High-temperature transverse fracture toughness of nicalon-fiber-reinforced CAS-II glass-ceramic matrix composite

Cracking parallel to the fibers in off-axis plies is usually the initial form of damage in composite laminates. This cracking process has been associated with the (transverse) fracture toughness, defined by the critical strain energy release rate, G_Ic. The measurement of G_Ic provides basic information about the transverse crack resistance. In this study, the utility of the double torsion (DT) test technique to determine G_Ic in a glass-ceramic matrix composite (Nicalon/ CAS-II) at temperatures up to 1000°C has been demonstrated. G_Ic did decrease moderately with increasing temperature (as does the bulk matrix); however, no evidence of an interphase oxidizing effect on crack growth (parallel to the fibers) could be found. The inevitable misalignment of fibers in the material was not very efficient at bridging the crack in the DT specimens, in contrast to the significant matrix crack interactions with the fibers reported for other geometries such as double cantilever beam and flexure specimens.