Interfacial shear strength studies of Nicalon fibers in epoxy matrix using single fiber composite test

Interfacial properties of Nicalon (SiC) fiber in epoxy matrices of varying stiffnesses were studied using the single fiber composite test, in conjunction with stress birefringence patterns. Extensive debonding was observed with hard epoxies, but transverse matrix cracks were found in the more flexible epo;bes, with the interface remaining intact. Micromechanical modeling and Monte Carlo simulation of the single fiber composite fragmentation process provided a basis to compute the interfacial shear stress from the final fragmentation length distribution. The interfacial shear stress appeared to decrease moderately with increasing matrix ductility. The large diameter Nicalon fibers create transverse cracks in the single fiber composite specimens made with flexible epoxies. Consequently, there is a high possibility of premature failure of the specimen before fiber break saturation is reached. This poses some difficulty in interpreting the results for flexible epoxies. It was also found that the interfacial shear stress values from the single fiber composite tests were always considerably higher than the ultimate shear stress values obtained from bulk epoxy (without fiber) tension tests. This effect is similar to what was seen earlier for single fiber composite tests based on graphite fibers and similar epoxy blends, though the difference between the two values was not as great.