Journal of the American Ceramic Society-Morscher et al. Vol. 88. No 2D 7.epc Onset Composite St 150- Slope=2.17 MPa m1/2 ZMI UNI 8 T300 UNI 6 ZMI XPLY Onset Minimatrix Stress 4 Slope= 1.07 MPa Rayon XPLY (a) 〔 Tow height)12,mm1l2 T300 XPLY 10 ZMI UNI Minimatrix Stress MPa Fig. 6. Estimated crack density plotted versus mini-matrix stress in the T300 and ZMI composites ranged from 0.08 to 0.19 mm (0. 14 mm on average), whereas the height for the maximum UNI height of a 90 tow in the rayon composites ranged from 0.08 to tows may be the cause for iage). The presence of thicker 90o 0. 16 mm(-0.12 mm on aver he lower matrix tunnel-cracking 占3E UNI resses in the XPLY region of the T300 and ZMI composites Note that for random lay-up architectures of 2D woven com- posites, tunnel cracking can also occur at relatively low mini 0 natrix stress(<20 MPa, see Fig. 6). For these 2D architecture contrast to the 3D architectures. there are often two contact Stress x Tow height 2, MPa-m142 a ng 90 tows with a combined height of up to c0.3 mm. These Fig. 7.(a)Onset stress for TTMC versus the square root of the inverse regions would of course not be through the width of the spec- tow height of the Z-direction tow for the UNi regions of the 3D or- imen; however, they are prime sights for low-stress tunnel or micro-crack formation thogonal composites. (b) Estimated crack density for UN regions versu There exists a 20 MPa separation in mini-matrix stress be- "stress intensity. tween the T300 and ZMI composites and the similar rayon and 2D composites(Fig. 6). There was a considerable amount of which is effectively the flaw size in the matrix. Assuming this estimation in the determination of mini-matrix stress for the to be the case, Fig. 7(a) plots the onset of TTMC versus the different regions, and error is expected even in the use of inverse square root of the height of the Z-fiber tow measured o processing parameters since some variation in local composite mm from the face of the composite for both the applied com- constituent composition will occur over the entire composite posite stress and the mini-matrix stress. A linear relationship panel. Nevertheless, it is probable that real TTMC differences exists for both stresses, confirming this implication. This can be do exist for the different XPLY regions of the 3D composites taken one step further. The estimated TTMC density can be due to the thinner 90 tow height of the rayon composite re- plotted versus a" stress-intensity"(applied stress versus the ulting in smaller local areas of unbridged matrix cracks square root of the Z-fiber tow height) as shown in Fig. 7(b) At least for crack densities below 5. the distribution of matrix 5) UNI Matrix Cracking cracks for all three 3D orthogonal composites converges for the For the UNI regions, the lack of convergence of the TTMC stress-intensity" parameter very well. As described above, significant tunnel cracking for UNI re- mation in the T300 and rayon 3D composites at stresses lower gions of the zMl composite occurred prior to Tno( ig than those required for the ZMI composites. Crack formation in the UNI regions is likely related to the size of the z-direction for the uni reg cracking prior to TTMC occurred and rayon composites trix between the 0 mini-composites. This implies the possibility of a Griffith-type relationship between the onset for TTMC acking and the Z-direction mini-composite size or height IV. Discussion In this study, two factors of 3D orthogonal composites were found that clearly dictate the nature of matrix cracking:(1) the Table Ill. 3D Architecture Average Dimensions(see Fig. 1(b) Fig. 1(b) size or height of the Z-fiber tow and (2)the local architecture of the composite. How these factors affect matrix cracking can be Z-direction fiber type iscerned by evaluating their impact on the onset stress for tun nel cracking. the onset stress for TTMC. and the stress distri- ZMI 1.50(0.12)0.14(0.02)1.5(0.23 bution for TTMC. As discussed in the following. these 1.54(0.25)0.1400.03)1.44(0.40) parameters are interrelated and key to the development of phys Rayon 1.62(0.24) 0.12(0.02) 1.34(0.39) ics-based damage and life models for textile-woven ceramic Standard deviation of at least 20 measurements composites in general and melt-infiltrated SiC/SiC compositesin the T300 and ZMI composites ranged from 0.08 to 0.19 mm (B0.14 mm on average), whereas the height for the maximum height of a 901 tow in the rayon composites ranged from 0.08 to 0.16 mm (B0.12 mm on average). The presence of thicker 901 tows may be