COATING OF CARBON-FIBERS - THE STRENGTH OF THE FIBERS

The 6k carbon fiber Torayca T800H was coated with pyrolytic carbon by a CVD process. Fiber bundles were tested and evaluated. By this proced ure, the whole distribution of the failure probability with respect to the fiber strength is obtained in a single experiment. The 50% streng th of the fiber bundle, i,e., the strength at which 50% of the fibers in the bundle are broken, is inversely proportional to the square root of the thickness of the coating. By relating the strength to the defe ct size according to linear-elastic fracture mechanics (LEFM), the pro bability density function of the defects was derived. It is Weibull-sh aped for the uncoated fiber and shows an increasing bimodal shape for the increasing coating thicknesses.