFTT) to Evaluate Strength Degredation Levels of Original and 14-Minute Oxidized T700-FOE Continuous Carbon Fibers

• Cayden Walker, Mechanical Engineering, University of Delaware

• Tekin Ozdemir, University of Delaware Center for Composite Materials, University of Delaware

TuFF is a unique process that uses short (3-5 mm) carbon fibers to produce well-aligned short fiber preforms and composites with strengths equal to continuous fibers. Recycled short carbon fibers tend to have groupings of fibers stuck together (“clumps”) because of the residue leftover after recycling processes such as depolymerization. These clumps degrade composite strength because fiber alignment and fiber volume fraction (FVF) are important to preserve mechanical properties. Optimized oxidation of recycled carbon fibers can be used to clean fibers and ensure good dispersion properties (important for manufacturing) and minimal strength degradation. However, higher levels of oxidation lead to more fiber strength loss. After determining that 14 minutes of oxidation leads to optimal dispersion and strength preservation, single fiber tensile testing (SFTT) is used to evaluate the strength of T700-FOE virgin and recycled fibers. Samples of single fibers of each type are prepared in sets of 20, firmly held in place by resin-filled end tabs. The diameter of each fiber is measured using a confocal microscope, and the sets of samples are tested using a Dia-Stron SFTT machine. Depolymerized and 14-min oxidized T700-F0E showed a fiber strength measured as 4640 MPa with an STD of 1437 MPa, compared to the virgin fiber’s measurements of 4864 MPa with an STD of 1372 MPa. The recycled strength retention of 94.7% is acceptable for the first iteration of oxidation processing.  In the future, the fibers will be re-infused and strength levels will be analyzed after multiple stages of oxidation.