the cause for the lower matrix tunnel-cracking stresses in the XPLY region of the T300 and ZMI composites. Note that for random lay-up architectures of 2D woven com￾posites, tunnel cracking can also occur at relatively low mini￾matrix stress (o20 MPa, see Fig. 6). For these 2D architectures, in contrast to the 3D architectures, there are often two contact￾ing 901 tows with a combined height of up to B0.3 mm. These regions would of course not be through the width of the spec￾imen; however, they are prime sights for low-stress tunnel or micro-crack formation. There exists a 20 MPa separation in mini-matrix stress be￾tween the T300 and ZMI composites and the similar rayon and 2D composites (Fig. 6). There was a considerable amount of estimation in the determination of mini-matrix stress for the different regions, and error is expected even in the use of processing parameters since some variation in local composite constituent composition will occur over the entire composite panel. Nevertheless, it is probable that real TTMC differences do exist for the different XPLY regions of the 3D composites due to the thinner 901 tow height of the rayon composite re￾sulting in smaller local areas of unbridged matrix cracks. (5) UNI Matrix Cracking For the UNI regions, the lack of convergence of the TTMC stress distributions appears to be due to the lack of crack for￾mation in the T300 and rayon 3D composites at stresses lower than those required for the ZMI composites. Crack formation in the UNI regions is likely related to the size of the Z-direction mini-composites, which act as flaws in the relatively dense ma￾trix between the 01 mini-composites. This implies the possibility of a Griffith-type relationship between the onset for TTMC cracking and the Z-direction mini-composite size or height, which is effectively the flaw size in the matrix. Assuming this to be the case, Fig. 7(a) plots the onset of TTMC versus the inverse square root of the height of the Z-fiber tow measured 0.5 mm from the face of the composite for both the applied com￾posite stress and the mini-matrix stress. A linear relationship exists for both stresses, confirming this implication. This can be taken one step further. The estimated TTMC density can be plotted versus a ‘‘stress-intensity’’ (applied stress versus the square root of the Z-fiber tow height) as shown in Fig. 7(b). At least for crack densities below B5, the distribution of matrix cracks for all three 3D orthogonal composites converges for the ‘‘stress-intensity’’ parameter very well. As described above, significant tunnel cracking for UNI re￾gions of the ZMI composite occurred prior to TTMC (Fig. 6), presumably due to the large tow height of the ZMI Z-tow. However, little if any tunnel cracking prior to TTMC occurred for the UNI regions of T300 and rayon composites. IV. Discussion In this study, two factors of 3D orthogonal composites were found that clearly dictate the nature of matrix cracking: (1) the size or height of the Z-fiber tow and (2) the local architecture of the composite. How these factors affect matrix cracking can be discerned by evaluating their impact on the onset stress for tun￾nel cracking, the onset stress for TTMC, and the stress distri￾bution for TTMC. As discussed in the following, these parameters are interrelated and key to the development of phys￾ics-based damage and life models for textile-woven ceramic composites in general and melt-infiltrated SiC/SiC composites 0 2 4 6 8 10 12 0 100 200 300 Minimatrix Stress, MPa Crack density, mm-1 ZMI UNI ZMI XPLY Rayon XPLY T300 UNI Rayon UNI 2D 7.9epcm model T300 XPLY Fig. 6. Estimated crack density plotted versus mini-matrix stress. Table III. 3D Architecture Average Dimensions (see Fig. 1(b)) Z-direction fiber type wx (mm) hx (mm) uy (mm) ZMI 1.50 (0.12)w 0.14 (0.02) 1.45 (0.23) T300 1.54 (0.25) 0.14 (0.03) 1.44 (0.40) Rayon 1.62 (0.24) 0.12 (0.02) 1.34 (0.39) w Standard deviation of at least 20 measurements. 0 50 100 150 200 0 0.5 1 1.5 2 2.5 3 (Tow height)-1/2, mm-1/2 Stress, MPa Onset Composite Stress Slope = 2.17 MPa m1/2 Onset Minimatrix Stress Slope = 1.07 MPa m1/2 0 2 4 6 8 10 12 01 3 5 2 4 67 Stress x Tow height1/2, MPa-m1/2 Estimated Crack Density, mm-1 ZMI UNI T300 UNI Rayon UNI (a) (b) Fig. 7. (a) Onset stress for TTMC versus the square root of the inverse tow height of the Z-direction tow for the UNI regions of the 3D or￾thogonal composites. (b) Estimated crack density for UNI regions versus ‘‘stress intensity.’’ 150 Journal of the American Ceramic Society—Morscher et al. Vol. 88, No. 1