EVALUATION OF THE TENSILE INTERFACE STRENGTH IN BRITTLE-MATRIX COMPOSITE SYSTEMS

Two test methods for determining the tensile (normal) interface bond s trength in brittle-matrix composites have been described: one geometry is the cruciform bend specimen, and the other is the embedded fiber s pecimen. Both specimens have the characteristic that free edge debondi ng is avoided, so that valid interface strength data can be obtained f rom the knee in the stress-strain curve. The techniques are applied to a Sigma SiC/7040 glass composite, and a bond strength of similar to 5 MPa has been obtained. The method of analysis is described. Normal in terface separation has been observed at or very close to the SiC/carbo n (primarily graphite) interface of the Sigma fiber and is consistent with the weak orientation of the graphitic layers. Two examples of int erface separation are cited in the as-fabricated composite, to show th e application and validation of the test results. In one instance, ten sile interface separation has been observed next to existing voids, an d in the other instance, interface separation has occurred near radial matrix cracks. Analyses show that the debonds are consistent with the low normal strength of the interface.