Estimation of interfacial shear strength: an application of a new statistical theory for single fiber composite test

A simple procedure is proposed for estimating the effect of different sizings on the strength of fiber/matrix interface. This procedure is based on a newly developed statistical theory for single-fiber composite tests. This procedure is used to evaluate the interfacial shear strength of HTA carbon fiber in an epoxy matrix with two different sizings. It is shown that if the experiment data are presented in the form of the normalized variables, l̄fδc and σσc, the two sets of data corresponding to the two different sizings can be collapsed into a single curve which can be obtained theoretically. Excellent agreement between experiments and theory is found. An argument is presented for the use of the interfacial shear strength as an engineering parameter to characterize interface adhesion. We also investigate the effect of residual stresses due to thermal-expansion coefficient mismatch between the fiber and the matrix on the interpretation of single-fiber composite tests. We found that the experimentally determined Weibull parameters are very sensitive to the effect of these residual stresses and must be taken into consideration when interpreting single-filament composite tests.