Defect Analysis of Brittle Fibers by Single Fiber Tensile Tests in Liquids

The tensile strength of brittle fibers in liquids can be lower than that in air. Based on Griffith’s theory, this phenomenon can be explained by a change in the surface crack state owing to chemical or physical interactions. Therefore, this phenomenon can be used to analyze the surface crack state. In this study, single fiber tensile tests of glass fibers (GFs), polyacrylonitrile-based carbon fibers (PAN-based CFs) and pitch-based carbon fiber (pitch-based CFs), and silicon carbide (SiC) fibers were performed in air and liquids with different surface-free energies. The strength reduction mode was different for each fiber: GF and PAN-based CF had strength reduction at low strength, pitch-based CF had no strength reduction, and SiC fiber had strength reduction at full strength. The ratio of surface to internal defects in each failure probability region was estimated based on the test results. It was found that the strengths of the PAN-based CFs and SiC fibers were controlled by the surface cracks, and the strengths of the GFs and pitch-based CFs were controlled by the internal cracks. If surface defects are difficult to observe for their size, this method has the potential to analyze the location and proportion of defects that cause brittle fiber fractures.