January 2005 Matrix Cracking in 3D Orthogonal Melt-Infiltrated Composites 2D 7.epcm 10 ZMI UNI Y-Direction ZMI XPLY EE8普6 T300 UNI 4 XPLY Rayon XPLY 0 Remains of rayon Z-fiber Fig 4. Estimated matrix crack density, based on acoustic emission ac- tivity. for the two regions of 3D Y-direction-oriented composites and a Most of the matrix cracking for the UNI region of a gi specimen occurred over a narrow stress range with the mid tress of the distribution increasing with decreasing Z-direction size. The height of the Z-direction tow is 0.15 mm for ZM 0. 1l mm for T300, and 0.03 mm for rayon when measured from polished longitudinal sections as in Fig. 5 where the Z- Y-direction direction tow is closest to the face of the composite. When measured in the interior of the composite, the tow height in- creases to 0.40, 0.28, and 0.15 mm for ZMI. T300, and rayon, respectively. Figure 5(b)shows the small rayon tow height. Note that the rayon fiber was expected to have decomposed during (b) the cvi bn step; however, some porous carbon char remained Fig. 5. Polished longitudinal section of (a)ZMI specimen and(b)rayon in the form of what appears to be an approximately 60"fiber ecimen tested in the y-direction orientation. arrows indicate matrix tow. Fibers in the rayon composite were very straight in both the cracks UN and XPLY regions compared with the other Z-toy posites, e.g- ZMI(Fig. 5(a)). This may also contribute to the balanced 2D architecture. the volume fraction of oo mini-com- high matrix crack stresses for this composite. It was quite re- markable that the very first cracks(as detected by AE) in the posites, minit is half the total fraction of fiber, BN, and CVI Si rayon composite occurred at 170 MPa, significantly higher(at ithin the composite. The elastic modulus of the mini-compos ites. Ein can be estimated via the rule-of-mixtures from the least 40 MPa) than the other two composites or any other 2D elastic moduli of each constituent of the mini-compo MI composite tested to date. (Er≈380GPa,EBN≈60GPa, and ecviSic≈425GPa)and the volume fraction of each constituent in the loading directio (3) Stress Dependence for Matrix Cracking Appendix a describes how this was accomplished for the 3D In a prior study concerning through-thickness matrix cracking composites TTMC) in similar 2D melt-infiltrated SiC/SiC composites, it Matrix crack density versus mini-matrix stress is plotted in was shown that an important parameter controlling cracking is Fig. 6 for the 3D composites tested in the y-direction as well as the effective stress acting on the 90 tows that exist in the matrix for the single-tow 2D composite(from Morscher) Matrix crack rich regions outside of the 0 mini-composites(hereafter termed densities of the different 3D XPLY composites and the 2D wo- the""mini-matrix"region). This mini-matrix stress, ominik-matrix. ven composite have a very similar dependence on mini-matrix can be determined from simple rule-of-mixtures theory stress. However, the stress range where matrix cracking occurs in the UNI regions appears to be dependent on the tow size of (oc+Oth)/Ec-fmini Emini the Z-fiber type, i.e., the smaller the tow size(rayon fibers), the Omini-matrix (1) higher the matrix-cracking stress range. where oe is the applied composite tensile stress, oth is the resid (4) XPLY Matrix Crackin ual compressive stress in the matrix determined from the hys- For the XPLY regions of the 3D composites a teresis loops of the tensile test, ",and Ec is the composite elastic amount of low energy ae was observed at stresses below the modulus measured from the tensile stress-strain curve. For a onset of high-energy AE activity. This low-stress low AE energy activity can be attributed to the formation of tunnel cracks Table ll. Matrix Crack densities thin the 90 X-fiber tows. Tunnel cracking began at a mini matrix stress of 20 MPa in the XPly regions for both the ZMI and T300 composites. However, for the rayon composite Composite Z-fiber type density (mm-) density(mm-) tunnel cracking in the XPLY region began at -60 MPa. The 90 x-direction SiC/SiC mini-composites in the rayon compos- ZMI (Y direction) 10.2 8.8 ites were noticeably longer and thinner than the x-direction T300(r direction) Rayon(Y direction 4.8 tows in the T300 and ZMI composites. The measured maximum height of a 90 tow(hx in Table Ill and Fig. I; see Appendix A)Most of the matrix cracking for the UNI region of a given specimen occurred over a narrow stress range with the mid￾stress of the distribution increasing