1654 Journal of the American Ceramic Society-Morscher and pujar Vol. 89. No. 5 Table Il. Tensile Creep Results Creep stress E(RT) E(1315)initial loading E(1315)after creep E(RT) after Minimum strain rate Creep strain Ultimate stress (GPa) (GPa) (GPa) I RT 1315FF 253+ 1315°C 2.80E-09 0.18 103 2.50E-09 267 A2 RT 二 232 41 1315FF 271t 1315°C 2.80E-10 1315°C 3.90E-090.3 1315°C 233 3.10E-09 0.23 1315°C 2.60E-080.44 270 1315°C 5.60E-10 1315°C 138 263 213 214 5.20E-090.36 247 Tested at 1315 C ' Did not fail in hot zone. Tested at room tempe Prior to the onset of tertiary creep. Each row corresponds to an individual tensile specimen. cracking is also evident. Second, the initial loading curves for the both the 103 and 138 MPa creep creep experiments exhibited a wide range of elastic moduli (Ta- fraction composites from panel a offset stress- ble ID). This could be because of variability in constituent con- es. Panel B with the higher Si cor higher offset tent from specimen to specimen or experimental error. Third, stress after creep; however, the increase was not as dramatic the elastic moduli measured from the fast-fracture curves after (only -20% higher) creep were slightly lower relative to moduli measured in the A specimen from panel A2 and a specimen from panel B that same specimens from the initial loading curves. This is discussed had survived 100 h, 138 MPa tensile creep were tested(unload- in more detail in the next section. Fourth, in all the specimens eload) to failure at room temperature. Figure 4(a) shows the that survived 100 h creep, no real loss in fast-fracture strength room temperature tensile stress-strain curves for the as-pro- was observed when compared with fast fracture of as-produced duced and after-creep A2 specimen, and Table IV compares the specimens at 1315.C. Fifth, the creep strain corresponded al- before-creep and after- creep properties. It is evident that the most exactly with the permanent displacement measured after "knee"in the after-creep curve is higher than the room temper- rapid u ture curve. Similarly, significant AE activity occurred at 140 MPa for the after-creep specimen, higher than the 115 MPa ob- served for the as-produced specimen (see dashed arrows in (2) Non-Linearity at Elevated and Room Temperature Fig 4(b). The stresses at which the 0.002% and 0.005% offset Table Ill lists the 0.002% and 0.005% offset-strain stresses de- strain curves intersect the stress-strain curve were 142 and 177 termined for tensile specimens tested at 1315C. Although there MPa, respectively, for the after-creep curve compared with 12 is some specimen-to-specimen variation in the measured offset and 147 MPa, respectively. for the as-produced curve(Fig 4(c) stresses, it is apparent that for panels Al and A2, which had Finally taking the average slope from the top portion of the relatively higher CVI SiC and lower Si content, the after-creep hysteresis loops(after Steen and Valles) one can determine a compressive stress for the crept specimen of approxi- in the intital loading of the creep experiment. This is true for 450 RT Fast Fracture 3.5E08 (hys loops remove 350 1315C Fast Fracture A1-103 MPa s 300(did not fail in hot A2-172 MPa E20E08 15E-08H B-138 MPa 00 138 Mpa; 100 hr; /1315C Fast Fracture 1315° C Creep 150 50E-09 方00E+00da 103Mpa;100hr; 040.50.60.7 1315° C Cree A2-138 MPa 10E-08 0.20.30.4 0.60.7 Total Strain. Fig. 2. Strain rate versus strain Fig 3. Stress-strain history for panel A2 specimenscracking is also evident. Second, the initial loading curves for the creep experiments exhibited a wide range of elastic moduli (Ta￾ble II). This could be because of variability in constituent con￾tent from specimen to specimen or experimental error. Third, the elastic moduli measured from the fast-fracture curves after creep were slightly lower relative to moduli measured in