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 estimat ion of the fibre/polymer interface shear strength in reinforced plasti cs. The test results are fibre fragment lengths from which the interfa ce shear strength is usually calculated by using a fibre strength esti mated with the help of Weibull statistics. This paper describes tensil e tests on two types of carbon and Kevlar fibres, and compares the str engths estimated by means of Weibull statistics with actual measured s trengths. It is shown that the use of Weibull statistics with carbon f ibres can be very misleading, and that there is unlikely to be a good alternative to measuring the fibre strength actually at the fragmentat ion length. Another important inference is that the use of any hypothe tical flaw distribution to explain statistical variations in fibre str ength is probably invalid. This is also true in the case of the Kevlar s which show statistical variations in strength over and above those d ue to diameter variations, but no significant dependence of strength o n length.