3730 HENAGER et al. SUBCRITICAL CRACK GROWTH: PART I 2.50 0.000l 三 ∠1398K 1473K 2 81 1510 g610 51 2 10 1273K CG-C 1373K 110421043104410451 Fig. 2. Displacement-time curves for a Nicalon-CG (CG-C) Fig 4. Displacement-time curves for identical Hi-C SENI range of 1273 he in 25 K temperature steps for 4XI0- four CG-C specimens tested at 1373 K in argon are included nt-time data were adjusted for each temperat and only the time-depen and T is the temperature in Kelvin. The 630 N load corresponds to an initial applied stress intensity (K) 9.6 MPa m/ at the initial normalized notch length f alW=0. 17(a=0.93 mm). We explored the tem perature dependence of this subcritical cracking for (a) the CFCC materials by performing identical tests over a range of temperatures(Figs 2-4). Initially, one specimen of the CG-C material was tested by taking it to progressively higher temperatures in argon, each exposure lasting for 4x10s(Fig. 2), from 1273 to 2 500 1398 K in 25 K increments and at a constant load of 630 N. The specimen was unloaded during tempera- 4001 ture increases and allowed to come to thermal equilib- rium at the new, higher temperature before it was Total plastic reloaded. Similar temperature-dependent data for the train at zero load CG-C material were obtained by performing identical 1373 K in argon where the specimens were held for elastic. ot tests on three separate specimens at 1 173, 1273, and 1030.000120.0016 longer times at constant loads corresponding to initial (b) K values of 10 MPa m(Fig. 3). The loading dis-70 placements of the samples are not shown--only the hours in Ar: min in O: le-dependent displacements under constant load are plotted in Figs 2-4 400 300 3105 l373K 2105010"21034105 11041.210-41.4104 273K Mid-Point Displacement (m) 1173K Fig. 5.(a)Load-displacement data showing unloading measured in flexure at 0. 5. 25. and 5 10 1105 1.510 2 10 ment on Hi-C material at 1448K in argon. Note that there is essentially no change in the appearance of the loops after 50 Fig. 3. Displacement-time curves for three identical CG-C cated(see Table 2).(b) Similar load-di The data were adjusted so that zero time coincides with th beginning of the time-dependent displacements. The loads on to 202 Pa O,(added each specimen correspond to a K,=10 MPa mn and loop widening, which is attributed to interphase recession.3730 HENAGER et al.: SUBCRITICAL CRACK GROWTH: PART I Fig. 2. Displacement–time curves for a Nicalon-CG (CG-C) SENB specimen tested at 600 N in argon over a temperature range of 1273 to 1398 K in 25 K temperature steps for 4×104 s per step. The displacement–time data were adjusted to zero for each temperature step and only the time-dependent dis￾placements are shown. and T is the temperature in Kelvin. The 630 N load corresponds to an initial applied stress intensity (Ka) of 9.6 MPa m1/2 at the initial normalized notch length of a/W = 0.17 (a = 0.93 mm). We explored the tem￾perature dependence of this subcritical cracking for the CFCC materials by performing identical tests over a range of temperatures (Figs 2–4). Initially, one specimen of the CG-C material was tested by taking it to progressively higher temperatures in argon, each exposure lasting for 4×104 s (Fig. 2), from 1273 to 1398 K in 25 K increments and at a constant load of 630 N. The specimen was unloaded during tempera￾ture increases and allowed to come to thermal equilib￾rium at the new, higher temperature before it was reloaded. Similar temperature-dependent data for the CG-C material were obtained by performing identical tests on three separate specimens at 1173, 1273, and 1373 K in argon where the specimens were held for longer times at constant loads corresponding to initial K values of 10 MPa m1/2 (Fig. 3). The loading dis￾placements of the samples are not shown—only the time-dependent displacements under constant load are plotted in Figs 2–4. Fig. 3. Displacement–time curves for three identical CG-C SENB specimens tested at 1373, 1273, and 1173 K in argon. The data were adjusted so that zero time coincides with the beginning of the time-dependent displacements. The loads on each specimen correspond to a Ka = 10 MPa m1/2. Fig. 4. Displacement–time curves for identical Hi-C SENB specimens tested at 1373 to 1473 K in argon. The curves for four CG-C specimens tested at 1373 K in argon are included for comparison. Fig. 5. (a) Load–displacement data showing unloading– reloading hysteresis loops measured in flexure at 0, 5, 25, and 50 h, respectively, during a subcritical-crack-growth experi￾ment on Hi-C material at 1448 K in argon. Note that there is essentially no change in the appearance of the loops after 50 h of crack growth. Elastic, inelastic, and plastic strains are indi￾cated (see Table 2). (b) Similar load–displacement data show￾ing unloading–reloading hysteresis loops at 0, 5, 25 h in argon at 1448 K but immediately followed by exposure during testing to 202 Pa O2 (added to argon gas feed). Note the slope decrease and loop widening, which is attributed to interphase recession