J. Martinez-Fernandez, G.N. Morscher / Journal of the European Ceramic Society 20(2000)2627-2636 stress of 25-50 MPa to the peak stress. The loading ultimate failure composite stress ranged from 280 to 350 requency was 0.01 Hz (100 s per cycle) for all the MPa; however, most of the minicomposites(approxi experiments. For the fatigue tests, the load-cycle was mately 75%)failed between 310 and 325 MPa, with an begun after reaching the set temperature. The peak load average strength of 317-+26 MPa. The average ultimate as applied prior to and during heating just as for the strength of the fibers determined by the cross-sectional rupture tests. The minicomposites were also precracked area of load-bearing fibers(fully loaded) was 2000_+200 prior to the cyclic test at 280 MPa. MPa. The strength of individual HN fibers (25.4 mm Some minicomposites were polished longitudinally gage length)is -2800 MPa whereas the average strength after testing, and the crack spacing was measured by of as-produced tows of hn(25.4 mm gage length)is optical microscopy. The fracture surfaces were observed 1700 MPa. 4 Based on Eq (2), the actual fully-loaded using a scanning electron microscope(SEM) Jeol 840A gage length is on the order of 30 to 50 mm, therefore, in (eol, Tokyo, Japan) comparison to single tow strength, no strength loss due to minicomposite fabrication occurred Fig. 2. shows the cumulative number of events recor- 3. Results ded normalized by the gage length of the minicomposite en. The nature of Ae activity was very co 3.1. Room temperature mechanical testing for the specimens tested. For several samples, the num ber of cracks was measured from polished longitudinal The load versus time curve for a typical monotonic sections. The crack spacing was determined from the loading experiment is shown in Fig. 1. The curves are number of cracks counted over a given length of linear up to approximately 84.4 N(200 MPa composite minicomposite. In the Appendix, Table Al shows stress), when matrix cracking starts to occur. The the nearly one-to-one correspondence between the occurrence of AE events(also shown in Fig. 1 )indi- number of highest energy events recorded and the cates that the change in slope is associated with matrix estimated number of cracks in the gage section of cracking. The cumulative number of AE events and the the minicomposite cumulative total energy increase at the same rate. The 3. 2. Elevated temperature mechanical testing The data from the constant load stress-rupture experiments are plotted in Fig. 3a and b as the stress applied (if fibers were fully loaded) versus the time to rupture( the arrows indicate the minicomposite did not fail). These data are plotted together with previous data from C-NIC minicomposites(Fig. 3a)and BN-HN minicomposites(Fig 3b).2 The C-HN minicomposites have longer survival times than the C-NIC mini- 50100150200250300 composites at 700 C although for higher loads the two curves tend to blend together. The C-hn mini al load vs. time curve for a monotonic loading experi. composites have shorter survival times than the BN- Imber and cumulative energy (in arbitary units)of the HN minicomposites especially at 700 and 950C. All the also shown samples broke in the hot region of the furnace. For hese tests the minicomposites were precracked at 280 MPa(composite stress). Stress-rupture tests were also performed using a lar ger precrack load In Fig 4, the C-Hn data from Fig 3 (where a precrack load of 280 MPa was used) are plot- 9 N Precrack ted compared with the data from samples precracked at 295 MPa. At 700 and 950C. the time of survival decreases with increasing precrack load. At 1200C, this 04 effect was not as pronounced as at the lower tempera ture tests. The use of a larger precrack load introduced a greater amount of scatter in the data. The precrack stresses of 280 MPa (load of 119 N) and 295 MPa(load of 126 N) correspond with 88 and 93% of the average Fig. 2. Number of events per mm vs load for several specimens tested strength, respectively. The precrack load used in a pre- at room temperature vious work for C-NIC and BN-hn was 60%(110 N)stress of 25±50 MPa to the peak stress. The loading frequency was 0.01 Hz (100 s per cycle) for all the experiments. For the fatigue tests, the load-cycle was begun after reaching the set temperature. The peak load was applied prior to and during heating just as for the rupture tests. The minicomposites were also precracked prior to the cyclic test at 280 MPa. Some minicomposites were polished longitudinally after testing, and the crack spacing was measured by optical microscopy. The fracture surfaces were observed using a scanning electron microscope (SEM) Jeol 840A (Jeol, Tokyo, Japan). 3. Results 3.1. Room temperature mechanical testing The load versus time curve for a typical monotonic loading experiment is shown in Fig. 1. The curves are linear up to approximately 84.4 N (200 MPa composite stress), when matrix cracking starts to occur. The occurrence of AE events (also shown in Fig. 1.) indi￾cates that the change in slope is associated with matrix cracking. The cumulative number of AE events and the cumulative total energy increase at the same rate. The ultimate failure composite stress ranged from 280 to 350 MPa; however, most of the minicomposites (approxi￾mately 75%) failed between 310 and 325 MPa, with an average strength of 31726 MPa. The average ultimate strength of the ®bers determined by the cross-sectional area of load-bearing ®bers (fully loaded) was 2000200 MPa. The strength of individual HN ®bers (25.4 mm gage length) is 2800 MPa whereas the average strength of as-produced tows of HN (25.4 mm gage length) is 1700 MPa.14 Based on Eq. (2), the actual fully-loaded gage length is on the order of 30 to 50 mm, therefore, in comparison to single tow strength, no strength loss due to minicomposite fabrication occurred. Fig. 2. shows the cumulative number of events recor￾ded normalized by the gage length of the minicomposite specimen. The nature of AE activity was very consistent for the specimens tested. For several samples, the num￾ber of cracks was measured from polished longitudinal sections. The crack spacing was determined from the number of cracks counted over a given length of minicomposite. In the Appendix, Table A1 shows the nearly one-to-one correspondence between the number of highest energy events recorded and the estimated number of cracks in the gage section of the minicomposite. 3.2. Elevated temperature mechanical testing The data from the constant load stress-rupture experiments are plotted in Fig. 3a and b as the stress applied (if ®bers were fully loaded) versus the time to rupture (the arrows indicate the minicomposite did not fail). These data are plotted together with previous data from C±NIC minicomposites (Fig. 3a) and BN±HN minicomposites (Fig. 3b).2 The C±HN minicomposites have longer survival times than the C±NIC mini￾composites at 700C although for higher loads the two curves tend to blend together. The C±HN mini￾composites have shorter survival times than the BN± HN minicomposites especially at 700 and 950C. All the samples broke in the hot region of the furnace. For these tests the minicomposites were precracked at 280 MPa (composite stress). Stress±rupture tests were also performed using a lar￾ger precrack load. In Fig. 4, the C±HN data from Fig. 3 (where a precrack load of 280 MPa was used) are plot￾ted compared with the data from samples precracked at 295 MPa. At 700 and 950C, the time of survival decreases with increasing precrack load. At 1200C, this e€ect was not as pronounced as at the lower tempera￾ture tests. The use of a larger precrack load introduced a greater amount of scatter in the data. The precrack stresses of 280 MPa (load of 119 N) and 295 MPa (load of 126 N) correspond with 88 and 93% of the average strength, respectively. The precrack load used in a pre￾vious work for C±NIC and BN±HN was 60% (110 N) Fig. 1. Typical load vs. time curve for a monotonic loading experi￾ment. The number and cumulative energy (in arbritary units) of the AE events are also shown. Fig. 2. Number of events per mm vs load for several specimens tested at room temperature. J. Marti nez-FernaÂndez, G.N. Morscher / Journal of the European Ceramic Society 20 (2000) 2627±2636 2629