Tensile Strengths of Carbon Fibres

Abstract

THE strength of carbon fibres prepared from commercially available polyacrylonitrile (PAN) fibres depends on the heat treatment temperature (HTT) and normally decreases¹ from a maximum after a HTT of about 1,500° C. Moreton² found that after all heat treatment temperatures (1,000–3,000° C) the strength is dependent on the gauge length indicative of a random distribution of flaws. Johnson³ and Thorne⁴ demonstrated that both surface and internal flaws are present, the latter due to foreign particles in the PAN precursor fibre reacting with the fibre during high temperature processing. Sharp and Burnay⁵ subsequently used high voltage electron microscopy to examine carbon fibres directly and concluded that, when the HTT is raised above about 1,800° C, inclusions react and volatilise to form voids. This results in the formation of misoriented three-dimensional graphite on the walls of the voids and decreases the strengths of the fibres.