THE STRENGTHS OF CARBON AND KEVLAR FIBERS AS A FUNCTION OF THEIR LENGTHS

The fragmentation test is one of the main methods used for the estimation of the fibre/polymer interface shear strength in reinforced plastics. The test results are fibre fragment lengths from which the interface shear strength is usually calculated by using a fibre strength estimated with the help of Weibull statistics. This paper describes tensile tests on two types of carbon and Kevlar fibres, and compares the strengths estimated by means of Weibull statistics with actual measured strengths. It is shown that the use of Weibull statistics with carbon fibres can be very misleading, and that there is unlikely to be a good alternative to measuring the fibre strength actually at the fragmentation length. Another important inference is that the use of any hypothetical flaw distribution to explain statistical variations in fibre strength is probably invalid. This is also true in the case of the Kevlars which show statistical variations in strength over and above those due to diameter variations, but no significant dependence of strength on length.