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

The present work reports a comparative study of alternative models to descr ibe the tensile strength of carbon fibers. The theoretical models were vali dated using data determined far a wide range of materials. Fibers derived f rom PAN and pitch, with different geometries (circular and ellipsoidal cros s section), with and without surface treatment, were tensile rested. A mixe d-mode Weibull distribution function, adapted for the length dependence of fiber strength, was used to model the tensile data, assuming the weakest li nk approximation. This function is capable of describing the effect of flaw s 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 mo st fibers studied at longer gauge lengths, a two-parameter Weibull distribu tion could adequately fit the strength data. Thus, in these cases, a domina nt flaw population may be responsible for fiber failure. Based on the resul ts of this study, criteria are proposed for selecting the most appropriate statistical models to predict the tensile strength of carbon fibers at smal l gauge lengths. (C) 2001 Elsevier Science Ltd. All rights reserved.