G.N. Morscher et al. Composites Science and Technology 67(2007)1009-1017 Table 2 Mechanical properties of specimens tested Specimen: fo E, GPa Ult. stress, Failure Residual stress, 0.002% offset First AE stress, AE onset stress, Pe, m stress. M Hi-Nicalon CV SiC composites 10 0.1725 0 0.16225416 0.1822 30 ply (C) 36 ply(C) 0.17231391 8033805287 466159894 6406086%0 2443332 I ply ( 0.13 2 ply (C) 0.14104 0 0.16109 0 E8Ply-8HS(BN)0.17118 Sy/-iBN CH SiC composites l10 10.3 0.18226433 7.9epcm(3) 7.9epcm(3) 98092 0.21289509 9.epcm 0.21293 Gage -42 588 128 04500 0.12260258 l10 0.12261298 43 115 a typical hysteresis stress-strain curve with the 8. 2 shows 3.2. Acoustic emission and matrix cracking the amount of residual stress in the specimen associated with that test. Following Steen and Valles [8]. The AE activity for all the composites is shown in Fig 3 the average slope of the top portion of the hysteresis loops as normalized cumulative AE energy versus stress and are extrapolated back towards zero. Where those lines strain. This is determined by normalizing the cumulative intersect indicates whether or not there is residual stress energy at a given stress-strain condition by the total energy in the matrix. Since the lines intersect in the positive stress of all the events that occurred during the entire test in the and positive strain quadrant, the matrix is in compression, gage section. Two aE properties are given in Table 2 for i.e., crack closure occurs at a positive stress upon unload- each specimen. The"First AE Stress"is the stress at which ng the specimen. The Sylramic-ibN composites typically the first AE event was recorded and represents the onset of exhibited some measure of residual compressive stress in microcrack formation. The"AE Onset Stress"is the stress did not. The Sylramic-ibN 94epcm specimen, with the rence of large energy AE events and is indicated by the highest fo and higher elastic modulus, had the highest drastic increase in AE activity in Fig. 3a and c. This is residual stress (Table 2) indicative of the onset of large matrix crack formation where fiber-bridged cracks propagate through-the-thick ness or at least across several plies For the hn composites( Fig 3a and b), the onset of AE E=293 Gpa 35008 activity and the range of stress or strain over which AE 3000 a activity occurs varies from composite to composite. How 500 w ever, for the higher-density HN composites(8, 30, and 36 a 0.035+0.005%. For the Syl-iBN composites(Fig. 3c 1500 and d), the strain for onset of significant AE activity was 1000 higher, -0.05+0.005%. However, the range of strain Residual Compressive Stress for the different Syl-iBN composites Normalized cumulative Ae energy has been shown to be Strain, an excellent measure of relative matrix crack density [6]. Fig 4 shows some typical matrix cracks from a polished Fig. 2. Stress-strain for a 9.epcm specimen, load-unload-reload hyster- section of the failed 7.95epcm(C) specimen. Multiplying esis tensile test with AE activity the final matrix crack density (Table 2)measured fromthe amount of residual stress in the specimen. Fig. 2 shows a typical hysteresis stress–strain curve with the AE activity associated with that test. Following Steen and Valles [8], the average slope of the top portion of the hysteresis loops are extrapolated back towards zero. Where those lines intersect indicates whether or not there is residual stress in the matrix. Since the lines intersect in the positive stress and positive strain quadrant, the matrix is in compression, i.e., crack closure occurs at a positive stress upon unload￾ing the specimen. The Sylramic-iBN composites typically exhibited some measure of residual compressive stress in the matrix whereas the Hi-Nicalon composites typically did not. The Sylramic-iBN 9.4epcm specimen, with the highest f 0 f and higher elastic modulus, had the highest residual stress (Table 2). 