G N. Morscher Composites Science and Technology 64 (2004)1311-1319 matrix compositions in order to maximize composite properties. Most of the development composite panels E荡 ave been processed with the sylramic- or the newer Sylramic-iBN fiber-type. The latter offers the strongest most creep-resistant, and most reliable composites to date. A variety of composite specimens were tested in this study from those developmental panels with a wide 语35 variation of the 2D woven, five-harness satin(5HS)ar- 寸R≥9g85g chitecture, e.g., changes in composite thickness, number of plies, number of tows per length, and the number of fibers per woven tow. An earlier work [14] compared the effects of these changes on the stress-strain curve in general. This study will concentrate on characterizing 甚ssss the effect these changes have on matrix cracking toward the development of a general relationship that can be used for purposes of component design and perfor- nance modeling 2. Experimental 6,55864,日 Unload-reload tensile hysteresis tests were performed R商送因后后 on ten different composite specimens, each from a dif ferent SiC/SiC composite panel, that varied in fiber tow ends per unit length, number of plies, and composite thickness. Two different fiber-types were used, Sylramic 上过过 Syl) and in situ-BN Sylramic(Syl-iBN) which is a modified Sylramic fiber that has been heat-treated [13] 兰 to form an in situ BN layer (iBN)on the fiber surface prior to composite fabrication. However, the differences 338 in the fiber-types are considered to not affect matrix cracking behavior. Both have 800 fibers per standard 当限喜 tow with an average fiber diameter of 10 um. For one composite panel, 041, two standard 800 fiber-count tows were woven together at 3.9 tow ends per cm(epcm)[15] In effect this was like having a woven tow with twice as many fibers, i.e., 1600, as the standard tow. However, 041 had the same fraction of fibers in the loading 号期可 direction as a single-tow weave of 7.9 epcm(011 in Table 1). Optical micrographs of polished longitudinal sections of the oll and the 041 composites are shown in ∞∞∞g必∞|s Fig. 1. It is evident that the double-tow woven com- posite essentially doubles the width dimension of the effective woven tow. The composites were processed by 5≥ the former Honeywell Advanced Composites(Newark, DE), currently known as General Electric Power Sys- [2]. Table 1 lists the consti ations for the composites tested. Note that there was considerable variation in fiber volume fraction and cvi 百 SiC volume fraction between the panels tested. Composite processing entails first stacking of bal anced 0/90 five-harness fabric woven from SYL or SYL iBN tows, a Bn interphase layer deposition (0.5 um) via CVi, a Sic interphase over-coating via CVI, Sic 自 3旨象 2 Dow Corning Corporation, Midland, MImatrix compositions in order to maximize composite properties. Most of the development composite panels have been processed with the Sylramic 2 or the newer Sylramic-iBN fiber-type. The latter offers the strongest, most creep-resistant, and most reliable composites to date. A variety of composite specimens were tested in this study from those developmental panels with a wide variation of the 2D woven, five-harness satin (5HS) ar￾chitecture, e.g., changes in composite thickness, number of plies, number of tows per length, and the number of fibers per woven tow. An earlier work [14] compared the effects of these changes on the stress–strain curve in general. This study will concentrate on characterizing the effect these changes have on matrix cracking towards the development of a general relationship that can be used for purposes of component design and perfor￾mance modeling. 2. Experimental Unload–reload tensile hysteresis tests were performed on ten different composite specimens, each from a dif￾ferent SiC/SiC composite panel, that varied in fiber tow ends per unit length, number of plies, and composite thickness. Two different fiber-types were used, Sylramic (Syl) and in situ-BN Sylramic (Syl-iBN) which is a modified Sylramic fiber that has been heat-treated [13] to form an in situ BN layer (iBN) on the fiber surface prior to composite fabrication. However, the differences in the fiber-types are considered to not affect matrix￾cracking behavior. Both have 800 fibers per standard tow with an average fiber diameter of 10 lm. For one composite panel, 041, two standard 800 fiber-count tows were woven together at 3.9 tow ends per cm (epcm) [15]. In effect this was like having a woven tow with twice as many fibers, i.e., 1600, as the standard tow. However, 041 had the same fraction of fibers in the loading direction as a single-tow weave of 7.9 epcm (011 in Table 1). Optical micrographs of polished longitudinal sections of the 011 and the 041 composites are shown in Fig. 1. It is evident that the double-tow woven com￾posite essentially doubles the width dimension of the effective woven tow. The composites were processed by the former Honeywell Advanced Composites (Newark, DE), currently known as General Electric Power Sys￾tems Composites [2]. Table 1 lists the constituent vari￾ations for the composites tested. Note that there was considerable variation in fiber volume fraction and CVI SiC volume fraction between the panels tested. Composite processing entails first stacking of bal￾anced 0/90 five-harness fabric woven from SYL or SYL￾iBN tows, a BN interphase layer deposition (
0.5 lm) via CVI, a SiC interphase over-coating via CVI, SiC Table 1 Properties of tested SiC/SiC specimens Specimen (type of debondinga) Sylramic fiber typeb Tow ends per cm No. of plies Specimen thickness (mm) Crack density after specimen failure, #/mm Fiber fractionc E (GPa) rth (MPa) BN fractionc CVI SiC fractionc fmini Emini Single-tow woven composites 007 (OD) AP 4.9 8 1.63 8.0 0.15 219 )35 0.04 0.11 0.30 357 016 (ID) iBN 4.9 8 2.04 11.6 0.12 279 )35 0.03 0.23 0.38 386 017 (ID) iBN 4.9 8 1.46 12.0 0.17 224 )53 0.04 0.14 0.35 366 012 (ID) iBN 7.1 6 1.99 7.2 0.14 289 )37 0.03 0.20 0.36 379 009 (MD) AP 7.9 8 2.18 9.0 0.18 246 )55 0.04 0.11 0.33 358 011 (MD) iBN 7.9 8 2.04 10.3 0.19 228 )57 0.04 0.12 0.34 361 018 (ID) iBN 7.9 4 1.37 5.0 0.14 235 )53 0.03 0.12 0.29 364 044 (OD) iBN 8.7 8 2.14 9.5 0.20 216 )35 0.03 0.13 0.35 369 068 (ID) IBN 8.7 8 2.21 10.4 0.20 277 )67 0.04 0.15 0.39 366 Double-tow woven composite 041 (OD) IBN 3.9(2)d 8 2.07 9.0 0.19 197 )50 0.03 0.11 0.33 363 a ID, inside debonding; OD, outside debonding; MD, mixed debonding. b AP, as-produced; iBN, in situ BN. Each fiber tow consisted of 800 fibers. The average fiber diameter was 10 lm. c Volume fraction of constituent in the load-bearing direction, the total fraction would be double this amount. dTwo tows were woven together into a 3.9 epcm fabric. In effect there were the same number of 800 count tows per length as the 7.9 epcm 011 specimen; however, the effective tow size for 041 was 1600 fibers. 2 Dow Corning Corporation, Midland, MI. 1312 G.N. Morscher / Composites Science and Technology 64 (2004) 1311–1319