International Journal of Applied Ceramic Technology-Morscher, et al. Vol.7,No.3,2010 3501+AUN ■3 DO Unbalanced 300 3DO Bal Fill 200 x LTLAI 0.06 150 ■+xo△口 Fig 3. Matrix cracking stress()and strain(b)versus fiber volume fraction in the loading direction for the different composites. the Z-direction reinforcement which was prone to low observe the matrix cracks. Because the matrix is in com- stress tunnel cracks confined to the ZMI minicomposite pression, matrix cracks are impossible to locate without which accounted for the nonlinearity. The degree of the plasma etching. Unfortunately plasma etching re- nonlinearity is dependent on several factors: the number moves most of the Si from the SiC-particulate, Si teroy of cracks and whether they are micro or macro in na- of the matrix so that cracks can only be observed in ture, volume fraction of fibers, fiber and matrix elastic CVI SiC of the minicomposites. Figure 4 shows some moduli, interfacial sliding stress, straightness of fibers, representative longitudinal sections of a 3D orthogonal tc. The graphical offset technique does not account for specimen and the AI-UNI specimen. Typical matrix the physical dependence of nonlinearity which will re- cracks are shown in Fig, 4c. Note that the cracks were ate to fiber-bridged matrix cracking differently for the very faint even after etching; however, the cracks were factors listed above and so it is not surprising that the observed to traverse through-the-thickness of the speci- two techniques vary for some cases. aE does measure men. The crack densities estimated for the 3DO-Un-R the occurrence of fiber-bridged matrix cracks and is and AI-UNI specimens were at least eight and nine considered to be the more desired method for deter- cracks/mm, respectively. These crack densities are similar ning the onset stress for through-thickness or fiber- to those measured in other Syl-iBN MI composites It should be noted that the plasma etch of the 3DO-Un The effect of fiber volume fraction on matrix crack- Z specimen was not as successful. Although some cracks ress and strain is shown in Fig. 3a and b, respec- could be observed, the surface was stained to a degree The general trend is that increasing volume that reliable crack density measurements could be made. fraction of fibers in the loading direction results in higher matrix cracking stresses and higher matrix crack ing strains, a roughly linear dependence for both.0. aNalysis However, there is considerable scatter in the data ±50 MPa in stress and~±0.03% in strain. There appears to be a general relationship between matrix cracking and fiber volume fraction of fibers in Optical Microscopy the load direction (or fibers that would be bridging a crack in the case of the braid panel). However, there still Longitudinal sections of the specimens were cut, is considerable scatter. A second effect related to matrix lished. and etched as in earlier studies 6. I0 in order to racking postulated in Morscher et al. was that largethe Z-direction reinforcement which was prone to low stress tunnel cracks confined to the ZMI minicomposite which accounted for the nonlinearity.6 The degree of nonlinearity is dependent on several factors: the number of cracks and whether they are micro or macro in na￾ture, volume fraction of fibers, fiber and matrix elastic moduli, interfacial sliding stress, straightness of fibers, etc. The graphical offset technique does not account for the physical dependence of nonlinearity which will re￾late to fiber-bridged matrix cracking differently for the factors listed above and so it is not surprising that the two techniques vary for some cases. AE does measure the occurrence of fiber-bridged matrix cracks and is considered to be the more desired method for deter￾mining the onset stress for through-thickness or fiber￾bridged matrix cracking. The effect of fiber volume fraction on matrix crack￾ing stress and strain is shown in Fig. 3a and b, respec￾tively. The general trend is that increasing volume fraction of fibers in the loading direction results in higher matrix cracking stresses and higher matrix crack￾ing strains, a roughly linear dependence for both.10,13 However, there is considerable scatter in the data, B750 MPa in stress and B70.03% in strain. Optical Microscopy Longitudinal sections of the specimens were cut, polished, and etched as in earlier studies6,10 in order to observe the matrix cracks. Because the matrix is in com￾pression, matrix cracks are impossible to locate without the plasma etching. Unfortunately plasma etching re￾moves most of the Si from the SiC-particulate, Si region of the matrix so that cracks can only be observed in the CVI SiC of the minicomposites. Figure 4 shows some representative longitudinal sections of a 3D orthogonal specimen and the AI-UNI specimen. Typical matrix cracks are shown in Fig. 4c. Note that the cracks were very faint even after etching; however, the cracks were observed to traverse through-the-thickness of the speci￾men. The crack densities estimated for the 3DO-Un-R and AI-UNI specimens were at least eight and nine cracks/mm, respectively. These crack densities are similar to those measured in other Syl-iBN MI composites.6,10 It should be noted that the plasma etch of the 3DO-Un￾Z specimen was not as successful. Although some cracks could be observed, the surface was stained to a degree that reliable crack density measurements could be made. Analysis There appears to be a general relationship between matrix cracking and fiber volume fraction of fibers in the load direction (or fibers that would be bridging a crack in the case of the Braid panel). However, there still is considerable scatter. A second effect related to matrix cracking postulated in Morscher et al. 6 was that larger 0 50 100 150 200 250 300 350 0 AE Onset Stress, MPa AI UNI 3DO Unbalanced braid 2D 5HS 2D 5HS - double tow 2D 5HS N24A 3DO Bal Y 3DO Bal Fill LTLAI 0 0.02 0.04 0.06 0.08 0.1 0.12 0 AE Onset Strain, % 3DO Unbalanced AI UNI 2D 5HS 2D 5HS - double tow 2D 5HS N24A braid 3DO Bal Y 3DO Bal Fill LTL AI fo 0.1 0.2 0.3 0.1 0.2 0.3 fo Fig. 3. Matrix cracking stress (a) and strain (b) versus fiber volume fraction in the loading direction for the different composites. 284 International Journal of Applied Ceramic Technology—Morscher, et al. Vol. 7, No. 3, 2010