A comparative analysis of alternative models to predict the tensile strength of untreated and surface oxidised carbon fibers

Abstract

The present work reports a comparative study of alternative models to describe the tensile strength of carbon fibers. The theoretical models were validated using data determined for a wide range of materials. Fibers derived from PAN and pitch, with different geometries (circular and ellipsoidal cross section), with and without surface treatment, were tensile tested. A mixed-mode Weibull distribution function, adapted for the length dependence of fiber strength, was used to model the tensile data, assuming the weakest link approximation. This function is capable of describing the effect of flaws and crystalline misalignment on fiber tensile strength. Additionally, the ‘end-effect’ model was employed to distinguish between these failures and those that result from the test method itself, namely the influence of the machine clamps. It was observed that the relative importance of end-effects increases as the length or the aspect ratio of the fiber decreases. For most fibers studied at longer gauge lengths, a two-parameter Weibull distribution could adequately fit the strength data. Thus, in these cases, a dominant flaw population may be responsible for fiber failure. Based on the results of this study, criteria are proposed for selecting the most appropriate statistical models to predict the tensile strength of carbon fibers at small gauge lengths.