3.2. Acoustic emission and matrix cracking The AE activity for all the composites is shown in Fig. 3 as normalized cumulative AE energy versus stress and strain. This is determined by normalizing the cumulative energy at a given stress–strain condition by the total energy of all the events that occurred during the entire test in the gage section. Two AE properties are given in Table 2 for each specimen. The ‘‘First AE Stress’’ is the stress at which the first AE event was recorded and represents the onset of microcrack formation. The ‘‘AE Onset Stress’’ is the stress at which significant AE activity occurs due to the occur￾rence of large energy AE events and is indicated by the drastic increase in AE activity in Fig. 3a and c. This is indicative of the onset of large matrix crack formation where fiber-bridged cracks propagate through-the-thick￾ness or at least across several plies. For the HN composites (Fig. 3a and b), the onset of AE activity and the range of stress or strain over which AE activity occurs varies from composite to composite. How￾ever, for the higher-density HN composites (8, 30, and 36 ply), the strain for onset of significant AE activity was 0.035 ± 0.005%. For the Syl-iBN composites (Fig. 3c and d), the strain for onset of significant AE activity was higher, 0.05 ± 0.005%. However, the range of strain and stress over which cumulative AE activity occurs varies for the different Syl-iBN composites. Normalized cumulative AE energy has been shown to be an excellent measure of relative matrix crack density [6]. Fig. 4 shows some typical matrix cracks from a polished section of the failed 7.95epcm(C) specimen. Multiplying the final matrix crack density (Table 2) measured from Table 2 Mechanical properties of specimens tested Specimen: Panel f 0 f E, GPa Ult. stress, MPa Failure location Residual stress, MPa 0.002% offset stress, MPa First AE stress, MPa AE onset stress, MPa qc, mm1 s, MPa Hi-Nicalon CVI SiC composites 8 ply (C) 0.14 199 300 Gage 10 68 24 61 2.2 14 8 ply (BN1) 0.17 258 415 Gage 0 90 66 94 4.6 35 8 ply (BN2) 0.16 225 416 Gage 0 73 63 70 4.3 31 8 ply (BN3) 0.18 225 367 Gage – 83 51 86 3.6 20 30 ply (C) 0.17 237 328 Radius – 88 15 70 3.4 30 36 ply (C) 0.17 231 391 Radius – 80 19 78 3.5 27 1 ply (C) 0.13 96 110 Grip – 55 28 56 2.0 25 2 ply (C) 0.14 104 274 Grip 0 92 29 95 10.6 – 3 ply (C) 0.16 109 380 Grip 0 83 24 80 11.6 – E8Ply-8HS(BN) 0.17 118 364 Gage 0 77 48 85 10.8 33 Syl-iBN CVI SiC composites 7.9epcm(1) .18 247 432 Gage – 138 69 110 10.3 48 7.9epcm(2) 0.18 254 424 Gage – 135 91 120 9.0 45 7.9epcm(2) 0.18 226 433 Gage 25 131 84 117 – – 7.9epcm(3) 0.18 278 445 Gage – 158 107 150 8.1 43 7.9epcm(3) 0.18 276 – Gage 30 153 97 145 – – 9.4epcm 0.21 289 509 Radius – 188 122 155 10.6 – 9.4epcm 0.21 293 500 Gage 42 180 128 150 10.1 59 5.5epcm 0.12 260 258 Gage – 140 110 110 8.3 – 5.5epcm 0.12 261 298 Gage 30 143 115 123 9.4 63 7.9epcm(C) 0.17 230 387 Gage 30 120 69 114 6.7 28 0 100 200 300 400 500 600 0 0.1 0.2 0.3 0.4 0.5 Strain, % Stress, MPa 0 500 1000 1500 2000 2500 3000 3500 4000 Cumulative AE Energy E = 293 Gpa fo = 0.21 Residual Compressive Stress Fig. 2. Stress–strain for a 9.4epcm specimen, load–unload–reload hyster￾esis tensile test with AE activity. 1012 G.N. Morscher et al. / Composites Science and Technology 67 (2007) 1009–1017