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 (∼40 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 90° 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.