1656 Journal of the American Ceramic Society-Morscher and pujar Vol. 89. No. 5 (a) 400PanelA2After Panel B 138 Mpa 100 h Creep As-produced creep strain =0.23% AS-Produced 250 (hys loops removed Panel B After 138 MPa 100 100 h Creep creep strain= 0.36% 50 Compressive Stress 0.1 0.2 0.304 0.1 Strain. 0.8 0.8 u0.7 60.5 E0.4 0.produced 20.3 After-Creep 0 100150200250300350 Stress, MP 125MPa0.002% Fig. 5.(a)Room temperature unload-reload hysteresis curves to fail- ure for panel B specimens. Note, the hysteresi from the as-produced specimen for clarity.(b) Acoustic emission gener- ated from the gage section for both tensile specimens 1201 Panel A2 As-Produced 0.005% offset stress (>0.3%). This suggests most of the composite creep observed n these specimens was primarily because of the creep of the constituents in combination with some minor bridged crack 0. 002% offset wth 40 20 0.005%oset 0.06 For this HNS/MI composite system, excellent creep resistance % as observed when loaded up to 103 MPa at 1315C in the pri- Fig 4.(a) Room ary fiber direction. The creep strains measured in this study are s Note, the hysteresis loops have been re. comparable with those measured for the Sylramic-iBN MI com- oved from the as. men for clarity.(b)Acoustic emission posites when tested under similar conditions. For example, the generated from th tion for both tensile specimens. (c)The con- offset stresses for the as-produced specimen and 103 MPa for 100 h were 0.10%+0.05%. similar to the ested to failur creep strains measured in this study for the same conditions Table ID). In addition, composite creep appears to lead to load relaxation in the matrix and consequently residual compressive Table IV. Properties Associated with Non-Linearity in Room Temperature Tested Composites As-Produced and After 138 MPa, 100 Panela Panel b Mechanical property As-produced specimen After creep specimen s-produced specimen After creep specimen 0.002% offset 125 MPa 142 MPa 151 MPa 150 MPa 0.005% offset 147 MPa 177 MPa 169 MPa 177 MPa Significant AE 115 MP 140 MPa 160 MPa 160 MPa Matrix compression 20 MP 60 MPa 35 MPa 55 MPa Creep strain 0.23% 0.36% RT elastic modul 232 GPa 233 Gpa 270 Gpa Ultimate strength 412 MPa 321 MP 362 MPa 247 MPa n refers to a different tensile specimen from the same panel, not the same specimen before and after creep.( 0.3%). This suggests most of the composite creep observed in these specimens was primarily because of the creep of the constituents in combination with some minor bridged crack growth. IV. Discussion For this HNS/MI composite system, excellent creep resistance was observed when loaded up to 103 MPa at 13151C in the pri￾mary fiber direction. The creep strains measured in this study are comparable with those measured for the Sylramic-iBN MI com￾posites when tested under similar conditions. For example, the creep strains of Sylramic-iBN MI composites tested at 13151C and 103 MPa for 100 h were 0.10%70.05%,10 similar to the creep strains measured in this study for the same conditions (Table II). In addition, composite creep appears to lead to load relaxation in the matrix and consequently residual compressive 0 50 100 150 200 250 300 350 400 450 0 0.1 0.2 0.3 0.4 0.5 0.6 Strain, % Stress, MPa Panel A2 After 138 Mpa 100 h Creep creep strain = 0.23% Panel A2 As-Produced (hys loops removed) Residual Compressive Stress 0 20 40 60 80 100 120 140 160 0 0.02 0.04 0.06 0.08 Strain, % Stress, MPa Panel A2 As-Produced (E = 232 GPa) 0.002% offset 147 MPa 0.005% offset stress 0.005% offset 125 MPa 0.002% offset stress (a) (b) (c) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Norm Cum AE As￾produced After Creep 50 100 150 200 250 300 350 Stress, MPa Fig. 4. (a) Room temperature unload–reload hysteresis curves to fail￾ure for panel A2 specimens. Note, the hysteresis loops have been re￾moved from the as-produced specimen for clarity. (b) Acoustic emission generated from the gage section for both tensile specimens. (c) The con￾struction used to determine offset stresses for the as-produced specimen tested to failure. 0 50 100 150 200 250 300 350 400 0 0.1 0.2 0.3 0.4 0.5 0.6 Strain, % Stress, MPa Panel B As-produced (hys loops removed) Panel B After 138 MPa 100 h Creep creep strain= 0.36% Residual Compressive Stress (b) (a) 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Norm Cum AE After-Creep As￾Produced 100 150 200 250 300 350 Stress, MPa Fig. 5. (a) Room temperature unload–reload hysteresis curves to fail￾ure for panel B specimens. Note, the hysteresis loops have been removed from the as-produced specimen for clarity. (b) Acoustic emission gener￾ated from the gage section for both tensile specimens. Table IV. Properties Associated with Non-Linearity in Room Temperature Tested Composites As-Produced and After 138 MPa, 100 H Creep Mechanical property Panel A Panel B As-produced specimenw After creep specimenw As-produced specimenw After creep specimenw 0.002% offset 125 MPa 142 MPa 151 MPa 150 MPa 0.005% offset 147 MPa 177 MPa 169 MPa 177 MPa Significant AE 115 MPa 140 MPa 160 MPa 160 MPa Matrix compression 20 MPa 60 MPa 35 MPa 55 MPa Creep strain — 0.23% — 0.36% RT elastic modulus 232 GPa 233 Gpa 270 Gpa 214 Gpa Ultimate strength 412 MPa 321 MPa 362 MPa 247 MPa w Each specimen refers to a different tensile specimen from the same panel, not the same specimen before and after creep. 1656 Journal of the American Ceramic Society—Morscher and Pujar Vol. 89, No. 5