In this study, the effect of fiber content on the flexural property of continuous carbon fiber/epoxy composites was investigated. Samples with four different fiber volume fractions, 50, 60, 70, and 80 vol.%, were fabricated. For comparisons, cast epoxy resin was also prepared. It was observed that the flexural strength and modulus of this material are enhanced with increasing fiber volume fractions in the range of 50–70 vol.%. Results show that the carbon fiber/epoxy composites possess the largest fracture force and displacement when the fiber volume fraction is 70 vol.%, which is mainly attributed to the effective stress transfer of fibers. This can restrict crack tip propagation and blunt the crack tip, then consume abundant deformation energy and result in an increase of fracture work. On the other hand, poor flexural property was observed when the sample with high fiber volume fraction (80 vol.%) was tested. Three different types of failure modes were observed according to the fiber content.

The most fundamental parameter that controls the properties of fiber/epoxy composites is fiber content. Thus, exact calculation of carbon fiber content is important for product quality control and process optimization. In this study, four methods for calculating fiber content of continuous carbon fiber/epoxy composites were investigated. These four methods are processing statistical (PS), optical microscopy (OM), thermogravimetric analysis (TGA), and carbonization-in-nitrogen (CIN). The results show that the CIN and PS methods have high repeatability with no more than 4.7 wt.% deviation, and a relatively exact estimation of fiber content can be obtained by using both of them. A larger difference was generated when the TGA and OM methods were used to determine the fiber content. This is because the small amount of samples required by these methods is not sufficiently representative of the whole carbon fiber composite structure. The PS and CIN methods can be used as approved ways to calculate fiber content of carbon fiber/epoxy composites effectively.