International Journal of Applied Ceramic Technolog-Morscher and pujar Vol.6,No.2,2009 0005%Ote fractions of fiber, BN, and CVI SiC(Table D). Emini was determined from the rule of mixtures of the fractions of First LOUd Al the three components of the minicomposite where the fiber moduli used were 380 GPa for Syl-iBN, SA, and HNS: 280 GPa for HN; and 200 GPa for ZMI. The bn modulus was 25 GPa, and the CVI SiC modulus was 425 GPa. Plotting NormCumAE versus ominimatris Fig 5 shows essentially that all of the aE behavior of the composites of this study now falls into a much narrower ominimatrix range of <+20 MPa. This reduced range is similar to what was found for the MI composites pro- 140 160180 cessed by a different vendor in a previous study 0. 005% Offset Stress, MPa For predictive modeling, an empirical two-param Fig 4 of0.005% offset stress, AE onset stress and eter Weibull distribution for Norm CumAE was best first Loud AE event for composite specimens tested in this study fitted to the data in Fig. 4 composites. As shown in Fig 4, the AE onset stress and NormCumAE 005% offset stress are close in magnitude; however 1-exp(ominimatrix/ominimatri) the 0.005% offset stress almost always exceeded the AE onset stress for the composites of this study. The AE onset stress is considered to be a better parameter for matrix cracking because it is a direct measure of when where ominimatris is a reference stress and m is the Weibull modulus. For this study, the distribution func- matrix cracks actually occur There is a large scatter in stress over which aE ac tion was only slightly different from that determined in ivity occurs for the different composite systems Morscher6(see Fig. 5). Also note that extrapolating the (+70 MPa for the composites tested in this study- slope of the near-linear region of the distribution rep- Fig 3). It has been shown in Morscher.that the ma- esents a general Ominimatrix for onset of high-energy AE activity, that is, through-thickness matrix crackin trix cracking behavior of 2D woven MI composites with The stress corresponding to the onset of through-thick- similar-sized tows could be correlated to the stress in the region of the composite outside the load-bearing o because these cracks allow relatively easy access for ox- 0 fiber tow and its associated interphase coating and idizing species into the composite and cause environ- CVI SIC matrix. To estimate the in situ stress on the minimatrix? "material, or the portion of the composite This siupgngirine outside of the load-bearing 0" minicomposite, including the 90 minicomposites and the surrounding slurry-cast and MI material, it was shown in Morscher that a sim- a reen= SA H le rule of mixtures relationship could be used 0.6 Purple = ZMI Model from reference 16: E E 1 -mini where oc is the applied composite stress; och is the re- w95 MPa Onset Minirmatrix Siress sidual stress within the as-fabricated composite; Ec is elastic modulus of the composite; Emini is the elastic modulus of the load-bearing 0 f mini is the fraction of 0 minicomposites in the com- Fig. 5. Normalized cumulative acoustic emission energy plotted posite. The composites of this study were all balanced versus the stress in the matrix outside of the load-bearing weaves;therefore, mini is simply half the combined total minicom at 1s. animatrix stresscomposites. As shown in Fig. 4, the AE onset stress and 0.005% offset stress are close in magnitude; however, the 0.005% offset stress almost always exceeded the AE onset stress for the composites of this study. The AE onset stress is considered to be a better parameter for matrix cracking because it is a direct measure of when matrix cracks actually occur. There is a large scatter in stress over which AE ac￾tivity occurs for the different composite systems (770 MPa for the composites tested in this study— Fig. 3). It has been shown in Morscher7,13 that the ma￾trix cracking behavior of 2D woven MI composites with similar-sized tows could be correlated to the stress in the region of the composite outside the load-bearing 01 minicomposite, where the minicomposite consists of the 01 fiber tow and its associated interphase coating and CVI SiC matrix. To estimate the in situ stress on the ‘‘minimatrix’’ material, or the portion of the composite outside of the load-bearing 01 minicomposite, including the 901 minicomposites and the surrounding slurry-cast and MI material, it was shown in Morscher7 that a sim￾ple rule of mixtures relationship could be used: sminimatrix ¼ ðsc þ sthÞ Ec Ec fminiEmini 1 fmini ð2Þ where sc is the applied composite stress; sth is the re￾sidual stress within the as-fabricated composite; Ec is elastic modulus of the composite; Emini is the elastic modulus of the load-bearing 01 minicomposite; and fmini is the fraction of 01 minicomposites in the com￾posite. The composites of this study were all balanced weaves; therefore, fmini is simply half the combined total fractions of fiber, BN, and CVI SiC (Table I). Emini was determined from the rule of mixtures of the fractions of the three components of the minicomposite where the fiber moduli used were 380 GPa for Syl-iBN, SA, and HNS; 280 GPa for HN; and 200 GPa for ZMI. The BN modulus was 25 GPa, and the CVI SiC modulus was 425 GPa. Plotting NormCumAE versus sminimatrix, Fig. 5 shows essentially that all of the AE behavior of the composites of this study now falls into a much narrower sminimatrix range of o720 MPa. This reduced range is similar to what was found for the MI composites pro￾cessed by a different vendor in a previous study.7,16 For predictive modeling, an empirical two-param￾eter Weibull distribution for NormCumAE was best fitted to the data in Fig. 4: NormCumAE ¼ 1 exp ðsminimatrix=so minimatrixÞ m ð3Þ where so minimatrixis a reference stress and m is the Weibull modulus. For this study, the distribution func￾tion was only slightly different from that determined in Morscher16 (see Fig. 5). Also note that extrapolating the slope of the near-linear region of the distribution rep￾resents a general sminimatrix for onset of high-energy AE activity, that is, through-thickness matrix cracking. The stress corresponding to the onset of through-thick￾ness matrix cracks is an important design parameter, because these cracks allow relatively easy access for ox￾idizing species into the composite and cause environ- 100 120 140 160 180 200 225 200 175 150 125 100 75 50 0.005% Offset Stress, MPa Stress, MPa 0.005% Offset Stress AE Onset Stress First Loud AE Event Fig. 4. Comparison of 0.005% offset stress, AE onset stress and first Loud AE event for composite specimens tested in this study. 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0 50 100 150 200 250 300 Minimatrix Stress, MPa Norm Cum AE Blue = Syl-iBN Red = HNS Green = SA Gray = HN Purple = ZMI Fig. 5. Normalized cumulative acoustic emission energy plotted versus the stress in the matrix outside of the load-bearing minicomposites, that is, minimatrix stress. 156 International Journal of Applied Ceramic Technology—Morscher and Pujar Vol. 6, No. 2, 2009