J. Martinez.Fernandez, G N Morscher Journal of the European Ceramic Society 20(2000)2627-2636 Table al AE and matrix crack data for several room temperature tested minicomposites Specimen Gage Speed of Number Failure Crack nated number length unda(m/s) of events load (N) with highest ks in ga two decades of AE energy 928 1.67 a Extensional wave the lead-break event. As a composite forms matrix The reduction in speed of sound is about 12%. Since cracks during testing the elastic modulus decreases and the speed of sound of the extensional wave is directly pro- the speed of sound decreases resulting in an increase in portional to the square root of the elastic modulus, the Alx. For the tensile experiment Ae data, the maximum elastic modulus is reduced by approximately 23%. This difference in time of arrival was determined as a func- is not as great a reduction in elastic modulus as in woven A2 shows the decrease in speed of sound of the exten The events that were determined to occur outside of sional wave with load. This corresponds to the reduc- the gage section, i.e. events with difference in time of tion in square root of the elastic modulus of the arrival, 4t, equal to 4tx, were removed from the data specimen as matrix cracking occurs set. The location of each event along the gage length of the specimen could then be determined based on At, Alr,and the sensor which received the signal first. 22 Fig A3 shows an example of event location as a func- tion of load. The acoustic energy was determined for 11400 each event. It was observed that for the specimens where only two sensors were used, all of the AE event 11200 energies were within two decades of energy from one another. However, for the specimens where four sensors were used, the spread in AE energies was over three orders of magnitude. Also, many AE events were trig 10400 gered by the more sensitive resonant frequency trans ducers. These events would never have triggered the wide-band sensors alone at the preamplification settings used. The events were therefore filtered according to ae Fig. A2. Speed of sound vs load for specimen h3hc70 energy where only the two highest decades of events were used to relate to the accumulated d Table Al shows the event statistics along with the sured crack spacings for five specimens. Note the excel lent agreement between the estimated number of cracks over the entire gage length of the samples compared to 0 he highest two orders of magnitude AE energy events The cumulative number and energies of the sorted AE events were used for Figs. 2 and 3 References L. Heredia, F. E, McNulty, J C, Zok, F. w. and Evans, A. G, Am. Ceram.Soc,1995,78.2097-2100 Fig. A3. Location of the ae events along the length of the mini 2. Morscher. G. N, Tensile stress rupture of SiCf/ Sicm mini composite(h3hc70)during a tensile monotonic loading test as a func- mposites with carbon and boron nitride interphases at elevatedthe lead-break event.22 As a composite forms matrix cracks during testing the elastic modulus decreases and the speed of sound decreases resulting in an increase in
tx. For the tensile experiment AE data, the maximum di€erence in time of arrival was determined as a func￾tion of load and the speed of sound determined from the distance between the two sensors divided by
tx. Fig. A2 shows the decrease in speed of sound of the exten￾sional wave with load. This corresponds to the reduc￾tion in square root of the elastic modulus of the specimen as matrix cracking occurs. The reduction in speed of sound is about 12%. Since the speed of sound of the extensional wave is directly pro￾portional to the square root of the elastic modulus, the elastic modulus is reduced by approximately 23%. This is not as great a reduction in elastic modulus as in woven macrocomposites,  60%, that reach a state of matrix crack saturation at relatively low composite stresses.22 The events that were determined to occur outside of the gage section, i.e. events with di€erence in time of arrival, Dt, equal to Dtx, were removed from the data set. The location of each event along the gage length of the specimen could then be determined based on
t,
tx, and the sensor which received the signal ®rst.22 Fig. A3 shows an example of event location as a func￾tion of load. The acoustic energy was determined for each event. It was observed that for the specimens where only two sensors were used, all of the AE event energies were within two decades of energy from one another. However, for the specimens where four sensors were used, the spread in AE energies was over three orders of magnitude. Also, many AE events were trig￾gered by the more sensitive resonant frequency trans￾ducers. These events would never have triggered the wide-band sensors alone at the preampli®cation settings used. The events were therefore ®ltered according to AE energy where only the two highest decades of energy events were used to relate to the accumulated damage. Table A1 shows the event statistics along with the mea￾sured crack spacings for ®ve specimens. Note the excel￾lent agreement between the estimated number of cracks over the entire gage length of the samples compared to the highest two orders of magnitude AE energy events. The cumulative number and energies of the sorted AE events were used for Figs. 2 and 3. References 1. Heredia, F. E., McNulty, J. C., Zok, F. W. and Evans, A. G., Oxidation embrittlement probe for ceramic-matrix composites. J. Am. Ceram. Soc., 1995, 78, 2097±2100. 2. Morscher, G. N., Tensile stress rupture of SiCf/SiCm mini￾composites with carbon and boron nitride interphases at elevated temperatures in air. J. Am. Ceram. Soc., 1997, 80, 2029±2042. Fig. A2. Speed of sound vs load for specimen h3hc70. Table A1 AE and matrix crack data for several room temperature tested minicomposites Specimen Gage length (mm) Speed of sounda (m/s) Number of events Failure load (N) Crack spacing (mm) No. of events with highest two decades of AE energy Estimated number of cracks in gage length h3hc39 140 13 861 212 151 ± 135 ± h3hc40 158 15 192 579 152 0.56 238 250 h3hc70 160 11 851 149 143 1.05 149 133 h3hc76 168 13 228 132 143 0.96 132 146 h3hc77 170 12 500 87 138 1.67 87 84 a Extensional wave. Fig. A3. Location of the AE events along the length of the mini￾composite (h3hc70) during a tensile monotonic loading test as a func￾tion of time. J. Marti nez-FernaÂndez, G.N. Morscher / Journal of the European Ceramic Society 20 (2000) 2627±2636 2635