G N. Morscher Composites Science and Technology 64 (2004)1311-1319 500 0+1 3.9(epcm; f 3501060:8pcm:t= 250 .epcm: f= 6:4epcm;f=0.12 50 0.5 0.6 0.7 Strain. Fig. 3. Stress-strain curves for the specimens tested and analyzed in his study. Note, unload-reload hysteresis loops have been removed for clarity Fig 4. Example of matrix cracks observed along a polished and etched longitudinal section of the tensile specimen(017) after failure at room temperatur Fig. 3 shows stress-strain curves. unload-reload loops removed, for a variety of different architecture composites,i.e, different panels. As is expected, the T12 posites tend to exhibit higher 649epcm;向=0.12 action com ultimate strengths, higher stresses for the"knee"in the 04t;3.92)epcm;017:49 epcm, I stress-strain curve, and steeper secondary slopes [14] There was considerable scatter in elastic moduli. some of 88068:8.epcm: 011;7.9 epcm: fe019 which was due to the anomalies described below The matrix crack density was determined fo 007: 4.9opcm: fat 012:7.1 eocm: f=0.13 of matrix cracking in these composites for a portion a p/ specimen after tensile testing. Fig 4 shows an example oeE pecimen cut and polished from the gage section fol lowed by a plasma-etch. No matrix cracks are visible, because of the residual compression in the matrix and 00050.10.15020.25 0.35 the higher interfacial shear stresses of composites with Strain. nic fibers, without plasma etching The energy of aE has shown to be a good measure of the relative crack density when the more accurate 017:4.9epcm0.17 "modal"AE approach is used for these types of com- 8.epcm, fa posite systems[17]. In other words, the relative amount 2 041:3.92pcm of cumulative AE energy is nearly directly related to the g relative number of matrix cracks formed. Therefore the 007:49epmo final matrix crack density measured from the composite test specimens was multiplied by the normalized cumu- lative AE energy(e.g, Fig. 2(b))for each specimen in 018:79epcm台=0.14 order to estimate the stress-dependent matrix crack distribution. the estimated crack distributions are shown in Fig. 5 for a number of specimens versus strain and stress. The strain and stress distribution for matrix 150200250300350 cracking varies from specimen to specimen considerably. ter to define the earliest formation of Fig. 5. Estimated matrix cracking(normalized AE measured during large matrix cracks is theonset"strain or stress at strain and (b)stress for standard single-tow woven composites and a which the rate of AE rapidly increases. The sudden double-tow woven composite(041). crease in AE activity is due to high-energy events that are associated with large bridged matrix cracks that with another matrix crack) of the specimen [16, 17] propagate through-the-thickness(or at least a significant Fig. 2(a) and(b) show the determination of Eonset and portion of the cross-section if a matrix crack links up onset from extrapolation of the initial high-rate AEFig. 3 shows stress–strain curves, unload–reload loops removed, for a variety of different architecture composites, i.e., different panels. As is expected, the higher volume fraction composites tend to exhibit higher ultimate strengths, higher stresses for the ‘‘knee’’ in the stress–strain curve, and steeper secondary slopes [14]. There was considerable scatter in elastic moduli, some of which was due to the anomalies described below. The matrix crack density was determined for each specimen after tensile testing. Fig. 4 shows an example of matrix cracking in these composites for a portion of a specimen cut and polished from the gage section fol￾lowed by a plasma-etch. No matrix cracks are visible, because of the residual compression in the matrix and the higher interfacial shear stresses of composites with Sylramic fibers, without plasma etching. The energy of AE has shown to be a good measure of the relative crack density when the more accurate ‘‘modal’’ AE approach is used for these types of com￾posite systems [17]. In other words, the relative amount of cumulative AE energy is nearly directly related to the relative number of matrix cracks formed. Therefore, the final matrix crack density measured from the composite test specimens was multiplied by the normalized cumu￾lative AE energy (e.g., Fig. 2(b)) for each specimen in order to estimate the stress-dependent matrix crack distribution. The estimated crack distributions are shown in Fig. 5 for a number of specimens versus strain and stress. The strain and stress distribution for matrix cracking varies from specimen to specimen considerably. One useful parameter to define the earliest formation of large matrix cracks is the ‘‘onset’’ strain or stress at which the rate of AE rapidly increases. The sudden in￾crease in AE activity is due to high-energy events that are associated with large bridged matrix cracks that propagate through-the-thickness (or at least a significant portion of the cross-section if a matrix crack links up with another matrix crack) of the specimen [16,17]. Fig. 2(a) and (b) show the determination of eonset and ronset from extrapolation of the initial high-rate AE Fig. 3. Stress–strain curves for the specimens tested and analyzed in this study. Note, unload–reload hysteresis loops have been removed for clarity. Fig. 4. Example of matrix cracks observed along a polished and etched longitudinal section of the tensile specimen (017) after failure at room temperature. Fig. 5. Estimated matrix cracking (normalized AE measured during the stress–strain test multiplied by measured crack density) versus (a) strain and (b) stress for standard single-tow woven composites and a double-tow woven composite (041). 1314 G.N. Morscher / Composites Science and Technology 64 (2004) 1311–1319