the same specimens from the initial loading curves. This is discussed in more detail in the next section. Fourth, in all the specimens that survived 100 h creep, no real loss in fast-fracture strength was observed when compared with fast fracture of as-produced specimens at 13151C. Fifth, the creep strain corresponded al￾most exactly with the permanent displacement measured after rapid unloading. (2) Non-Linearity at Elevated and Room Temperature Table III lists the 0.002% and 0.005% offset-strain stresses de￾termined for tensile specimens tested at 13151C. Although there is some specimen-to-specimen variation in the measured offset stresses, it is apparent that for panels A1 and A2, which had relatively higher CVI SiC and lower Si content, the after-creep offset stresses are nearly doubled compared with those observed in the intital loading of the creep experiment. This is true for both the 103 and 138 MPa creep tests. The higher fiber volume fraction composites from panel A2 had the highest offset stress￾es. Panel B with the higher Si content also had a higher offset stress after creep; however, the increase was not as dramatic (only B20% higher). A specimen from panel A2 and a specimen from panel B that had survived 100 h, 138 MPa tensile creep were tested (unload– reload) to failure at room temperature. Figure 4(a) shows the room temperature tensile stress–strain curves for the as-pro￾duced and after-creep A2 specimen, and Table IV compares the before-creep and after-creep properties. It is evident that the ‘‘knee’’ in the after-creep curve is higher than the room temper￾ature curve. Similarly, significant AE activity occurred at 140 MPa for the after-creep specimen, higher than the 115 MPa ob￾served for the as-produced specimen (see dashed arrows in Fig. 4(b)). The stresses at which the 0.002% and 0.005% offset strain curves intersect the stress–strain curve were 142 and 177 MPa, respectively, for the after-creep curve compared with 125 and 147 MPa, respectively, for the as-produced curve (Fig. 4(c)). Finally, taking the average slope from the top portion of the hysteresis loops (after Steen and Valles14) one can determine a matrix compressive stress for the crept specimen of approxi- −1.0E−08 −5.0E−09 0.0E+00 5.0E−09 1.0E−08 1.5E−08 2.0E−08 2.5E−08 3.0E−08 3.5E−08 4.0E−08 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Strain, % Strain Rate, mm/mm/sec A1-103 MPa A2-138 MPa B-138 MPa A2-172 MPa MPa B-103 MPa rupture Fig. 2. Strain rate versus strain. Table II. Tensile Creep Results Panel Experiment Creep stress (MPa) E (RT) (GPa) E (1315) initial loading (GPa) E (1315) after creep (GPa) E (RT) after creep Minimum strain rate (s
1 ) Creep strain (%) Ultimate stress (MPa) A1 RT — 262 — — — — — 349 1315FF — — 189 — — — — 253w,z 13151C creep 103 — NA 209 — 2.80E–09 0.18 256w 13151C creep 103 — 223 209 — 2.50E–09 0.09 267w A2 RT — 232 — — — — 412 1315FF — — 182 — — — — 271w,z 13151C creep 103 — 225 198 — 2.80E
10 0.05 295w 13151C creep 138 — 184 157 — 3.90E
09 0.3 291w 13151C creep 138 — 203 — 233 3.10E
09 0.23 321y 13151C creep 172 — 177 — — 2.60E
08 0.44z — B RT — 270 — — — — — 362 13151C creep 103 — 217 209 — 5.60E
10 0.07 255w 13151C creep 138 263 213 — 214 5.20E
09 0.36 247y w Tested at 13151C. z Did not fail in hot zone. y Tested at room temperature. z Prior to the onset of tertiary creep. Each row corresponds to an individual tensile specimen. 0 50 100 150 200 250 300 350 400 450 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 Total Strain, % Stress, MPa RT Fast Fracture (hys loops removed) 1315°C Fast Fracture (did not fail in hot zone) 138 Mpa;100 hr; 1315°C Creep 1315°C Fast Fracture after Creep Creep strain 103 Mpa;100 hr; 1315°C Creep Fig. 3. Stress–strain history for panel A2 specimens. 1654 Journal of the American Ceramic Society—Morscher and Pujar Vol. 89, No. 5