Abstract: The effect of diameter of carbon fiber on the mechanical properties is dramatic. The diameter of carbon fiber present obvious lateral and vertical in-homogeneity. Conventional testing technique cannot characterize the diameter of carbon fiber accurately and the deviation made by testing technique is often ignored for performance of carbon fibers and composite products. In this study, two conventional and one image processing methods for testing diameter were analyzed in order to evaluate the effect of the testing technique on Weibull modulus and fracture toughness, the effects of irregular shape on the properties of the carbon fiber were studied, and scheme for correction and calibration was given. The results showed that the diameters tested by the three technique were different and in the inverse proportional relationship fracture toughness, Weibull modulus and diameter had not a linear relationship, but could prove that the image processing method could correctly represent the diameter of the carbon fiber and the mechanical properties. The data tested by method of image processing could be considered as realistic values, and the data tested by method of SEM and calculation could be corrected through SPSS statistical software, and improve the accuracy of the carbon fiber mechanical properties effectively.

Key words: carbon fibers, weibull, diameter, fracture toughness, X-ray diffraction