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Effect of Process-Induced Resin Bead on the Measured Tensile Strength of Polyacrylonitrile-Based Carbon Fibers

The aim of this work is to investigate the effect of a process-induced resin bead on the measured tensile strength of polyacrylonitrile-based carbon fibers. It is found that the resin bead was formed on the surface of carbon fiber specimens if the carbon fibers impregnated with resin were positioned horizontally during the tensile specimen preparation. Mechanical testing results indicated that the measured tensile strength of carbon fibers noticeably decreased by about 10 % due to the formation of the resin bead, and the initial fracture of carbon fibers usually occurred around the resin bead. Moreover, the bending of the filaments in the resin bead had been observed by optical microscope. It is concluded that the flowing and curing of excess resin lead to the bending of loose filaments in carbon fiber tow during specimen preparation; thus, internal stress was subsequently formed in the fiber specimen, which is detrimental to the measured tensile strength. Finally, based on the microscopic observations, theoretical analysis under the maximum stress criterion was employed to predict the tensile strength of carbon fibers with a resin bead, and the theoretical results agree well with the experimental results. It is verified that the bending angle and the fraction of the bending filaments in the resin bead play important roles in deteriorating the measured tensile strength. These conclusions can help to improve the tensile testing method for accurately measuring the tensile strength of high modulus carbon fibers.