Matrix cracking in 3D orthogonal melt-infiltrated SiC/SiC composites with various Z-fiber types

The occurrence of matrix cracks in melt-infiltrated SiC/SiC composites with a three-dimensional (3D) orthogonal architecture was determined at room temperature for specimens tested in tension parallel to the Y-direction (perpendicular to Z-bundle weave direction). The fiber types were Sylramic and Sylramic-iBN in the X- and Y-directions and lower modulus ZMI, T300, and rayon in the Z-direction. Acoustic emission (AE) was used to monitor the matrix-cracking activity. For Y-direction composites, the AE data were used to determine the location (+/- 0.25 mm) where matrix cracks occurred in the 3D orthogonal architecture. This enabled the determination of the stress-dependent matrix crack distributions for small but repeatable matrix-rich "unidirectional" and the matrix-poor "cross-ply" regions within the architecture. Matrix cracking initiated at very low stresses (similar to40 MPa) in the "unidirectional" regions for the largest Z-direction fiber tow composites. Decreasing the size of the Z-fiber bundle increased the stress for matrix cracking in the "unidirectional" regions. Matrix cracking was analyzed on the basis that the source for through-thickness matrix cracks (TTMC) originated in the 90degrees or Z-fiber tows. It was found that matrix cracking in the "cross-ply" regions was very similar to two-dimensional cross-woven composites. However, in the "unidirectional" regions, matrix cracking followed a Griffith-type relationship, where the stress-distribution for TTMC was inversely proportional to the square root of the height of the Z-fiber tows.