Quantitative Measurement of Fiber Pull-out by Laser Scanning Confocal Microscopy

Abstract

The mechanical properties and failure behavior of carbon/carbon composites are strongly influenced by the bonding between fiber and matrix which has a strong impact on fiber pull-out. A new method of determining quantitatively the distribution of fiber pull-out length is presented. Fracture surfaces of carbon/carbon (c/c) composites – polyacrylonitrile (PAN)-based carbon fibers infiltrated with pyrolytic carbon – are investigated by laser scanning confocal microscopy (LSCM) to measure the three-dimensional surface topography and thereby the fiber pull-out length of carbon fibers. The distribution of fiber pull-out lengths is successfully determined, and it is shown that the average fiber pull-out length increases with the temperature of heat treatment before tensile testing. C/c composites can exhibit excellent mechanical properties, such as high strength, and particularly, sufficient fracture toughness even at temperatures up to 2000 °C. These properties are strongly influenced by the strength of the fiber/matrix interface, therefore knowledge of the interfacial shear strength is of fundamental importance. While experiments on a single fiber,[1, 2] or a fiber bundle,[3, 4] in a matrix can be performed to determine interfacial shear strength, a statistically meaningful result can only be achieved by repeating such a complex experiment many times. An alternative method is based on the quantitative determination of fiber pull-out length which, in principle, allows the calculation of interfacial shear strengthJRC.F.2-Energy Conversion and Storage Technologie