Strength of carbon-carbon composites (C/Cs) is extremely lower than that of CFRP. One of the main factors of this fact is that the fiber/matrix interfacial strength of C/Cs is too high. The strong interface causes high stress concentration owing to load transfer from broken fibers onto adjacent intact fibers resulting in tensile fracture units into large bundles and low tensile strength in C/Cs. In author's previous work, it has been verified that the C/C with poor interface was improved the tensile strength. This was only qualitative insight, so that the results should be quantified. In this paper, an elementary model describing the C/Cs' tensile fracture is presented. The model has two steps ; 1) the tensile strength of bundle C/Cs is derived from Weibull distribution of the single fiber strength as a function of the bundle thickness, 2) C/C specimen strength is estimated using the bundle strength distribution. Thus, the model expresses the failure stress of C/Cs in terms of the Weibull distribution of the single fiber strength and the size of the fracture bundle. The analytical result show reasonable agreement with the experimental results in spite of several rough assumptions adopted in this model.
