A STATISTICAL THEORY OF INTERFACIAL SHEAR STRENGTH DISTRIBUTION, AND ITS APPLICATIONS TO CARBON-FIBER REINFORCED POLYMER COMPOSITES

The objectives of this research are to develop a statistical theory of interfacial shear strength (IFSS), and to apply this theory to diagnose the effect of electrodeposited coatings on carbon fiber on the IFSS in a single filament composite. KR138S, titanium di(dioctyl pyrophosphate) oxyacetate, and P-30 (ammonium polyphosphate), which have been shown to minimize fiber lofting, were selected for electrodeposition and evaluation of their effect on IFSS.; The statistical theory of IFSS was developed by treating both the fiber fracture stress and the ultimate fragmentation length as dependent random variables. The single filament composite technique was used to get the fragmentation length distribution of the carbon fiber embedded in the epoxy matrix. Various models were tried to fit the empirical strength distribution and the empirical fragmentation length distribution. Kolmogorov-Smirnov goodness-of-fit test was used to test the goodness-of-fit of the proposed models.; A bimodal lognormal model fits the gauge-length dependent strength distribution best, and a unimodal lognormal model fits the fragmentation-length distribution best for Fortafil 3 fibers. Furthermore, the interflaw spacing is inferred from a statistical analysis of the fiber strength at different gauge lengths.; From the observed distributions for fiber strength and fragmentation lengths, an analytical solution of the IFSS distribution is obtained as a lognormal distribution. The electrodeposition of KR138S on carbon fiber increases IFSS and interflaw spacing; washing off the protective KR138S coating further increases IFSS, but decreases interflaw spacing. The compatability of the KR138S interlayer with the epoxy matrix, and the significant role of other effects, such as surface oxidation, during electrodeposition are thus indicated.; The theory is not limited to carbon-fiber-epoxy composite, but has general applicability to composites with elastic fibers in a plastic matrix.