Crack Initiation in Borosilicate Glass-SiC Fiber Composites

The initiation of matrix microcracking was investigated in unidirectional glass matrix composites having controlled fiber spacing. Observations were taken from composites consisting of regular arrays of TiB₂-coated SIGMA 1240 and carbon-coated SCS-6 monofilament SiC fibers in a series of borosilicate glasses. The thermal expansion mismatch between the fibers and glass matrix was varied such that the resulting radial stresses after processing ranged from tensile to compressive. The glass strongly bonds to the TiB₂-coated SIGMA 1240 fiber but weakly bonds to the carbon coating of the SCS-6 fiber, allowing the investigation of the effects of bonding at the fiber/matrix interface. The observed crack initiation stresses of the various composites are compared to predictions based on a previously developed semiempirical model and used to study the influence of the volume fraction of fibers, residual stress state and interface strength.