The effect of shape on the tensile strength of pitch-based carbon fibers

This dissertation concerns the characterization of the effect of cross-sectional shape on the tensile strength of pitch-based carbon fibers. Round and trilobal pitch-based carbon fibers were produced using similar processing conditions, and strength data was obtained from single filament testing. X-ray diffraction, transmission electron microscopy, and scanning electron microscopy revealed that the fibers exhibited similar microstructures and could be compared on the basis of shape alone. In a companion study, the tensile strengths and microstructures of two commercial round fibers were also compared.;The application of a variety of distributions to the strength data indicated two sources of failure, one source being the accentuation of end effects at short gauge lengths. A new mixed distribution, the end effect distribution, was proposed to account for these effects and applied to the experimental data. The end effect model provided an excellent description of the strength distributions of all experimental fibers and one of the commercial fibers. An adaptation of the model was necessary to describe the strength distribution of the remaining fiber, and the adaptation suggests the manner in which this fiber's microstructure might influence its tensile strength.;The end effect distribution is not complex and is based on sound physical assumptions. It quantifies a recognized inadequacy of the test method which has not previously been accounted for, and it allows separation of end effects from the true fiber strength distribution. Our results indicate that end effects can be an important concern for gauge lengths as long as 40 mm. In addition, the model provided a good description of the observed data even when end effects were relatively unimportant.;Use of this model revealed that, in the absence of end effects, all fibers failed due to macroscopic flaws; thus, varying the fiber geometry does not result in an unusual failure mechanism. The advantage to producing noncircular fibers is that the tensile strength of the noncircular fiber is less dependent on fiber size. Thus, noncircular fibers can be produced at higher mass flow rates, decreasing filament breakage and increasing precursor conversion.