3188 Journal of the American Ceramic Society-Morscher et al. VoL. 90. No. 10 Table Il. Matrix Cracking Properties of Composite Specimens DFLS,0002% DFLS.0.005% First AE First loud ae AE onset before AE onset stress- Final matrix crack Panel offset(MPa) offset(MPa stress(MPa stress(MPa) stress(MPa otal #f loud events density(#/mm) Orthogonal oriented composites 147 174 190 4-141 l30 173 182 1-141 135 157 2-134 9.0 Off-axis oriented composites 10 4.0 4.9 195 231 193 210 3-134 7. 9 epcm [±45 7.9 epcm [/9o 8.7 epcm 0/901 8.7 epcm double-tot [0/90] 3.95 epcm Double Tow [0/901 p 2 [O/+67] braid 8.7 epcm, 00 Fig 4. Acoustic emission activity versus composite stress Fig. 5. Estimated matrix crack density with stress based on acoustic 2 and 4 and in Table I and Il will be mechanistically analyzed and compared with other data in the literature to better ur dividual 90" minicomposites and the size of two 90% minicom- tand their scientific and technical significan osites that happen to be adjacent to one another. Whenever this back-to-back tow circumstance exists. the effective width of an unbridged tunnel crack would be approximately twice the (1) Matrix Cracking in 0/90 2D-Woren Composites Tested in back-to-back individual 90 minicomposites is much more com- osite. This characteristic of the o direction non in the double-tow woven 3.95 epcm composite panel com- A variety of microscopic studies of [o/90]2D-woven CMC spec- pared with single-tow 7.9 epcm woven panels. Not only are there mens have shown that at lower stresses. initial transverse matrix nore regions of multiple 90@ minicomposites but also the length cracks are usually either"tunnel"microcracks, which occur in of these regions(distance the tow is woven over four tows before the 90 minicomposites oriented perpendicular to the 00loadir axis, and or nonsteac ady-state microcracks that are partially it is woven under the fifth tow in the five-harness satin archi- bridged due to sufficient fiber traction in the matrix crack tecture)would be approximately twice the length of single-tow to stop matrix crack propagation through-thickness. At woven composites for the five- harness satin weave because the higher stresses, these microcracks propagate through-thickness epcm of the double-tow CMC was one-half that of the single or link up with other microcracks to form TTMC over a range tow CMC. As a result, as shown in Fig 4, the double-tow woven AE methodologies have been successfully used not only uantify but also to understand and model the occurrence and tress-strain dependence of microcrack and TTMc behavior. Initial low-energy events generally correspond to tunnel micro crack formation in 90 minicomposites perpendicular to the loading direction. High-energy events, those in the upper loga- rithmic decade of energy, correspond to either large microcracks 1st Loud AE event raid &[0/90] tribution for TTMC is controlled by(I) the size distribution of 1st AE even 90 minicomposites perpendicular to the load-bearing 0 mini 5 0.31 Braid &(0r-9o) composites and (2)the in situ stress in the region of the com- 0 posite outside the load-bearing 0 minicomposite, i.e., the portion of the composite composed of 90 minicomposites and the MI matrix. For panels fabricated from the random lay-up of standard single-tow-woven fabric plies, the size distribution of 90 minicomposites can crudely vary between the size of Stress. MPa ' For the CMC of this study, a minicom of a single multi fiber tow, the CvI Fig. 6. Acoustic emission(AE) events and onset stress determination. The arrows below the x-axis indicate ae onset stress2 and 4 and in Table I and II will be mechanistically analyzed and compared with other data in the literature to better under￾stand their scientific and technical significance. (1) Matrix Cracking in 0/90 2D-Woven Composites Tested in the 01 Direction A variety of microscopic studies of [0/90] 2D-woven CMC spec￾imens have shown that at lower stresses, initial transverse matrix cracks are usually either ‘‘tunnel’’ microcracks,12 which occur in the 901 minicompositesy oriented perpendicular to the 01 loading axis, and/or nonsteady-state microcracks that are partially bridged due to sufficient fiber traction in the matrix crack wake to stop matrix crack propagation through-thickness. At higher stresses, these microcracks propagate through-thickness or link up with other microcracks to form TTMC over a range of stress levels. AE methodologies have been successfully used not only to quantify but also to understand and model the occurrence and stress–strain dependence of microcrack and TTMC behavior. Initial low-energy events generally correspond to tunnel micro￾crack formation in 901 minicomposites perpendicular to the loading direction. High-energy events, those in the upper loga￾rithmic decade of energy, correspond to either large microcracks and/or TTMC.13 For 2D-woven 0/90 MI SiC/SiC panels tested in the 01 direction, it has been demonstrated that the stress dis￾tribution for TTMC is controlled by (1) the size distribution of 901 minicomposites perpendicular to the load-bearing 01 mini￾composites and (2) the in situ stress in the region of the com￾posite outside the load-bearing 01 minicomposite, i.e., the portion of the composite composed of 901 minicomposites and the MI matrix.7,14 For panels fabricated from the random lay-up of standard single-tow-woven fabric plies, the size distribution of 901 minicomposites can crudely vary between the size of in￾dividual 901 minicomposites and the size of two 901 minicom￾posites that happen to be adjacent to one another. Whenever this back-to-back tow circumstance exists, the effective width of an unbridged tunnel crack would be approximately twice the crack width of a single minicomposite. This characteristic of back-to-back individual 901 minicomposites is much more com￾mon in the double-tow woven 3.95 epcm composite panel com￾pared with single-tow 7.9 epcm woven panels. Not only are there more regions of multiple 901minicomposites but also the length of these regions (distance the tow is woven over four tows before it is woven under the fifth tow in the five-harness satin archi￾tecture) would be approximately twice the length of single-tow woven composites for the five-harness satin weave because the epcm of the double-tow CMC was one-half that of the single￾tow CMC. As a result, as shown in Fig. 4, the double-tow woven Table II. Matrix Cracking Properties of Composite Specimens Panel DFLS, 0.002% offset (MPa) DFLS, 0.005% offset (MPa) First AE stress (MPa) First loud AE stress (MPa) AE onset stress (MPa) No. of loud events before AE onset stress— total # loud events Final matrix crack density (#/mm) Orthogonal oriented composites A1 147 174 132 170 190 4–141 — A2 130 173 100 159 182 1–141 10.3 A3 135 176 128 138 157 2–134 9.0 Off-axis oriented composites A4 210 225 56 197 220 1–65 4.0 B1 232 259 83 187 215 2–95 4.9 195 231 135 193 210 3–134 — AE, acoustic emmision. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 100 200 300 400 500 Stress, MPa Norm Cum AE Energy 7.9 epcm [0/90] [0/+67] braid 8.7 epcm, [+45] 3.95 epcm double-tow [0/90] 8.7 epcm [0/90] Fig. 4. Acoustic emission activity versus composite stress. 0 2 4 6 8 10 12 0 100 200 300 400 500 Composite Stress, MPa Estimated Crack Density, #/mm 7.9 epcm; [0/90] 3.95 epcm Double Tow [0/90] 8.7 epcm [0/90] [0/+67] braid 8.7 epcm, [+45] Fig. 5. Estimated matrix crack density with stress based on acoustic emission. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 100 200 300 400 500 Stress, MPa Norm Cum AE Energy 8.7 epcm [0/90] 1st AE event Braid & [0/90] 1st Loud AE event Braid & [0/90] [0/+67] braid Fig. 6. Acoustic emission (AE) events and onset stress determination. The arrows below the x-axis indicate AE onset stress. y For the CMC of this study, a minicomposite consists of a single multi fiber tow, the CVI BN interphase coating, and the initial CVI SiC matrix coating associated with tow. 3188 Journal of the American Ceramic Society—Morscher et al. Vol. 90, No. 10