J. Haslam et al. Journal of the European Ceramic Society 20(2000)607-618 to the phenomenon of sequential, delamination failure A slight reduction in stiffness was observed near the Tensile Failures peak loads. At the first load drop, cracking was usually cracks became apparent after loading beyond the initial load drop Levi et al, referene 3. 2. In-plane flexure testing 200 C1 hr. /HCI Table I reports the in-plane mechanical properties for 1200 C/5 hrs /HCI un-notched 00/90 and +/-45 oriented composite 1050C/2 hrs /Air, 1250C/5 hrs. /HCI pecimens processed in HCI for different conditions The strength of the 0/90 specimens was >160 MPa Span to Thickness 9 10 and the strength of the +/-45o specimens was >80 MPa consistent with those reported by Levi et al. for the pre- Fig3. Plot of interlaminar shear strength against span to vious method of processing the porous matrix composites thickness ratio for a variety of samples. The 1250C/5 h/HCI samples Fig. 6 shows representative stress-strain curves for 00/90 failed in a tensile failure mode which implies that the interlaminar and +/-450 composites(1250C/5h/HCD); their respec shear strength is greater than these values tive failure strains were 0.25 and 0.2% Table 2 reports the in-plane mechanical properties of the notched specimens 0/90 and +1-45 oriented composite specimens processed with the same condi tions as the un-notched specimens (Table 1). Strength values are reported for a'net cross-sectional area'spe cimen, i.e. assuming that the bar dimensions used to E:5 calculate the maximum tensile stress at failure [Eq. (2) does not include the notched portion of the bar. As shown, the strength of the notched bars, based on the net cross-sectional area is 70% of the un-notched bars, regardless of fiber orientation. Fig. 7 shows the load/ displacement curves for representative 0 /90 and +/-45 notched specimens. It can be seen that the failure was not catastrophic, and the work of fracture, reported 0,0% 20% in Table 2, could be determined by measurements of the Nominal flexure strain area under the curve. The work of fracture for the 0o/ 90 specimens was 3-4 times greater than the +/-45o orientation sintered in H 200c strength test result: 0/90 fiber oriented Fig 8, a photograph of one of fractured, notched 0/90 oriented specimen, illustrates that interlocking fibers still hold the bar together after the applied load is removed. It was observed that both notched and un-notched specimens did not fall apart after the load was removed Table I Properties of in-plane un-notched composite 3-point bend tests Fiber Flexure Strength Modulu let stopped GPa 1250°c/5h/HCl 1250°C/5h/HCl 1200°c/5h/HCl Fig. 5. "Nominal Interlaminar shear strength test result: 0/90 fiber +/-45 1250°C/5h/HCl +/-45 orientation sintered in Hcl at 12500C. 5h 1250°C/5b/HClto the phenomenon of sequential, delamination failure.23 A slight reduction in sti€ness was observed near the peak loads. At the ®rst load drop, cracking was usually not evident under low magni®cation (2±3X), but the cracks became apparent after loading beyond the initial load drop. 3.2. In-plane ¯exure testing Table 1 reports the in-plane mechanical properties for un-notched 0/90 and +/ÿ45 oriented composite specimens processed in HCI for di€erent conditions. The strength of the 0/90 specimens was >160 MPa, and the strength of the +/ÿ45 specimens was >80 MPa consistent with those reported by Levi et al.5 for the pre￾vious method of processing the porous matrix composites. Fig. 6 shows representative stress±strain curves for 0/90 and +/ÿ45 composites (1250C/5h/HCl); their respec￾tive failure strains were 0.25 and 0.2%. Table 2 reports the in-plane mechanical properties of the notched specimens 0/90 and +/ÿ45 oriented composite specimens processed with the same condi￾tions as the un-notched specimens (Table 1). Strength values are reported for a `net cross-sectional area' spe￾cimen, i.e. assuming that the bar dimensions used to calculate the maximum tensile stress at failure [Eq. (2)] does not include the notched portion of the bar. As shown, the strength of the notched bars, based on the `net cross-sectional area' is 70% of the un-notched bars, regardless of ®ber orientation. Fig. 7 shows the load/displacement curves for representative 0/90 and +/ÿ45 notched specimens. It can be seen that the failure was not catastrophic, and the work of fracture, reported in Table 2, could be determined by measurements of the area under the curve. The work of fracture for the 0/ 90 specimens was 3±4 times greater than the +/ÿ45 oriented specimens. Fig. 8, a photograph of one of the fractured, notched 0/90 oriented specimen, illustrates that interlocking ®bers still hold the bar together after the applied load is removed. It was observed that both notched and un-notched specimens did not fall apart after the load was removed. Fig. 4. `Nominal' Interlaminar shear strength test result: 0/90 ®ber orientation sintered in HCl at 1200C, 5s. Fig. 3. Plot of `nominal' interlaminar shear strength against span to thickness ratio for a variety of samples. The 1250C/5 h/HCl samples failed in a tensile failure mode which implies that the interlaminar shear strength is greater than these values. Fig. 5. `Nominal' Interlaminar shear strength test result: 0/90 ®ber orientation sintered in HCl at 1250C, 5h. Table 1 Properties of in-plane un-notched composite 3-point bend tests Fiber Sintering Flexure Strength MPa Elastic Modulus GPa 0/90 1250C/5 h/HCl 165 64.4 0/90 1250C/5 h/HCl 168 61.6 0/90 1200C/5 h/HCl 152 50.2 +/ÿ45 1250C/5 h/HCl 85 45.9 +/ÿ45 1250C/5 h/HCl 88 41.9 612 J.J. Haslam et al. / Journal of the European Ceramic Society 20 (2000) 607±618