IN-SITU FIBER STRENGTHS IN CERAMIC-MATRIX COMPOSITES FROM FRACTURE MIRRORS

The in situ breaking stresses of pulled-out fibers on the fracture sur face of a fiber-reinforced composite can be determined ex post facto f rom a measurement of the fracture mirrors on the surfaces of the fiber s. Here, the relationship between the resultant distribution of breaki ng stresses and the true in situ fiber strength distribution, as chara cterized by a Weibull modulus m and characteristic strength sigma(c), is derived for composites that fail after the first matrix crack has p ropagated. Specifically, it is demonstrated that, if the true in situ parameters are m and sigma(c), then the distribution of breaking stres ses is well approximated by a Weibull form with apparent Weibull modul us m and apparent characteristic strength S*. The relationships betwe en m and m, and S* and sigma(c), are presented, and these results all ow for the conversion of fracture mirror data, or any fracto-graphic d etermination of fiber strength, to the true strength parameters necess ary for predicting/interpreting composite performance.