with decreasing Z-direction tow size. The height of the Z-direction tow is B0.15 mm for ZMI, 0.11 mm for T300, and 0.03 mm for rayon when measured from polished longitudinal sections as in Fig. 5 where the Z￾direction tow is closest to the face of the composite. When measured in the interior of the composite, the tow height in￾creases to 0.40, 0.28, and 0.15 mm for ZMI, T300, and rayon, respectively. Figure 5(b) shows the small rayon tow height. Note that the rayon fiber was expected to have decomposed during the CVI BN step; however, some porous carbon char remained in the form of what appears to be an approximately 60 ‘‘fiber’’ tow. Fibers in the rayon composite were very straight in both the UNI and XPLY regions compared with the other Z-tow com￾posites, e.g., ZMI (Fig. 5(a)). This may also contribute to the high matrix crack stresses for this composite. It was quite re￾markable that the very first cracks (as detected by AE) in the rayon composite occurred at B170 MPa, significantly higher (at least 40 MPa) than the other two composites or any other 2D MI composite tested to date. (3) Stress Dependence for Matrix Cracking In a prior study concerning through-thickness matrix cracking (TTMC) in similar 2D melt-infiltrated SiC/SiC composites,7,8 it was shown that an important parameter controlling cracking is the effective stress acting on the 901 tows that exist in the matrix￾rich regions outside of the 01 mini-composites (hereafter termed the ‘‘mini-matrix’’ region). This mini-matrix stress, smini-matrix, can be determined from simple rule-of-mixtures theory: smini-matrix ¼ ð Þ sc þ sth Ec Ec fminiEmini 1 fmini
ð1Þ where sc is the applied composite tensile stress, sth is the resid￾ual compressive stress in the matrix determined from the hys￾teresis loops of the tensile test,7,19 and Ec is the composite elastic modulus measured from the tensile stress–strain curve. For a balanced 2D architecture, the volume fraction of 01 mini-com￾posites, fmini, is half the total fraction of fiber, BN, and CVI SiC within the composite. The elastic modulus of the mini-compos￾ites, Emini, can be estimated via the rule-of-mixtures from the elastic moduli of each constituent of the mini-composite (Ef
380 GPa, EBN
60 GPa, and ECVI-SiC
425 GPa) and the volume fraction of each constituent in the loading direction. Appendix A describes how this was accomplished for the 3D composites. Matrix crack density versus mini-matrix stress is plotted in Fig. 6 for the 3D composites tested in the Y-direction as well as for the single-tow 2D composite (from Morscher7 ). Matrix crack densities of the different 3D XPLY composites and the 2D wo￾ven composite have a very similar dependence on mini-matrix stress. However, the stress range where matrix cracking occurs in the UNI regions appears to be dependent on the tow size of the Z-fiber type, i.e., the smaller the tow size (rayon fibers), the higher the matrix-cracking stress range. (4) XPLY Matrix Cracking For the XPLY regions of the 3D composites a significant amount of low energy AE was observed at stresses below the onset of high-energy AE activity. This low-stress low AE energy activity can be attributed to the formation of tunnel cracks within the 901 X-fiber tows. Tunnel cracking began at a mini￾matrix stress of B20 MPa in the XPLY regions for both the ZMI and T300 composites. However, for the rayon composite tunnel cracking in the XPLY region began at B60 MPa. The 901 X-direction SiC/SiC mini-composites in the rayon compos￾ites were noticeably longer and thinner than the X-direction tows in the T300 and ZMI composites. The measured maximum height of a 901 tow (hx in Table III and Fig. 1; see Appendix A) 0 2 4 6 8 10 12 0 100 200 300 400 500 Composite Stress, MPa Crack density, mm-1 ZMI UNI T300 UNI Rayon UNI ZMI XPLY 2D 7.9epcm T300 XPLY Rayon XPLY I Fig. 4. Estimated matrix crack density, based on acoustic emission ac￾tivity, for the two regions of 3D Y-direction-oriented composites and a representative 2D woven composite. Table II. Matrix Crack Densities Composite Z-fiber type Average UNI crack density (mm1 ) Average XPLY crack density (mm1 ) ZMI (Y direction) 10.2 8.8 T300 (Y direction) 7.4 4.3 Rayon (Y direction) 4.8 4.8 Y-Direction Matrix cracks i 1 mm 1mm Y-direction Remains of rayon Z-fiber (a) (b) Fig. 5. Polished longitudinal section of (a) ZMI specimen and (b) rayon specimen tested in the Y-direction orientation. Arrows indicate matrix cracks. January 2005 Matrix Cracking in 3D Orthogonal Melt-Infiltrated Composites 149