wwceramics. org/ACT Efects of Fiber Architecture on SiC/SiC Composites (a) 90 Syl-iBN Stress Direction Z Rayon tow SyriAN 1 mm R L ZM Stress Direction CVI SIC Matrix Cracks microstructures of longitudinal polished sections of failed(a)3D orthogonal co 3DO-Un-R) and (b)through-the-thickness angle interlock composite with ZMI and Rayon warp weaver(Al-UND. Two faint matrix cracks propagating through the CVI portion of a minicomposite for an Al-UNI sPecimen are shown in(c). For(c), the image brightness had to be decreased and contrast increased significantly in order to enhance the cracks. The specimens were plasma etched which is the reason for the rather dull appearance of the MI region of the matrix. unbridged regions of transverse fiber tow minicompos- the 90 tow minicomposites at stresses just below ites perpendicular to the loading direction would result TTMC formation. This was vindicated to some of in lower matrix cracking stress if the fiber volume frac tent for the 3D orthogonal"Balanced"composites tion was the same. The rationale for this was that the Morscher et al6 where it was demonstrated that the 3D observation that the sources of TTMC matrix cracks in orthogonal composites woven with larger Z fiber tows nse MI composites are the tunnel cracks formed in had lower matrix cracking stresses than 3D orthogonalunbridged regions of transverse fiber tow minicompos￾ites perpendicular to the loading direction would result in lower matrix cracking stress if the fiber volume frac￾tion was the same. The rationale for this was that the observation that the sources of TTMC matrix cracks in dense MI composites are the tunnel cracks formed in the 901 tow minicomposites at stresses just below TTMC formation.21 This was vindicated to some ex￾tent for the 3D orthogonal ‘‘Balanced’’ composites of Morscher et al. 6 where it was demonstrated that the 3D orthogonal composites woven with larger Z fiber tows had lower matrix cracking stresses than 3D orthogonal Fig. 4. Representative microstructures of longitudinal polished sections of failed (a) 3D orthogonal composite with Rayon as the Z fiber (3DO-Un-R) and (b) through-the-thickness angle interlock composite with ZMI and Rayon warp weavers (AI-UNI). Two faint matrix cracks propagating through the CVI portion of a minicomposite for an AI-UNI specimen are shown in (c). For (c), the image brightness had to be decreased and contrast increased significantly in order to enhance the cracks. The specimens were plasma etched which is the reason for the rather dull appearance of the MI region of the matrix. www.ceramics.org/ACT Effects of Fiber Architecture on SiC/SiC Composites 285