It has been observed that the tensile strength of carbon fibers is reduced in liquids, although the exact reason for this phenomenon has not been elucidated yet. In this study, this phenomenon has been measured in various liquids such as glycerin, water, ethylene-glycol, n-heptane, and mixtures of ethyleneglycol and water. This reduction in strength has been attributed to the reduction in free energy at the crack surface due to contact with liquids. Using this phenomenon, fractions of surface and internal defects in the fiber have been estimated and an intrinsic strength (exhibited in the absence of defects) was predicted from the tensile strength of notched fibers. For the polyacrylonitrile-based carbon fiber used in this study, the severe defects tended to be located at the fiber surface, and the intrinsic strength was estimated to be 9.3 GPa, which was almost twice the measured tensile strength of this fiber, i.e., 4.7 GPa.
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